Lecture XVI The Origin of the Peloric Toad-Flax
I have tried to show previously that species, in the ordinary sense of the word, consist of distinct groups of units. In systematic works these groups are all designated by the name of varieties, but it is usually granted that the units of the system are not always of the same value. Hence we have distinguished between elementary species and varieties proper. The first are combined into species whose common original type is now lost or unknown, and from their characters is derived an hypothetical image of what the common ancestor is supposed to have been. The varieties proper are derived in most cases from still existing types, and therefore are subjoined to them. A closer investigation has shown that this derivation is ordinarily produced by the loss of some definite attribute, or by the re-acquisition of an apparently [460] lost character. The elementary species, on the other hand, must have arisen by the production of new qualities, each new acquisition constituting the origin of a new elementary form.
Moreover we have seen, that such improvements and such losses constitute sharp limits between the single unit-forms. Every type, of course, varies around an average, and the extremes of one form may sometimes reach or even overlap those of the nearest allies, but the offspring of the extremes always return to the type. The transgression is only temporary and a real transition of one form to another does not come within ordinary features of fluctuating variability. Even in the cases of eversporting varieties, where two opposite types are united within one race, and where the succeeding individuals are continually swinging from one extreme to the other, passing through a wide range of intermediate steps, the limits of the variety are as sharply defined and as free from real transgression as in any other form.
In a complete systematic enumeration of the real units of nature, the elementary species and varieties are thus observed to be discontinuous and separated by definite gaps. Every unit may have its youth, may lead a long life in the adult state and may finally die. But through [461] the whole period of its existence it remains the same, at the end as sharply defined from its nearest allies as in the beginning. Should some of the units die out, the gaps between the neighboring ones will become wider, as must often have been the case. Such segregations, however important and useful for systematic distinctions, are evidently only of secondary value, when considering the real nature of the units themselves.
We may now take up the other side of the problem. The question arises as to how species and varieties have originated. According to the Darwinian theory they have been produced from one another, the more highly differentiated ones from the simpler, in a graduated series from the most simple forms to the most complicated and most highly organized existing types. This evolution of course must have been regular and continuous, diverging from time to time into new directions, and linking all organisms together into one common pedigree. All lacunae in our present system are explained by Darwin as due to the extinction of the forms, which previously filled them.
Since Lamarck first propounded the conception of a common origin for all living beings, much has been done to clear up our ideas as to the real nature of this process. The broader [462] aspect of the subject, including the general pedigree of the animal and vegetable kingdom, may be said to have been outlined by Darwin and his followers, but this phase of the subject lies beyond the limits of our present discussion.
The other phase of the problem is concerned with the manner in which the single elementary species and varieties have sprung from one another. There is no reason to suppose that the world is reaching the end of its development, and so we are to infer that the production of new species and varieties is still going on. In reality, new forms are observed to originate from time to time, both wild and in cultivation, and such facts do not leave any doubt as to their origin from other allied types, and according to natural and general laws.
In the wild state however, and even with cultivated plants of the field and garden, the conditions, though allowing of the immediate observation of the origination of new forms, are by no means favorable for a closer inquiry into the real nature of the process. Therefore I shall postpone the discussion of the facts till another lecture, as their bearing will be more easily understood after having dealt with more complete cases.
These can only be obtained by direct experimentation. Comparative studies, of course, [463] are valuable for the elucidation of general problems and broad features of the whole pedigree, but the narrower and more practical question as to the genetic relation of the single forms to one another must be studied in another way, by direct experiment. The exact methods of the laboratory must be used, and in this case the garden is the laboratory. The cultures must be guarded with the strictest care and every precaution taken to exclude opportunities for error. The parents and grandparents and their offspring must be kept pure and under control, and all facts bearing upon the birth or origin of the new types should be carefully recorded.
Two great difficulties have of late stood in the way of such experimental investigation. One of them is of a theoretical, the, other of a practical nature. One is the general belief in the supposed slowness of the process, the other is the choice of adequate material for experimental purposes. Darwin’s hypothesis of natural selection as the means by which new types arise, is now being generally interpreted as stating the slow transformation of ordinary fluctuating divergencies from the average type into specific differences. But in doing so it is overlooked that Quetelet’s law of fluctuating variability was not yet discovered at the time, when Darwin propounded his theory. So there [464] is no real and intimate connection between these two great conceptions. Darwin frequently pointed out that a long period of time might be needed for slow improvements, and was also a condition for the occurrence of rare sports. In any case those writers have been in error, according to my opinion, who have refrained from experimental work on the origin of species, on account of this narrow interpretation of Darwin’s views. The choice of the material is quite another question, and obviously all depends upon this choice. Promising instances must be sought for, but as a rule the best way is to test as many plants as possible. Many of them may show nothing of interest, but some might lead to the desired end.
For to-day’s lecture I have chosen an instance, in which the grounds upon which the choice was based are very evident. It is the origin of the peloric toad-flax (Linaria vulgaris peloria).
The ground for this choice lies simply in the fact that the peloric toad-flax is known to have originated from the ordinary type at different times and in different countries, under more or less divergent conditions. It had arisen from time to time, and hence I presumed that there was a chance to see it arise again. If this should happen under experimental circumstances [465] the desired evidence might easily be gathered. Or, to put it in other words, we must try to arrange things so as to be present at the time when nature produces another of these rare changes.
There was still another reason for choosing this plant for observational work. The step from the ordinary toad-flax to the peloric form is short, and it appears as if it might be produced by slow conversion. The ordinary species produces from time to time stray peloric flowers. These occur at the base of the raceme, or rarely in the midst of it. In other species they are often seen at the summit. Terminal pelories are usually regular, having five equal spurs. Lateral pelories are generally of zygomorphic structure, though of course in a less degree than the normal bilabiate flowers, but they have unequal spurs, the middle one being of the ordinary length, the two neighboring being shorter, and those standing next to the opposite side of the flower being the shortest of all. This curious remainder of the original, symmetrical structure of the flower seems to have been overlooked hitherto by the investigators of peloric toad-flaxes.
The peloric variety of this plant is characterized by its producing only peloric flowers. No single bilabiate or one-spurred flower remains.
[466] I once had a lot of nearly a hundred specimens of this fine variety, and it was a most curious and beautiful sight to observe the many thousands of nearly regular flowers blooming at the same time. Some degree of variability was of course present, even in a large measure. The number of the spurs varied between four and six, transgressing these limits in some instances, but never so far as to produce really one-spurred flowers. Comparing this variety with the ordinary type, two ways of passing over from the one to the other might be imagined. One would entail a slow increase of the number of the peloric flowers on each plant, combined with a decrease of the number of the normal ones, the other a sudden leap from one extreme to the other without any intermediate steps. The latter might easily be overlooked in field observations and their failure may not have the value of direct proof. They could never be overlooked, on the other hand, in experimental culture.
The first record of the peloric toad-flax is that of Zioberg, a student of Linnaeus, who found it in the neighborhood of Upsala. This curious discovery was described by Rudberg in his dissertation in the year 1744. Soon afterwards other localities were discovered by Link near Gottingen in Germany about 1791 and afterwards [467] in the vicinity of Berlin, as stated by Ratzeburg, 1825. Many other localities have since been indicated for it in Europe, and in my own country some have been noted of late, as for instance near Zandvoort in 1874 and near Oldenzaal in 1896. In both these last named cases the peloric form arose spontaneously in places which had often been visited by botanists before the recorded appearance, and therefore, without any doubt, they must have been produced directly and independently by the ordinary species which grows in the locality. The same holds good for other occurrences of it. In many instances the variety has been recorded to disappear after a certain lapse of time, the original specimens dying out and no new ones being produced. Linaria is a perennial herb, multiplying itself easily by buds growing on the roots, but even with this means of propagation its duration seems to have definite limits.
There is one other important point arguing strongly for the independent appearance of the peloric form in its several localities. It is the difficulty of fertilization and the high degree of sterility, even if artificially pollinated. Bees and bumble-bees are unable to crawl into the narrow tubular flowers, and to bring the fertilizing pollen to the stigma. Ripe capsules with seeds [468] have never been seen in the wild state. The only writer who succeeded in sowing seeds of the peloric variety was Wildenow and he got only very few seedlings. But even in artificial pollination the result is the same, the anthers seeming to be seriously affected by the change. I tried both self-fertilization and cross-pollination, and only with utmost care did I succeed in saving barely a hundred seeds. In order to obtain them I was compelled to operate on more than a thousand flowers on about a dozen peloric plants.
The variety being wholly barren in nature, the assumption that the plants in the different recorded localities might have a common origin is at once excluded. There must have been at least nearly as many mutations as localities. This strengthens the hope of seeing such a mutation happen in one’s own garden. It should also be remembered that peloric flowers are known to have originated in quite a number of different species of Linaria, and also with many of the allied species within the range of the Labiatiflorae.
I will now give the description of my own experiment. Of course this did not give the expected result in the first year. On the contrary, it was only after eight years’ work that I had the good fortune of observing the mutation. [469] But as the whole life-history of the preceding generations had been carefully observed and recorded, the exact interpretation of the fact was readily made.
My culture commenced in the year 1886. I chose some plants of the normal type with one or two peloric flowers besides the bilabiate majority which I found on a locality in the neighborhood of Hilversum in Holland. I planted the roots in my garden and from them had the first flowering generation in the following summer. From their seeds I grew the second generation in three following years. They flowered profusely and produced in 1889 only one, and in 1890 only two peloric structures. I saved the seeds in 1889 and had in 1890-1891 the third generation. These plants likewise flowered only in the second year, and gave among some thousands of symmetrical blossoms, only one five-spurred flower. I pollinated this flower myself, and it produced abundant fruit with enough seeds for the entire culture in 1892, and they only were sown.
Until this year my generations required two years each, owing to the perennial habit of the plants. In this way the prospects of the culture began to decrease, and I proposed to try to heighten my chances by having a new generation yearly. With this intention I sowed the [470] selected seeds in a pan in the glasshouse of my laboratory and planted them out as soon as the young stems had reached a length of some few centimeters. Each seedling was put in a separate pot, in heavily manured soil. The pots were kept under glass until the beginning of June, and the young plants produced during this period a number of secondary stems from the curious hypocotylous buds which are so characteristic of the species. These stems grew rapidly and as soon as they were strong enough, the plants were put into the beds. They all, at least nearly all, some twenty specimens, flowered in the following month.
I observed only one peloric flower among the large number present. I took the plant bearing this flower and one more for the culture of the following year, and destroyed all others. These two plants grew on the same spot, and were allowed to fertilize each other by the agency of the bees, but were kept isolated from any other congener. They flowered abundantly, but produced only one-spurred bilabiate flowers during the whole summer. They matured more than 10 cu. cm. of seeds.
It is from this pair of plants that my peloric race has sprung. And as they are the ancestors of the first closely observed case of peloric mutation, [471] it seems worth while to give some details regarding their fertilization.
Isolated plants of Linaria vulgaris do not produce seed, even if freely pollinated by bees. Pollen from other plants is required. This requirement is not at all restricted to the genus Linaria, as many instances are known to occur in different families. It is generally assumed that the pollen of any other individual of the same species is capable of producing fertilization, although it is to be said that a critical examination has been made in but few instances.
This, however, is not the case, at least not in the present instance. I have pollinated a number of plants, grown from seed of the same strain and combined them in pairs, and excluded the visits of insects, and pollen other than that of the plant itself and that of the specimen with which it was paired. The result was that some pairs were fertile and others barren. Counting these two groups of pairs, I found them nearly equal in number, indicating thereby that for any given individual the pollen of half of the others is potent, but that of the other half impotent. From these facts we may conclude the presence of a curious case of dimorphy, analogous to that proposed for the primroses, but without visible differentiating marks in the flowers. At least such opposite characters [472] have as yet not been ascertained in the case of our toad-flax.
In order to save seed from isolated plants it is necessary, for this reason, to have at least two individuals, and these must belong to the two physiologically different types. Now in the year 1892, as in other years, my plants, though separated at the outset by distances of about 20 cm. from each other, threw out roots of far greater length, growing in such a way as to abolish the strict isolation of the individuals. Any plot may produce several stems from such roots, and it is manifestly impossible to decide whether they all belong to one original plant or to the mixed roots of several individuals. No other strains were grown on the same bed with my plants however, and so I considered all the stems of the little group as belonging to one plant. But their perfect fertility showed, according to the experience described, that there must have been at least two specimens mingled together.
Returning now to the seeds of this pair of plants, I had, of course, not the least occasion to ascribe to it any higher value than the harvest of former years. The consequence was that I had no reason to make large sowings, and grew only enough young plants to have about 50 in bloom in the summer of 1894. Among [473] these, stray peloric flowers were observed in somewhat larger number than in the previous generations, 11 plants bearing one or two, or even three such abnormalities. This however, could not be considered as a real advance, since such plants may occur in varying, though ordinarily small numbers in every generation.
Besides them a single plant was seen to bear only peloric flowers; it produced racemes on several stems and their branches. All were peloric without exception. I kept it through the winter, taking care to preserve a complete isolation of its roots. The other plants were wholly destroyed. Such annihilation must include both the stems and roots and the latter of course requires considerable labor. The following year, however, gave proof of the success of the operation, since my plant bloomed luxuriously for the second time and remained true to the type of the first year, producing peloric flowers exclusively.
Here we have the first experimental mutation of a normal into a peloric race. Two facts were clear and simple. The ancestry was known for over a period of four generations, living under the ordinary care and conditions of an experimental garden, isolated from other toad-flaxes, but freely fertilized by bees or at times by myself. This ancestry was quite constant as to [474] the peloric peculiarity, remaining true to the wild type as it occurs everywhere in my country, and showing in no respect any tendency to the production of a new variety.
The mutation took place at once. It was a sudden leap from the normal plants with very rare peloric flowers to a type exclusively peloric. No intermediate steps were observed. The parents themselves had borne thousands of flowers during two summers, and these were inspected nearly every day, in the hope of finding some pelories and of saving their seed separately. Only one such flower was seen. If there had been more, say a few in every hundred flowers, it might be allowable to consider them as previous stages, showing a preparation of the impending change. But nothing of this kind was observed. There was simply no visible preparation for the sudden leap.
This leap, on the other hand, was full and complete. No reminiscence of the former condition remained. Not a single flower on the mutated plant reverted to the previous type. All were thoroughly affected by the new attribute, and showed the abnormally augmented number of spurs, the tubular structure of the corolla and the round and narrow entrance of its throat. The whole plant departed absolutely from the old type of its progenitors.
[475] Three ways were open to continue my experiment. The first was indicated by the abundant harvest from the parent-plants of the mutation. It seemed possible to compare the numerical proportion of the mutated seeds with those of normal plants. In order to ascertain this proportion I sowed the greatest part of my 10 cu. cm. of seed and planted some 2,000 young plants in little pots with well-manured soil. I got some 1,750 flowering plants and observed among them 16 wholly peloric individuals. The numerical proportion of the mutation was therefore in this instance to be calculated equal to about 1% of the whole crop.
This figure is of some importance. For it shows that the chance of finding mutations requires the cultivation of large groups of individuals. One plant in each hundred may mutate, and cultures of less than a hundred specimens must therefore be entirely dependent on chance for the appearance of new forms, even if such should accidentally have been produced and lay dormant in the seed. In other cases mutations may be more numerous, or on the contrary, more rare. But the chance of mutative changes in larger numbers is manifestly much reduced by this experiment, and they may be expected to form a very small proportion of the culture.
[476] The second question which arose from the above result was this. Could the mutation be repeated? Was it to be ascribed to some latent cause which might be operative more than once? Was there some hidden tendency to mutation, which, ordinarily weak, was strengthened in my cultures by some unknown influence? Was the observed mutation to be explained by a common cause with the other cases recorded by field-observations? To answer this question I had only to continue my experiment, excluding the mutated individuals from any intercrossing with their brethren. To this end I saved the seeds from duly isolated groups in different years and sowed them at different times. For various causes I was not prepared to have large cultures from these seeds, but notwithstanding this, the mutation repeated itself. In one instance I obtained two, in another, one peloric plant with exclusively many-spurred flowers. As is easily understood, these were related as “nieces” to the first observed mutants. They originated in quite the same way, by a sudden leap, without any preparation and without any intermediate steps.
Mutation is proved by this experience to be of an iterative nature. It is the expression of some concealed condition, or as it is generally [477] called, of some hidden tendency. The real nature of this state of the hereditary qualities is as yet wholly unknown. It would not be safe to formulate further conclusions before the evidence offered by the evening-primroses is considered.
Thirdly, the question arises, whether the mutation is complete, not only as to the morphologic character, but also as to the hereditary constitution of the mutated individuals. But here unfortunately the high degree of sterility of the peloric plants, as previously noted, makes the experimental evidence a thing of great difficulty. During the course of several years I isolated and planted together the peloric individuals already mentioned, all in all some twenty plants. Each individual was nearly absolutely sterile when treated with its own pollen, and the aid of insects was of no avail. I intercrossed my plants artificially, and pollinated more than a thousand flowers. Not a single one gave a normal fruit, but some small and nearly rudimentary capsules were produced, bearing a few seeds. From these I had 119 flowering plants, out of which 106 were peloric and 13 one-spurred. The great majority, some 90%, were thus shown to be true to their new type. Whether the 10% reverting ones were truly atavists, or whether they were [478] only vicinists, caused by stray pollen grains from another culture, cannot of course be decided with sufficient certitude.
Here I might refer to the observations concerning the invisible dimorphous state of the flowers of the normal toad-flax. Individuals of the same type, when fertilized with each other, are nearly, but not absolutely, sterile. The yield of seeds of my peloric plants agrees fairly well with the harvest which I have obtained from some of the nearly sterile pairs of individuals in my former trial. Hence the suggestion is forced upon us that perhaps, owing to some unknown cause, all the peloric individuals of my experiment belonged to one and the same type, and were sterile for this reason only. If this is true, then it is to be presumed that all previous investigators have met the same condition, each having at hand only one of the two required types. And this discussion has the further advantage of showing the way, in which perhaps a full and constant race of peloric toad-flaxes may be obtained. Two individuals of different type are required to start from. They seem as yet never to have arisen from one group of mutations. But if it were possible to combine the products of two mutations obtained in different countries and under different conditions, there would be a chance [479] that they might belong to the supposed opposite types, and thus be fertile with one another. My peloric plants are still available, and the occurrence of this form elsewhere would give material for a successful experiment. The probability thereof is enhanced by the experience that my peloric plants bear large capsules and a rich harvest of seeds when fertilized from plants of the normal one-spurred race, while they remain nearly wholly barren by artificial fertilization with others. I suppose that they are infertile with the normal toad-flaxes of their own sexual disposition, but fertile with those of the opposite constitution. At all events the fact that they may bear abundant seed when properly pollinated is an indication of successful experiments on the possibility of gaining a hereditary race with exclusively peloric flowers. And such a race would be a distinct gain for sundry physiologic inquiries, and perhaps not wholly destitute of value from an horticultural point of view.
Returning now to the often recorded occurrence of peloric toad-flaxes in the wild state and recalling our discussion about the improbability of a dispersion from one locality to another by seed, and the probability of independent origin for most of these cases, we are confronted with the conception that a latent [480] tendency to mutation must be universally present in the whole species. Another observation, although it is of a negative character, gains in importance from this point of view. I refer to the total lack of intermediate steps between normal and peloric individuals. If such links had ordinarily been produced previous to the purely peloric state they would no doubt have been observed from time to time. This is so much the more probable as Linaria is a perennial herb, and the ancestors of a mutation might still be in a flowering condition together with their divergent offspring. But no such intermediates are on record. The peloric toad-flaxes are, as a rule, found surrounded by the normal type, but without intergrading forms. This discontinuity has already been insisted upon by Hofmeister and others, even at the time when the theory of descent was most under discussion, and any link would surely have been produced as a proof of a slow and continuous change. But no such proof has been found, and the conclusion seems admissible that the mutation of toad-flaxes ordinarily, if not universally, takes place by a sudden step. Our experiment may simply be considered as a thoroughly controlled instance of an often recurring phenomenon. It teaches us how, in the [481] main, the peloric mutations must be assumed to proceed.
This conception may still be broadened. We may include in it all similar occurrences, in allied and other species. There is hardly a limit to the possibilities which are opened up by this experience. But it will be well to refrain from hazardous theorizing, and consider only those cases which may be regarded as exact repetitions of the same phenomenon and of which our culture is one of the most recent instances on record. We will limit ourselves to the probable origin of peloric variations at large, of which little is known, but some evidence may be derived from the recorded facts. Only one case can be said to be directly analogous to our observations.
This refers to the peloric race of the common snapdragon, or Antirrhinum majus of our gardens. It is known to produce peloric races from time to time in the same way as does the toadflax. But the snapdragon is self-fertile and so is its peloric variety. Some cases are relatively old, and some of them have been recorded and in part observed by Darwin. Whence they have sprung and in what manner they were produced, seems never to have been noted. Others are of later origin, and among these one or two varieties have been accidentally produced [482] in the nursery of Mr. Chr. Lorenz in Erfurt, and are now for sale, the seeds being guaranteed to yield a large proportion of peloric individuals. The peloric form in this case appeared at once, but was not isolated, and was left free to visiting insects, which of course crossed it with the surrounding varieties. Without doubt the existence of two color-varieties of the peloric type, one of a very dark red, indicating the “Black prince” variety as the pollen-parent, and the other with a white tube of the corolla, recalling the form known as “Delila,” is due to these crossings. I had last year (1903) a large lot of plants, partly normal and partly peloric, but evidently of hybrid origin, from seeds from this nursery, showing moreover all intermediate steps between nearly wholly peloric individuals and apparently normal ones. I have saved the seeds of the isolated types and before seeing the flowers of their offspring, nothing can be said about the purity and constancy of the type, when freed from hybrid admixtures. The peloric snapdragon has five small unequal spurs at the base of its long tube, and in this respect agrees with the peloric toad-flax.
Other pelories are terminal and quite regular, and occur in some species of Linaria, where I observed them in Linaria dalmatica. The [483] terminal flowers of many branches were large and beautifully peloric, bearing five long and equal spurs. About their origin and inheritance nothing is known.
A most curious terminal pelory is that of the common foxglove or Digitalis purpurea. As we have seen in a previous lecture, it is an old variety. It was described and figured for the first time by Vrolik of Amsterdam, and the original specimens of his plates are still to be seen in the collections of the botanic garden of that university. Since his time it has been propagated by seed as a commercial variety, and may be easily obtained. The terminal flower of the central stem and those of the branches only are affected, all other flowers being wholly normal. Almost always it is accompanied by other deviations, among which a marked increase of the number of the parts of the corolla and other whorls is the most striking. Likewise supernumerary petals on the outer side of the corolla, and a production of a bud in the center of the capsule may be often met with. This bud as a rule grows out after the fading away of the flower, bursting through the green carpels of the unripe fruit, and producing ordinarily a secondary raceme of flowers. This raceme is a weak but exact repetition of the first, bearing symmetrical foxgloves all [484] along and terminating in a peloric structure. On the branches these anomalies are more or less reduced, according to the strength of the branch, and conforming to the rule of periodicity, given in our lecture on the “five-leaved” clover. Through all this diminution the peloric type remains unchanged and therefore becomes so much the purer, the weaker the branches on which it stands.
I am not sure whether such peloric flowers have ever been purely pollinated and their seed saved separately, but I have often observed that the race comes pure from the seed of the zygomorphic flowers. It is as yet doubtful whether it is a half race or a double race, and whether it might be purified and strengthened by artificial selection. Perhaps the determination of the hereditary percentage described when dealing with the tricotyls might give the clue to the acquisition of a higher specialized race. The variety is old and widely disseminated, but must be subjected to quite a number of additional experiments before it can be said to be sufficiently understood.
The most widespread peloric variety is that of Gloxinia. It has erect instead of drooping flowers; and with the changed position the structure is also changed. Like other pelories it has five equal stamens instead of four unequal [485] ones, and a corolla with five equal segments instead of an upper and a lower lip. It shows the peloric condition in all of its flowers and is often combined with a small increase of the number of the parts of the whorls. It is for sale under the name of erecta, and may be had in a wide range of color-types. It seems to be quite constant from seed.
Many other instances of peloric flowers are on record. Indian cress or Tropaeolum majus loses the spur in some double varieties and with it most of its symmetrical structure; it seems to be considered justly as a peloric malformation. Other species produce such anomalies only from time to time and nothing is known about their hereditary tendency. One of the most curious instances is the terminal flower of the raceme of the common laburnum, which loses its whole papilionaceous character and becomes as regularly quinate as a common buttercup.
Some families are more liable to pelorism than others. Obviously all the groups, the flowers of which are not symmetrical, are to be excluded. But then we find that labiates and their allies among the dicotyledonous plants, and orchids among the monocotyledonous ones are especially subjected to this alteration. In both groups many genera and a long list of species [486] could be quoted as proof. The family of the labiates seems to be essentially rich in terminal pelories, as for instance in the wild sage or Salvia and the dead-nettle or Lamium. Here the pelories have long and straight corolla-tubes, which are terminated by a whorl of four or five segments. Such forms often occur in the wild state and seem to have a geographic distribution as narrowly circumscribed as in the case of many small species. Those of the labiates chiefly belong to southern Europe and are unknown at least in some parts of the other countries. On the contrary terminal pelories of Scrophularia nodosa are met with from time to time in Holland. Such facts clearly point to a common origin, and as only the terminal flowers are affected by the malformation, the fertility of the whole plant is evidently not seriously infringed upon.
Before leaving the labiates, we may cite a curious instance of pelorism in the toad-flax, which is quite different from the ordinary peloric variety. This latter may be considered from a morphologic standpoint to be owing to a five-fold repetition of the middle part of the underlip. This conception would at once explain the occurrence of five spurs and of the orange border all around the corolla-tube. We might readily imagine that any other of the five [487] parts of the corolla could be repeated five-fold, in which case there would be no spur, and no orange hue on the upper corolla-ring. Such forms really occur, though they seem to be more rare than the five-spurred pelories. Very little is known about their frequency and hereditary qualities.
Orchids include a large number of peloric monstrosities and moreover a wild pelory which is systematically described not only as a separate species but even as a new genus. It bears the name of Uropedium lindenii, and is so closely related to Cypripedium caudatum that many authors take it for the peloric variety of this plant. It occurs in the wild state in some parts of Mexico, where the Cypripedium also grows. Its claims to be a separate genus are lessened by the somewhat monstrous condition of the sexual organs, which are described as quite abnormal. But here also, intermediates are lacking, and this fact points to a sudden origin.
Many cases of pelorism afford promising material for further studies of experimental mutations. The peloric toad-flax is only the prototype of what may be expected in other cases. No opportunity should be lost to increase the as yet too scanty, evidence on this point.
[488]
LECTURE XVII
THE PRODUCTION OF DOUBLE FLOWERS
Mutations occur as often among cultivated plants as among those in the wild state. Garden flowers are known to vary markedly. Much of their variability, however, is due to hybridism, and the combination of characters previously separate has a value for the breeder nearly equal the production of really new qualities. Nevertheless there is no doubt that some new characters appear from time to time.
In a previous lecture we have seen that varietal characters have many features in common. One of them is their frequent recurrence both in the same and in other, often very distantly related, species. This recurrence is an important factor in the choice of the material for an experimental investigation of the nature of mutations.
Some varieties are reputed to occur more often and more readily than others. White-colored varieties, though so very common, seem for the most part to be of ancient date, but only few [489] have a known origin, however. Without any doubt many of them have been found in a wild state and were introduced into culture. On the other hand double flowers are exceedingly rare in the wild state, and even a slight indication of a tendency towards doubling, the stray petaloid stamens, are only rarely observed growing wild. In cultivation, however, double flowers are of frequent occurrence; hence the conclusion that they have been produced in gardens and nurseries more frequently than perhaps any other type of variety.
In the beginning of my experimental work I cherished the hope of being able to produce a white variety. My experiments, however, have not been successful, and so I have given them up temporarily. Much better chances for a new double variety seemed to exist, and my endeavors in this direction have finally been crowned with success.
For this reason I propose to deal now with the production of double flowers, to inquire what is on record about them in horticultural literature, and to give a full description of the origin thereof in an instance which it was my good fortune to observe in my garden.
Of course the historical part is only a hasty survey of the question and will only give such evidence as may enable us to get an idea of the [490] chances of success for the experimental worker. In the second half of the seventeenth century (1671), my countryman, Abraham Munting, published a large book on garden plants with many beautiful figures. It is called “Waare Oeffeninge der Planters,” or “True Exercises With Plants.” The descriptions pertain to ordinary typical species in greater part, but garden varieties receive special attention. Among these a long list of double flowers are to be seen. Double varieties of poppies, liverleaf (Hepatica), wallflowers (Cheiranthus), violets, Caltha, Althaea, Colchicum, and periwinkles (Vinca), and a great many other common flowers were already in cultivation at that time.
Other double forms have been since added. Many have been introduced from Japan, especially the Japanese marigold, Chrysanthemum indicum. Others have been derived from Mexico, as for instance the double zinnias. The single dahlias only seem to have been originally known to the inhabitants of Mexico. They were introduced into Spain at about 1789, and the first double ones were produced in Louvain, Belgium, in 1814. The method of their origin has not been described, and probably escaped the originators themselves. But in historical records we find the curious statement that it took place after three years’ work. This indicates [491] a distinct plan, and the possibility of carrying it to a practical conclusion within a few years’ time.
Something more is known about other cases. Garden anemones, Anemone coronaria, are said to have become double in the first half of the last century in an English nursery. The owner, Williamson, observing in his beds a flower with a single broadened stamen, saved its seeds separately, and in the next generations procured beautifully filled flowers. These he afterwards had crossed by bees with a number of colored varieties, and in this way succeeded in producing many new double types of anemone.
The first double petunia is known to have suddenly and accidentally arisen from ordinary seed in a private garden at Lyons about 1855. From this one plant all double races and-varieties have been derived by natural and partly by artificial crosses. Carriere, who reported this fact, added that likewise other species were known at that time to produce new double varieties rapidly. The double fuchsias originated about the same time (1854) and ten years later the range of double varieties of this plant had become so large that Carriere found it impossible to enumerate all of them.
Double carnations seem to be relatively old, double corn-flowers and double blue-bells being [492] of a later period. A long list could easily be made, to show that during the whole history of horticulture double varieties have arisen from time to time. As far as we can judge, such appearances have been isolated and sudden. Sometimes they sprang into existence in the full display of their beauty, but most commonly they showed themselves for the first time, exhibiting only spare supernumerary petals. Whenever such sports were worked up, a few years sufficed to reach the entire development of the new varietal attribute.
From this superficial survey of historical facts, the inference is forced upon us that the chance of producing a new double variety is good enough to justify the attempt. It has frequently succeeded for practical purposes, why should it not succeed as well for purely scientific investigation? At all events the type recommends itself to the student of nature, both on account of its frequency, and of the apparent insignificance of the first step, combined with the possibility of rapidly working up from this small beginning of one superfluous petal towards the highest degree of duplication.
Compared with the tedious experimental production of the peloric toad-flax, the attempt to produce a double flower has a distinct attraction. The peloric toad-flax is nothing new; the [493] experiment was only a repetition of what presumably takes place often within the same species. To attempt to produce a double variety we may choose any species, and of course should select one which as yet has not been known to produce double flowers. By doing so we will, if we succeed, produce something new. Of course, it does not matter whether the new variety has an horticultural interest or not, and it seems preferable to choose a wild or little cultivated species, to be quite sure that the variety in question is not already in existence. Finally the prospect of success seems to be enhanced if a species is chosen, the nearest allies of which are known to have produced double flowers.
For these reasons and others I chose for my experiment the corn-marigold, or Chrysanthemum segetum. It is also called the golden cornflower. In the wheat and rye fields of central Europe it associates with the blue-bottle or blue corn-flower. It is sometimes cultivated and the seeds are offered for sale by many nurserymen. It has a cultivated variety, called grandiflorum, which is esteemed for its brilliancy and long succession of golden bloom. This variety has larger flower-heads, surrounded with a fuller border of ray-florets. The species belongs to a genus many species of which have produced [494] double varieties. One of them is the Japanese marigold, others are the carinatum and the imbricatum species. Nearly allied are quite a number of garden-plants with double flower-heads, among which are the double camomiles.
My attention was first drawn to the structure of the heads and especially to the number of the ray-florets of the corn-marigold. The species appertains to that group of composites which have a head of small tubular florets surrounded by a broad border of rays. These rays, when counted, are observed to occur in definite numbers, which are connected with each other by a formula, known as “the series” of Braun and Schimper. In this formula, which commences with 1 and 2, each number is equal to the sum of the two foregoing figures. Thus 5, 8 and 13 are very frequent occurrences, and the following number, 21, is a most general one for apparently full rays, such as in daisies, camomiles, Arnica and many other wild and cultivated species.
These numbers are not at all constant. They are only the averages, around which the real numbers fluctuate. There may even be an overlapping of the extremes, since the fluctuation around 13 may even go beyond 8 and 21, and so on. But such extremes are only found in stray flowers, occurring on the same [495] individuals with the lesser degrees of deviation.
Now the marigold averages 13, and the grandiflorum 21 rays. The wild species is pure in this respect, but the garden-variety is not. The seeds which are offered for sale usually contain a mixture of both forms and their hybrids. So I had to isolate the pure types from this mixture and to ascertain their constancy and mutual independency. To this end I isolated from the mixture first the 13-rayed, and afterwards the 21-rayed types. As the marigolds are not sufficiently self-fertile, and are not easily pollinated artificially, it seemed impossible to carry on these two experiments at the same time and in the same garden. I devoted the first three years to the lower form, isolated some individuals with 12-13 rays out of the mixture of 1892 and counted the ray-florets on the terminal head of every plant of the ensuing generation next year. I cultivated and counted in this way above 150 individuals and found an average of exactly 13 with comparatively few individuals displaying 14 or only 12 rays, and with the remainder of the plants grouped symmetrically around this average. I continued the experiment for still another year and found the same group of figures. I was then satisfied as to the purity of the isolated strain. Next year I sowed a new mixture in [496] order to isolate the reputed pure grandiflorum type. During the beginning of the flowering period I ruthlessly threw away all plants displaying less than 21 rays in the first or terminal head. But this selection was not to be considered as complete, because the 13-rayed race may eventually transgress its boundary and come over to the 21 and more. This made a second selection necessary. On the selected plants all the secondary heads were inspected and their ray-florets counted. Some individuals showed an average of about 13 and were destroyed. Others gave doubtful figures and were likewise eliminated, and only 6 out of a lot of nearly 300 flowering plants reached an average of 21 for all of the flowers.
Our summer is a short one, compared with the long and beautiful summer of California, and it was too late to cut off the faded and the open flowers, and await new ones, which might be purely fertilized after the destruction of all minor plants. So I had to gather the seed from flowers, which might have been partially fertilized by the wrong pollen. This however, is not so great a drawback in selection experiments as might be supposed at first sight. The selection of the following year is sure to eliminate the offspring of such impure parentage.
[497] A far more important principle is that of the hereditary percentage, already discussed in our lecture on the selection of monstrosities. In our present case it had to be applied only to the six selected plants of 1895. To this end the seeds of each of them were sown separately, the ray-florets of the terminal heads of each of the new generation were counted, and curves and averages were made up for the six groups. Five of them gave proof of still being mixtures and were wholly rejected. The children of the sixth parent, however, formed a group of uniform constitution, all fluctuating around the desired average of 21. All in all the terminal heads of over 1,500 plants have been subjected to the somewhat tedious work of counting their ray-florets. And this not in the laboratory, but in the garden, without cutting them off. Otherwise it would obviously have been impossible to recognize the best plants for preservation. I chose only two plants which in addition recommended themselves by the average number of rays on their secondary heads, sowed their seeds next year separately and compared the numerical constitution of their offspring. Both groups averaged 21 and were distributed very symmetrically around this mean. This result [498] showed that no further selection could be of any avail, and that I had succeeded in purifying the 21-rayed grandiflorum variety.
It is from this grandiflorum that I have finally produced my double variety. In the year 1896 I selected from among the above quoted 1,500 plants, 500 with terminal heads bearing 21 or more rays. On these I counted the rays of all the secondary heads about the middle of August (1896) and found that they had, as a rule, retrograded to lower figures. On many thousands of heads only two were found having 22 rays. All others had the average number of 21 or even less. I isolated the individual which bore these two heads, allowed them to be fertilized by insects with the pollen of some of the best plants of the same group, but destroyed the remainder.
This single exceptional plant has been the starting point of my double variety. It was not remarkable for its terminal head, which exhibited the average number of rays of the 21-rayed race. Nor was it distinguished by the average figure for all its heads. It was only selected because it was the one plant which had some secondary heads with one ray more than all the others. This indication was very slight, and could not have been detected save by the counting of the rays of thousands of heads.
[499] But the rarity of the anomaly was exactly the indication wanted, and the same deviation would have had no signification whatever, had it occurred in a group fluctuating symmetrically around the average figure. On the other hand, the observed anomaly was only an indication, and no guarantee of future developments.
Here it should be remarked that the indication alluded to was not the appearance of the expected character of doubling in ever so slight a measure. It was only a guide to be followed in further work. The real character of double flower-heads among composites lies in the production of rays on the disk. No increase of the number of the outer rays can have the same significance. A hasty inspection of double flower heads may convey the idea that all rays are arranged around a little central cluster of disk-florets, the remainder of the original disk-florets but a closer investigation will always reveal the fallacy of this conclusion. Hidden between the inner rays, and covered by them, lie the little tubular and fertile florets everywhere on the disk. They may not be easily seen, but if the supernumerary rays are pulled out, the disk may be seen to bear numerous small florets at intervals. But these intervals are not at all numerous, showing thereby that only a relatively small number of tubes has been [500] converted into rays. This conversion is obviously the true mark of the doubling, and before traces of it are found, no assertion whatever can be given as to the issue of the pedigree experiment.
Three more years were required before this first, but decisive trace was discovered. During these years I subjected my strain to the same sharp selection as has already been described. The chosen ancestor of the race had flowered in 1896, and the next year I sowed its seeds only. From this generation I chose the one plant with the largest number of rays in its terminal head, and repeated this in the following year.
The consequence was that the average number of rays increased rapidly, and with it the absolute maximum of the whole strain. The average came up from 21 to 34. Brighter and brighter crowns of the yellow rays improved my race, until it became difficult and very time consuming to count all the large rays of the borders. The largest numbers determined in the succeeding generations increased by leaps from 21 to 34 in the first year, and thence to 48 and 66 in the two succeeding summers. Every year I was able to save enough seed from the very best plant and to use it only for the continuance of the race. Before the selected plants were allowed to open the flowers from which the seed [501] was to be gathered, nearly the whole remaining culture was exterminated, excepting only some of the best examples, in order to have the required material for cross-pollination by insects. Each new generation was thereby as sharply selected as possible with regard to both parents.
All flower-heads were of course closely inspected. Not the slightest indication of real doubling was discovered, even in the summer of 1899 in the fourth generation of my selected race. But among the best the new character suddenly made its appearance. It was at the commencement of September (1899), too late to admit of the seeds ripening before winter. An inspection of the younger heads was made, which revealed three heads with some few rays in the midst of the disk on one plant, the result of the efforts of four years. Had the germ of the mutation lain hidden through all this time? Had it been present, though dormant in the original sample of seed? Or had an entirely new creation taken place during my continuous endeavors? Perhaps as their more or less immediate result? It is obviously impossible to answer these questions, before further and similar experiments shall have been performed, bringing to light other details that will enable us to reach a more definite conclusion.
[502] The fact that the origination of such forms is accessible to direct investigation is proven quite independently of all further considerations. The new variety came into existence at once. The leap may have been made by the ancestor of the year 1895, or by the plant of 1899, which showed the first central rays, or the sport may have been gradually built up during those four years. In each case there was a leap, contrasting with the view which claims a very long succession of years for the development of every new character.
Having discovered this first trace of doubling, it was to be expected that the new variety would be at once as pure and as rich as other double composites usually are. Some effect of the crossing with the other seed-bearing individuals might still disturb this uniformity in the following year, but another year’s work would eliminate even this source of impurity.
These two years have given the expected result. The average number of the rays, which had already arisen from 13 to 34 now at once came up to 47 and 55, the last figure being the sum of 21 and 34 and therefore the probable uttermost limit to be reached before absolute doubling. The maximum numbers came as high as 100 in 1900, and reached even 200 in 1901. Such heads are as completely double as are the [503] brightest heads of the most beautiful double commercial varieties of composites. Even the best white camomiles (Chrysanthemum inodorum) and the gold-flowers or garden-marigolds (Calendula officinalis) do not come nearer to purity since they always have scores of little tubular florets between the rays on their disks.
Real atavists or real reversionists were seen no more after the first purification of the race. I have continued my culture and secured last summer (1903) as many and as completely doubled heads as previously. The race has at once become permanent and constant. It has of course a wide range of fluctuating variability, but the lower limit has been worked up to about 34 rays, a figure never reached by the grandiflorum parent, from which my new variety is thus sharply separated.
Unfortunately the best flowers and even the best individuals of my race are wholly barren. Selection has reached its practical limit. Seeds must be saved from less dense heads, and no way has been found of avoiding it. The ray-florets are sterile, even in the wild species, and when growing in somewhat large numbers on the disk, they conceal the fertile flowers from the visiting insects, and cause them also to be sterile. The same is the case with the best cultivated forms. Their showiest individuals are [504] barren, and incapable of the reproduction of the race.
This last is therefore, of necessity, always continued by means of individuals whose deviation from the mean average is the least. But in many cases the varieties are so highly differentiated that selection has become quite superfluous for practical purposes. I have already discussed the question as to the actual moment, in which the change of the grandiflorum variety into the new plenum form must be assumed to have taken place. In this respect some stress is to be laid on the fact that the improvement through selection has been gradual and continuous, though very rapid from the first moment. But with the appearance of the first stray rays within the disk, this continuity suddenly changed. All the children of this original mutated plant showed the new character, the rays within the disk, without exception. Not on all the heads, nor even on the majority of the heads on some individuals, but on some heads all gave clear proof of the possession of the new attribute. This was present in all the representatives of the new race, and had never been seen in any of their parents and grandparents. Here there was evidently a sudden leap, at least in the external form of the plants. And it seems to me to be the most simple conception, [505] that this visible leap directly corresponded to that inner change, which brought about the complete inheritability of the new peculiarity. It is very interesting to observe how completely my experience agrees with the results of the observations of breeders at large. No doubt a comparison is difficult, and the circumstances are not adequate to a close study.
Isolation and selection have been applied commonly only so far as was consistent with the requirements of practical horticulture, and of course a determination of the hereditary percentage was never made. The disregard of this feature made necessary a greater length of time and a larger number of generations to bring about the desired changes. Notwithstanding this, however, it has been seen that double varieties are produced suddenly. This may have occurred unexpectedly or after a few years’ effort toward the end desired. Whether this sudden appearance is the consequence of a single internal differentiating step, or of the rapid succession of lesser changes, cannot yet be made out. The extreme variability of double flowers and the chance of their appearance with only slight indications of the previous petaloid alterations of a few stamens may often result in their origin being overlooked, while subsequent generations may come in for full notice. [506] In the greater number of cases recorded it remains doubtful whether the work said to be done to obtain a new double variety was done before the appearance of these preliminary indications or afterward.
In the first case, it would correspond with our selection of large numbers of florets in the outer rays, in the second however, with the ordinary purification of new races from hybrid mixtures.
In scientific selection-experiments such crosses are of course avoided, and the process of purification is unnecessary, even as in the Chrysanthemum culture. The first generation succeeding the original plant with disk-rays was in this respect wholly uniform and true to the new type.
In practice the work does not start from such slight indications, and is done with no other purpose in view than to produce double flowers in species in which they did not already exist. Therefore it is of the highest importance to know the methods used and the chances of success. Unfortunately the evidence is very scanty on both points.
Lindley and other writers, on horticultural theory and practice assert that a large amount of nourishment tends to produce double flowers, while a culture under normal conditions, [507] even if the plants are very strong and healthy, has no such effect. But even here it remains doubtful whether it applies to the period before or after the internal mutation. On the other hand success is not at all to be relied upon, nor is the work to be regarded as easy. The instances of double flowers said to be obtainable at will, are too rare in comparison with the number of cases, where the first indication of them was found accidentally.
Leaving all these doubtful points, which will have to be cleared up by further scientific investigation, the high degree of variability requires further discussion. It may be considered from three different points of view according to the limit of the deviation from the average, to the dependency on external conditions and to periodicity. It seems best to take up the last two points first.
On a visit to a nursery at Erfurt I once inspected an experiment with a new double variety of the common blue-bottle or blue corn-flower. The plants were dependent on the weather to a high degree. Bad weather increased the number of poorly filled flower-heads, while warm and sunny days were productive of beautiful double flowers. The heads that are borne by strong branches have a greater tendency to become double than those of the weaker ones, [508] and towards the autumn, when all those of the first group are faded away, and only a weak though large section of the heads is still flowering, the whole aspect of the variety gradually retrogrades. The same law of dependency and periodicity is prevalent everywhere. In my own cultures of the improved field-marigold I have observed it frequently. The number of the ray-florets may be considered as a direct response to nourishment, both when this is determined by external circumstances, and when it depends on the particular strength of the branch, which bears the head in question. It is a case exactly similar to that of the supernumerary carpels of the pistilloid poppy, and the deductions arrived at with that variety may be applied directly to double flowers.
This dependency upon nourishment is of high practical importance in combination with the usual effect of the doubling which makes the flowers sterile. It is a general rule that the most perfect flowers do not produce seed. At the height of the flowering period the external circumstances are the most favorable, and the flowering branches still constitute the stronger axes of the plants. Hence we may infer that sterility will prevail precisely in this period. Many varieties are known to yield only seeds from the very last flowers, as for instance some [509] double begonias. Others bear only seed on their weaker lateral branches, as the double camomile, or become fertile only towards the fall, as is often the case with the above quoted Erfurt variety of the blue-bottle. As far as I have been able to ascertain, such seeds are quite adequate for the reproduction and perpetuation of the double varieties, but the question whether there are differences between the seeds of the more or less double flowers of the same plants still remains open. It is very probable, from a theoretical point of view, that such differences exist, but perhaps they are so slight, as to have practically no bearing on the question.
On the ground of their wide range of variability, the double varieties must be regarded as pertaining to the group of ever-sporting forms. On one side they fluctuate in the direction towards such petalomanous flowers as are borne by the stocks and others, which we have previously discussed. Here no trace of the fertile organs is left. But this extreme is never reached by petaloid double flowers. A gap remains which, often overlooked, always exists, and which sharply separates the two types. On the other hand the alteration of the stamens gradually relapses to perfectly single flowers. Here the analogy with the pistillody of the poppies and with the “five-leaved” clover is obvious.
[510] This conception of the inner nature of double flowers explains the fact that the varietal mark is seldom seen to be complete throughout larger groups of individuals, providing these have not been already selected by this character. Tagetes africana is liable to produce some poorly filled specimens, and some double varieties of carnations are offered for sale with the note that the seed yields only 80% of doubles. With Chrysanthemum coronarium and blue-bottles this figure is often announced to be only about 50%. No doubt it is partly due to impurities, caused by vicinism, but it is obviously improbable that the effect of these impurities should be so large.
Some cases of partial reversion may be interpreted in the same way. Among the garden anemones, Anemone coronaria, there is a variety called the “Bride,” on account of its pure white dowers. It is for sale with single and with double flowers, and these two forms are known to sport into one another, although they are multiplied in the vegetative way. Such cases are known to be of quite ordinary occurrence. Of course such sports must be considered as partial, and the same stem may bear both types of flowers. It even happens that some particular flower is partly double and partly single. Mr. Krelage, of Haarlem, had the kindness to [511] send me such a curious flower. One half of it was completely double, while the other half was entirely single, bearing normal and fertile stamens in the ordinary number.
The same halfway doubling is recorded to occur among composites sometimes, and from the same source I possess in my collection a head of Pyrethrum roseum, bearing on half of its disk elongated corolla tubes, and on the other half the small disk-florets of the typical species.
It is a current belief, that varieties are improved by continued culture. I have never been able to ascertain the grounds on which this conviction rests. It may be referred either to the purity of the race or to the complete development of the varietal character. In the first case it is a question of hybrid mixtures from which many young varieties must be freed before being placed on the market. But as we have already seen in a former lecture, this requires only three or four years, and afterwards the degree of purity is kept up to the point which proves to be the most suitable for practical purposes. The complete development of the varietal character is a question restricted to ever-sporting varieties, since in white flowers and other constant varieties this degree is variable in a very small and unimportant measure. [512] Hence the double flowers seem to afford a very good example for this discussion.
It can be decided by two facts. First by a consideration of the oldest double varieties, and secondly by that of the very youngest. Are the older ones now in a better condition than at the outset? Have they really been gradually improved during the centuries of their existence? Obviously this can only be answered by a comparison of the figures given by older writers, with the varieties as they are now in culture. Munting’s drawings and descriptions are now nearly two centuries and a half old, but I do not find any real difference between his double varieties and their present representatives. So it is in other cases in which improvements by crossing or the introduction of new forms does not vitiate the evidence. Double varieties, as a rule, are exactly the same now, as they were at the time of their first introduction.
If this were otherwise one would expect that young double varieties should in the main display only slight grades of the anomaly, and that they would require centuries to reach their full development. Nothing of the kind is on record. On the contrary the newest double sorts are said to be not only equal to their predecessors, but to excel them. As a rule such claims may be exaggerated, but not to any great extent. [513] This is proven in the simplest way by the result of our own experiment.
In the double field-marigold we have the very first generation of a variety of pure and not hybrid origin. It shows the new attribute in its full development. It has flower-heads nearly as completely filled as the best double varieties of allied cultivated composites. In the second generation it reached heads with 200 rays each, and much larger numbers will seldom be seen in older species on heads of equal size. I have compared my novelty with the choicest double camomiles and others, but failed to discover any real difference. Improvement of the variety developed in the experiments carried on by myself seems to he excluded by the fact that it comes into conflict with the same difficulty that confronts the older cultivated species, viz.: the increasing sterility of the race.
It is perfectly evident that this double marigold is now quite constant. Continuously varying about a fixed average it may live through centuries, but the mean and the limits will always remain the same, as in the case of the ever-sporting varieties.
Throughout this lecture I have spoken of double flowers and double flower-heads of composites as of one single group. They are as nearly related from the hereditary point of [514] view, as they are divergent in other respects. It would be superfluous to dwell any longer upon the difference between heads and flowers. But it is as well to point out, that the term double flowers indicates a motley assemblage of different phenomena. The hen-and-chicken daisy, and the corresponding variety of the garden cineraria (Cineraria cruenta), are extremes on one side. The hen-and-chicken type occurs even in other families and is known to produce most curious anomalies, as with Scabiosa, the supernumerary heads of which may be produced on long stalks and become branched themselves in the same manner.
Petalody of the stamens is well known to be the ordinary type of doubling. But it is often accompanied by a multiplication of the organs, both of the altered stamens and of the petals themselves. This proliferation may consist in median or in lateral cleavages, and in both cases the process may be repeated one or more times. It would be quite superfluous to give more details, which may be gathered from any morphologic treatise on double flowers. But from the physiologic point of view all these cases are to be considered as one large group, complying with previously given definitions of the ever-sporting varieties. They are very variable and wholly permanent. Obviously this [515] permanency agrees perfectly with the conception of their sudden origin.
[516]
LECTURE XVIII
NEW SPECIES OF OENOTHERA
In our experiments on the origin of peloric varieties and double flowers we were guided in the choice of our material by a survey of the evidence already at hand. We chose the types known to be most commonly produced anew, either in the wild state or under the conditions of cultivation. In both instances our novelty was a variety in the ordinary sense of the word. Our pedigree-culture was mainly an experimental demonstration of the validity of conclusions, which had previously been deduced from such observations as can be made after the accidental birth of new forms.
From these facts, and even from these pedigree-experiments, it is scarcely allowable to draw conclusions as to the origin of real species. If we want to know how species originate, it is obviously necessary to have recourse to direct observation. The question is of the highest importance, both for the theory of descent, and for our conception of the real nature of [517] systematic affinities at large. Many authors have tried to solve it on the ground of comparative studies and of speculations upon the biologic relations of plants and animals. But in vain. Contradiction and doubt still reign supreme. All our hopes now rest on the result of experiments.
Unfortunately such experiments seemed simply impossible a few years ago. What is to guide us in the choice of the material? The answer may only be expected from a consideration of elementary species. For it is obvious that they only can be observed to originate, and that the systematic species, because they are only artificial groups of lower unities, can never become the subject of successful experimental inquiry.
In previous lectures we tried to clear up the differences existing between nearly related elementary species. We have seen that they affect all of the attributes of the plants, each of them changing in some measure all of the organs. Nevertheless they were due to distinct unities and of the lowest possible degree. Such unit-steps may therefore be expected to become visible some time or other by artificial means. On the other hand, mutations as a rule make their appearance in groups, and there are many systematic species which on close inspection [518] have been shown to be in reality composite assemblages. Roses and brambles, hawkweeds and willows are the best known examples. Violets and Draba verna, dandelions and helianthemums and many other instances were dealt with in previous lectures. Even wheat and barley and corn afford instances of large groups of elementary species. Formerly mixed in the fields, they became separated during the last century, and now constitute constant races, which, for brevity’s sake, are dealt with under the name of varieties.
In such groups of nearly allied forms the single members must evidently be of common origin. It is not necessary for them to have originated all in the same place or at the same time. In some cases, as with Draba verna, the present geographic distribution points to a common birthplace, from whence the various forms may about the same period have radiated in all directions. The violets on the other hand seem to include widely diffused original forms, from which branches have started at different times and in different localities.
The origin of such groups of allied forms must therefore be the object of our research. Perhaps we might find a whole group, perhaps only part of it. In my opinion we have the right to assume that if Draba and violets and [519] others have formerly mutated in this way, other species must at present be in the same changeable condition. And if mutations in groups, or such periodic mutations should be the rule, it is to be premised that these periods recur from time to time, and that many species must even now be in mutating condition, while others are not.
It is readily granted that the constant condition of species is the normal one, and that mutating periods must be the exception. This fact does not tend to increase our prospect of discovering a species in a state of mutability. Many species will have to be tested before finding an instance. On the other hand, a direct trial seems to be the only way to reach the goal. No such special guides as those that led us to the choice of pelories and double flowers are available. The only indication of value is the presumption that a condition of mutability might be combined with a general state of variability at large, and that groups of plants of very uniform features might be supposed to be constant in this respect too. On the contrary, anomalies and deviations if existent in the members of one strain, or found together in one native locality of a species, might be considered as an indication in the desired direction.
Few plants vary in the wild state in such a [520] measure as to give distinct indications. All have to be given a trial in the garden under conditions as similar as possible to their natural environments. Cultivated plants are of course to be excluded. Practically they have already undergone the experience in question and can not be expected to change their habits soon enough. Moreover they are often of hybrid origin. The best way is to experiment with the native plants of one’s own country.
I have made such experiments with some hundred species that grow wild in Holland. Some were very variable, as for instance, the jointed charlock (Raphanus Raphanistrum) and the narrow-leaved plantain (Plantago lanceolata). Others seemed more uniform, but many species, collected without showing any malformation, subsequently produced them in my garden, either on the introduced plants themselves or among their offspring. From this initial material I have procured a long series of hereditary races, each with some peculiar anomaly for its special character. But this result was only a secondary gain, a meager consolation for the negative fact that no real mutability could be discovered.
My plants were mostly annuals or biennials, or such perennials as under adequate treatment might produce flowers and seeds during their [521] first summer. It would be of no special use to enumerate them. The negative result does not apply to the species as such, but only to the individual strain, which I collected and cultivated. Many species, which are quite constant with us, may be expected to be mutable in other parts of their range.
Only one of all my tests met my expectations. This species proved to be in a state of mutation, producing new elementary forms continually, and it soon became the chief member of my experimental garden. It was one of the evening primroses.
Several evening-primroses have at different times been introduced into European gardens from America. From thence they have spread into the vicinity, becoming common and exhibiting the behavior of indigenous types. Oenothera biennis was introduced about 1614 from Virginia, or nearly three centuries ago. O. muricata, with small corollas and narrow leaves, was introduced in the year 1789 by John Hunneman, and O. suaveolens, or sweet-scented primrose, a form very similar to the biennis, about the same time, in 1778, by John Fothergill. This form is met with in different parts of France, while the biennis and muricata are very common in the sandy regions of Holland, where I have observed them for [522] more than 40 years. They are very constant and have proven so in my experiments. Besides these three species, the large-flowered evening-primrose, or Oenothera lamarckiana, is found in some localities in Holland and elsewhere. We know little concerning its origin. It is supposed to have come from America in the same way as its congeners, but as yet I have not been able to ascertain on what grounds this supposition rests. As far as I know, it has not been seen growing wild in this country, though it may have been overlooked. The fact that the species of this group are subject to many systematic controversies and are combined by different writers into systematic species in different ways, being often considered as varieties of one or two types, easily accounts for it having been overlooked. However, it would be of great interest to ascertain whether O. lamarckiana yet grows in America, and whether it is in the same state of mutability here as it is in Holland.
The large-flowered evening-primrose was also cultivated about the beginning of the last century in the gardens of the Museum d’Histoire Naturelle, at Paris, where it was noticed by Lamarck, who at once distinguished it as an undescribed species. He wrote a complete description [523] of it and his type specimens are still preserved in the herbarium of the Museum, where I have compared them with the plants of my own culture. Shortly afterwards it was renamed by Seringe, in honor of its eminent discoverer, whose name it now bears. So Lamarck unconsciously discovered and described himself the plant, which after a century, was to become the means of an empirical demonstration of his far-reaching views on the common origin of all living beings.
Oenothera lamarckiana is considered in Europe as a garden-plant, much prized for parks and ornamental planting. It is cultivated by seed-merchants and offered for sale. It has escaped from gardens, and having abundant means for rapid multiplication, has become wild in many places. As far as I know its known localities are small, and it is to be presumed that in each of them the plant has escaped separately from culture. It was in this state that I first met with this beautiful species.
Lamarck’s evening-primrose is a stately plant, with a stout stem, attaining often a height of 1.6 meters and more. When not crowded the main stem is surrounded by a large circle of smaller branches, growing upwards from its base so as often to form a dense bush. These branches in their turn have numerous lateral [524] branches. Most of them are crowned with flowers in summer, which regularly succeed each other, leaving behind them long spikes of young fruits. The flowers are large and of a bright yellow color, attracting immediate attention, even from a distance. They open towards evening, as the name indicates, and are pollinated by humble-bees and moths. On bright days their duration is confined to one evening, but during cloudy weather they may still be found open on the following morning. Contrary to their congeners they are dependent on visiting insects for pollination. O. biennis and O. muricata have their stigmas in immediate contact with the anthers within the flower-buds, and as the anthers open in the morning preceding the evening of the display of the petals, fecundation is usually accomplished before the insects are let in. But in O. lamarckiana no such self-fertilization takes place. The stigmas are above the anthers in the bud, and as the style increases in length at the time of the opening of the corolla, they are elevated above the anthers and do not receive the pollen. Ordinarily the flowers remained sterile if not visited by insects or pollinated by myself, although rare instances of self-fertilization were seen.
In falling off, the flowers leave behind them a stout ovary with four cells and a large number [525] of young seeds. The capsule when ripe, opens at its summit with four valves, and contains often from two to three hundred seeds. A hundred capsules on the main stem is an average estimate, and the lateral branches may ripen even still more fruits, by which a very rapid dissemination is ensured.
This striking species was found in a locality near Hilvers, in the vicinity of Amsterdam, where it grew in some thousands of individuals. Ordinarily biennial, it produces rosettes in the first, and stems in the second year. Both the stems and the rosettes were at once seen to be highly variable, and soon distinct varieties could be distinguished among them.
The first discovery of this locality was made in 1886. Afterwards I visited it many times, often weekly or even daily during the first few years, and always at least once a year up to the present time. This stately plant showed the long-sought peculiarity of producing a number of new species every year. Some of them were observed directly on the field, either as stems or as rosettes. The latter could be transplanted into my garden for further observation, and the stems yielded seeds to be sown under like control. Others were too weak to live a sufficiently long time in the field. They were discovered by sowing seed from indifferent plants [526] of the wild locality in the garden. A third and last method of getting still more new species from the original strain, was the repetition of the sowing process, by saving and sowing the seed which ripened on the introduced plants. These various methods have led to the discovery of over a dozen new types, never previously observed or described.
Leaving the physiologic side of the relations of these new forms for the next lecture, it would be profitable to give a short description of the several novelties. To this end they may be combined under five different heads, according to their systematic value. The first head includes those which are evidently to be considered as varieties, in the narrower sense of the word, as previously given. The second and third heads indicate the real progressive elementary species, first those which are as strong as the parent-species, and secondly a group of weaker types, apparently not destined to be successful. Under the fourth head I shall include some inconstant forms, and under the last head those that are organically incomplete.
Of varieties with a negative attribute, or real retrograde varieties, I have found three, all of them in a flowering condition in the field. I have given them the names of laevifolia, brevistylis and nanella.
[527] The laevifolia, or smooth-leaved variety, was one of the very first deviating types found in the original field. This was in the summer of 1887, seventeen years ago. It formed a little group of plants growing at some distance from the main body, in the same field. I found some rosettes and some flowering stems and sowed some seed in the fall. The variety has been quite constant in the field, neither increasing in number of individual plants nor changing its place, though now closely surrounded by other Lamarckianas. In my garden it has proved to be constant from seed, never reverting to the original lamarckiana, provided intercrossing was excluded.
It is chiefly distinguished from Lamarck’s evening-primrose by its smooth leaves, as the name indicates. The leaves of the original form show numerous sinuosities in their blades, not at the edge, but anywhere between the veins. The blade shows numbers of convexities on either surface, the whole surface being undulated in this manner; it lacks also the brightness of the ordinary evening-primrose or Oenothera biennis.
These undulations are lacking or at least very rare on the leaves of the new laevifolia. Ordinarily they are wholly wanting, but at times single leaves with slight manifestations of this [528] character may make their appearance. They warn us that the capacity for such sinuosities is not wholly lost, but only lies dormant in the new variety. It is reduced to a latent state, exactly as are the apparently lost characters of so many ordinary horticultural varieties.
Lacking the undulations, the laevifolia leaves are smooth and bright. They are a little narrower and more slender than those of the lamarckiana. The convexities and concavities of leaves are said to be useful in dry seasons, but during wet summers, such as those of the last few years, they must be considered as very harmful, as they retain some of the water which falls on the plants, prolonging the action of the water on the leaves. This is considered by some writers to be of some utility after slight showers, but was observed to be a source of weakness during wet weather in my garden, preventing the leaves from drying. Whether the laevifolia would do better under such circumstances, remains to be tested.
The flowers of the laevifolia are also in a slight degree different from those of lamarckiana. The yellow color is paler and the petals are smoother. Later, in the fall, on the weaker side branches these differences increase. The laevifolia petals become smaller and are often not emarginated at the apex, becoming ovate [529] instead of obcordate. This shape is often the most easily recognized and most striking mark of the variety. In respect to the reproductive organs, the fertility and abundance of good seed, the laevifolia is by no means inferior or superior to the original species.
O. brevistylis, or the short-styled evening primrose, is the most curious of all my new forms. It has very short styles, which bring the stigmas only up to the throat of the calyx tube, instead of upwards of the anthers. The stigmas themselves are of a different shape, more flattened and not cylindrical. The pollen falls from the anthers abundantly on them, and germinates in the ordinary manner.
The ovary which in lamarckiana and in all other new forms is wholly underneath the calyx-tube, is here only partially so. This tube is inserted at some distance under its summit. The insertion divides the ovary into two parts: an upper and a lower one. The upper part is much reduced in breadth and somewhat attenuated, simulating a prolongation of the base of the style. The lower part is also reduced, but in another manner. At the time of flowering it is like the ovary of lamarckiana, neither smaller nor larger. But it is reached by only a very few pollen-tubes, and is therefore always incompletely fertilized. It does [530] not fall off after the fading away of the flower, as unfertilized ovaries usually do; neither does it grow out, nor assume the upright position of normal capsules. It is checked in its development, and at the time of ripening it is nearly of the same length as in the beginning. Many of them contain no good seeds at all; from others I have succeeded in saving only a hundred seeds from thousands of capsules.
These seeds, if purely pollinated, and with the exclusion of the visits of insects, reproduce the variety, entirely and without any reversion to the lamarckiana type.
Correlated with the detailed structures is the form of the flower-buds. They lack the high stigma placed above the anthers, which in the lamarckiana, by the vigorous growth of the style, extends the calyx and renders the flower bud thinner and more slender. Those of the brevistylis are therefore broader and more swollen. It is quite easy to distinguish the individuals by this striking character alone, although it differs from the parent in other particulars.
The leaves of the O. brevistylis are more rounded at the tip, but the difference is only pronounced at times, slightly in the adult rosettes, but more clearly on the growing summits of the stems and branches. By this character, the plants [531] may be discerned among the others, some weeks before the flowers begin to show themselves. But the character by which the plants may be most easily recognized from a distance in the field is the failure of the fruits. They were found there nearly every year in varying, but always small numbers.
Leaving the short-styled primrose, we come now to the last of our group of retrograde varieties. This is the O. nanella, or the dwarf, and is a most attractive little plant. It is very short of stature, reaching often a height of only 20-30 cm., or less than one-fourth of that of the parent. It commences flowering at a height of 10-15 cm., while the parent-form often measures nearly a meter at this stage of its development. Being so very dwarfed the large flowers are all the more striking. They are hardly inferior to those of the lamarckiana, and agree with them in structure. When they fade away the spike is rapidly lengthened, and often becomes much longer than the lower or vegetative part of the stem.
The dwarfs are one of the most common mutations in my garden, and were observed in the native locality and also grown from seeds saved there. Once produced they are absolutely constant. I have tried many thousands of seeds from various dwarf mutants, and never observed [532] any trace of reversion to the lamarckiana type. I have also cultivated them in successive generations with the same result. In a former lecture we have seen that contrary to the general run of horticultural belief, varieties are as constant as the best species, if kept free from hybrid admixtures. This is a general rule, and the exceptions, or cases of atavism are extremely rare. In this respect it is of great interest to observe that this constancy is not an acquired quality, but is to be considered as innate, because it is already fully developed at the very moment when the original mutation takes place.
From its first leaves to the rosette period, and through this to the lengthening of the stem, the dwarfs are easily distinguished from any other of their congeners. The most remarkable feature is the shape of the leaves. They are broader and shorter, and especially at the base they are broadened in such a way as to become apparently sessile. The stalk is very brittle, and any rough treatment may cause the leaves to break off. The young seedlings are recognizable by the shape of the first two or three leaves, and when more of them are produced, the rosettes become dense and strikingly different from others. Later leaves are more nearly like the parent-type, but the petioles remain short. The bases of the blades are frequently [533] almost cordate, the laminae themselves varying from oblong-ovate to ovate in outline. The stems are often quite unbranched, or branched only at the base of the spike. Strong secondary stems are a striking attribute of the lamarckiana parent, but they are lacking, or almost so in the dwarfs. The stem is straight and short, and this, combined with the large crown of bright flowers, makes the dwarfs eminently suitable for bed or border plants. Unfortunately they are very sensitive, especially to wet weather.
Oenothera gigas and O. rubrinervis, or the giant, and the red-veined evening-primroses, are the names given to two robust and stout species, which seem to be equal in vigor to the parent-plant, while diverging from it in striking characters. Both are true elementary species, differentiated from lamarckiana in nearly all their organs and qualities, but not showing any preponderating character of a retrograde nature. Their differences may be compared with those of the elementary species of other genera, as for instance, of Draba, or of violets, as will be seen by their description.
The giant evening-primrose, though not taller in stature than O. lamarckiana, deserves its name because it is so much stouter in all respects. [534] The stems are robust, often with twice the diameter of lamarckiana throughout. The internodes are shorter, and the leaves more numerous, covering the stems with a denser foliage. This shortness of the internodes extends itself to the spike, and for this reason the flowers and fruits grow closer together than on the parent-plant. Hence the crown of bright flowers, opening each evening, is more dense and more strikingly brilliant, so much the more so as the individual flowers are markedly larger than those of the parents. In connection with these characters, the flower-buds are seen to be much stouter than those of lamarckiana. The fruits attain only half the normal size, but are broader and contain fewer, but larger seeds.
The rubrinervis is in many respects a counterpart to the gigas, but its stature is more slender. The spikes and flowers are those of the lamarckiana, but the bracts are narrower. Red veins and red streaks on the fruits afford a striking differentiating mark, though they are not absolutely lacking in the parent-species. A red hue may be seen on the calyx, and even the yellow color of the petals is somewhat deepened in the same way. Young plants are often marked by the pale red tinge of the mid-veins, but in adult rosettes, or from lack of sunshine, this hue is often very faint.
[535] The leaves are narrow, and a curious feature of this species is the great brittleness of the leaves and stems, especially in annual individuals, especially in those that make their stem and flowers in the first year. High turgidity and weak development of the mechanical and supporting tissues are the anatomical cause of this deficiency, the bast-fibers showing thinner walls than those of the parent-type under the microscope. Young stems of rubrinervis may be broken off by a sharp stroke, and show a smooth rupture across all the tissues, while those of lamarckiana are very tough and strong.
Both the giant and the red-veined species are easily recognized in the rosette-stage. Even the very young seedlings of the latter are clearly differentiated from the lamarckiana, but often a dozen leaves are required, before the difference may be seen. Under such circumstances the young plants must reach an age of about two months before it is possible to discern their characters, or at least before these characters have become reliable enough to enable us to judge of each individual without doubt. But the divergencies rapidly become greater. The leaves of O. gigas are broader, of a deeper green, the blade more sharply set off against the stalk, all the rosettes [536] becoming stout and crowded with leaves. Those of O. rubrinervis on the contrary are thin, of a paler green and with a silvery white surface; the blades are elliptic, often being only 2 cm. or less in width. They are acute at the apex and gradually narrowed into the petiole.
It is quite evident that such pale narrow leaves must produce smaller quantities of organic food than the darker green and broad organs of the gigas. Perhaps this fact is accountable partly, at least, for the more robust growth of the giant in the second year. Perhaps also some relation exists between this difference in chemical activity and the tendency to become annual or biennial. The gigas, as a rule, produces far more, and the rubrinervis far less biennial plants than the lamarckiana. Annual culture for the one is as unreliable as biennial culture for the other. Rubrinervis may be annual in apparently all specimens, in sunny seasons, but gigas will ordinarily remain in the state of rosettes during the entire first summer. It would be very interesting to obtain a fuller insight into the relation of the length of life to other qualities, but as yet the facts can only be detailed as they stand.
Both of these stout species have been found [537] quite constant from the very first moment of their appearance. I have cultivated them from seed in large numbers, and they have never reverted to the lamarckiana. From this they have inherited the mutability or the capacity of producing at their turn new mutants. But they seem to have done so incompletely, changing in the direction of more absolute constancy. This was especially observed in the case of rubrinervis, which is not of such rare occurrence as O. gigas, and which it has been possible to study in large numbers of individuals. So for instance, the “red-veins” have never produced any dwarfs, notwithstanding they are produced very often by the parent-type. And in crossing experiments also the red-veins gave proof of the absence of a mutative capacity for their production.
Leaving the robust novelties, we may now take up a couple of forms, which are equally constants and differentiated from the parent species in exactly the same manner, though by other characters, but which are so obviously weak as to have no manifest chance of self maintenance in the wild state. These are the whitish and the oblong-leaved evening-primroses or the Oenothera albida and oblonga.
Oenothera albida is a very weak species, with whitish, narrow leaves, which are evidently incapable [538] of producing sufficient quantities of organic food. The young seedling-plants are soon seen to lag behind, and if no care is taken of them they are overgrown by their neighbors. It is necessary to take them out, to transplant them into pots with richly manured soil, and to give them all the care that should be given to weak and sickly plants. If this is done fully grown rosettes may be produced, which are strong enough to keep through the winter. In this case the individual leaves become stronger and broader, with oblong blades and long stalks, but retain their characteristic whitish color.
In the second year the stems become relatively stout. Not that they become equal to those of lamarckiana, but they become taller than might have been expected from the weakness of the plants in the previous stages. The flowers and racemes are nearly as large as those of the parent-form, the fruits only a little thinner and containing a smaller quantity of seed. From these seeds I have grown a second and a third generation, and observed that the plants remain true to their type.
O. oblonga may be grown either as an annual, or as a biennial. In the first case it is very slender and weak, bearing only small fruits and few seeds. In the alternative case however, it [539] becomes densely branched, bearing flowers on quite a number of racemes and yielding a full harvest of seeds. But it always remains a small plant, reaching about half the height of that of lamarckiana.
When very young it has broader leaves, but in the adult rosettes the leaves become very narrow, but fleshy and of a bright green color. They are so crowded as to leave no space between them unoccupied. The flowering spikes of the second year bear long leaf-like bracts under the first few flowers, but those arising later are much shorter. Numerous little capsules cover the axis of the spike after the fading away of the petals, constituting a very striking differentiating mark. This species also was found to be quite constant, if grown from pure seed.
We have now given the descriptions of seven new forms, which diverge in different ways from the parent-type. All were absolutely constant from seed. Hundreds or thousands of seedlings may have arisen, but they always come true and never revert to the original O. lamarckiana type. From this they have inherited the condition of mutability, either completely or partly, and according to this they may be able to produce new forms themselves. But this occurs only rarely, and combinations of more than one [540] type in one single plant seem to be limited to the admixture of the dwarf stature with the characters of the other new species.
These seven novelties do not comprise the whole range of the new productions of my O. lamarckiana. But they are the most interesting ones. Others, as the O. semilata and the O. leptocarpa are quite as constant and quite as distinct, but have no special claims for a closer description. Others again were sterile, or too weak to reach the adult stage and to yield seeds, and no reliable description or appreciation can be given on the ground of the appearance of a single individual.
Contrasted with these groups of constant forms are three inconstant types which we now take up. They belong to two different groups, according to the cause of their inconstancy. In one species which I call O. lata, the question of stability or instability must remain wholly unsolved, as only pistillate flowers are produced, and no seed can be fertilized save by the use of the pollen of another form, and therefore by hybridization. The other head comprises two fertile forms, O. scintillans and O. elliptica, which may easily be fertilized with their own pollen, but which gave a progeny only partly similar to the parents.
The Oenothera lata is a very distinct form [541] which was found more than once in the field, and recently (1902) in a luxuriant flowering specimen. It has likewise been raised from seeds collected in different years at the original station. It is also wholly pistillate. Apparently the anthers are robust, but they are dry, wrinkled and nearly devoid of contents. The inner wall of cells around the groups of pollen grow out instead of being resorbed, partly filling the cavity which is left free by the miscarriage of the pollen-grains. This miscarriage does not affect all the grains in the same degree, and under the microscope a few of them with an apparently normal structure may be seen. But the contents are not normally developed, and I have tried in vain to obtain fertilization with a large number of flowers. Only by cross-fertilization does O. lata produce seeds, and then as freely as the other species when self-fertilized. Of course its chance of ever founding a wild type is precluded by this defect.
O. lata is a low plant, with a limp stem, bent tips and branches, all very brittle, but with dense foliage and luxuriant growth. It has bright yellow flowers and thick flower-buds. But for an unknown reason the petals are apt to unfold only partially and to remain wrinkled throughout the flowering time. The stigmas are slightly divergent from the normal type, [542] also being partly united with one another, and laterally with the summit of the style, but without detriment to their function.
Young seedlings of lata may be recognized by the very first leaves. They have a nearly orbicular shape and are very sharply set off against their stalk. The surface is very uneven, with convexities and concavities on both sides. This difference is lessened in the later leaves, but remains visible throughout the whole life of the plant, even during the flowering season. Broad, sinuate leaves with rounded tips are a sure mark of O. lata. On the summits of the stems and branches they are crowded so as to form rosettes.
Concerning inheritance of these characteristics nothing can be directly asserted because of the lack of pollen. The new type can only be perpetuated by crosses, either with the parent form or some other mutant. I have fertilized it, as a rule, with lamarckiana pollen, but have often also used that from nanella and others. In doing so, the lata repeats its character in part of its offspring. This part seems to be independent of the nature of the pollen used, but is very variable according to external circumstances. On the average one-fourth of the offspring become lata, the others assuming the type of the pollen-parent, if this was a lamarckiana or [543] partly this type and partly that of any other of the new species derived from lamarckiana, that might have been used as the pollen-parent. This average seems to be a general rule, recurring in all experiments, and remaining unchanged through a long series of successive generations. The fluctuations around this mean go up to nearly 50% and down nearly to 1%, but, as in other cases, such extreme deviations from the average are met with only exceptionally.
The second category includes the inconstant but perfectly fertile species. I have already given the names of the only two forms, which deserve to be mentioned here.
One of them is called scintillans or the shiny evening-primrose, because its leaves are of a deep green color with smooth surfaces, glistening in the sunshine. On the young rosettes these leaves are somewhat broader, and afterwards somewhat narrower than those of O. lamarckiana at the corresponding ages. The plants themselves always remain small, never reaching the stature of the ancestral type. They are likewise much less branched. They can easily be cultivated in annual generations, but then do not become as fully developed and as fertile, as when flowering in the second year. The flowers have the same structure as those of the lamarckiana, but are of a smaller size.
[544] Fertilizing the flowers artificially with their own pollen, excluding the visiting insects by means of paper bags, and saving and sowing the seed of each individual separately, furnishes all the requisites for the estimation of the degree of stability of this species. In the first few weeks the seed-pans do not show any unequality, and often the young plants must be replanted at wider intervals, before anything can be made out with certainty. But as soon as the rosettes begin to fill it becomes manifest that some of them are more backward than others in size. Soon the smaller ones show their deeper green and broader leaves, and thereby display the attributes of the scintillans. The other grow faster and stronger and exhibit all the characteristics of ordinary lamarckianas.
The numerical proportion of these two groups has been found different on different occasions. Some plants give about one-third scintillans and two-thirds lamarckiana, while the progeny of individuals of another strain show exactly the reverse proportion.
Two points deserve to be noticed. First the progeny of the scintillans appears to be mutable in a large degree, exceeding even the lamarckiana. The same forms that are produced most often by the parent-family are also most ordinarily [545] met with among the offspring of the shiny evening-primrose. They are oblonga, lata and nanella. Oblonga was observed at times to constitute as much as 1% or more of the sowings of scintillans, while lata and nanella were commonly seen only in a few scattering individuals, although seldom lacking in experiments of a sufficient size.
Secondly the instability seems to be a constant quality, although the words themselves are at first sight, contradictory. I mean to convey the conception that the degree of instability remains unchanged during successive generations. This is a very curious fact, and strongly reminds us of the hereditary conditions of striped-flower varieties. But, on the contrary, the atavists, which are here the individuals with the stature and the characteristics of the lamarckiana, have become lamarckianas in their hereditary qualities, too. If their seed is saved and sown, their progeny does not contain any scintillans, or at least no more than might arise by ordinary mutations.
One other inconstant new species is to be noted, but as it was very rare both in the field and in my cultures, and as it was difficult of cultivation, little can as yet be said about it. It is the Oenothera elliptica, with narrow elliptical leaves and also with elliptical petals. It repeats [546] its type only in a very small proportion of its seed.
All in all we thus have a group of a dozen new types, springing from an original form in one restricted locality, and seen to grow there, or arising in the garden from seeds collected from the original locality. Without any doubt the germs of the new types are fully developed within the seed, ready to be evolved at the time of germination. More favorable conditions in the field would no doubt allow all of the described new species to unfold their attributes there, and to come into competition with each other and with the common parents. But obviously this is only of secondary importance, and has no influence on the fact that a number of new types, analogous to the older swarms of Draba, Viola and of many other polymorphous species, have been seen to arise directly in the wild state.
[547]
LECTURE XIX
EXPERIMENTAL PEDIGREE-CULTURES
The observation of the production of mutants in the field at Hilversum, and the subsequent cultivation of the new types in the garden at Amsterdam, gives ample proof of the mutability of plants. Furthermore it furnishes an analogy with the hypothetical origin of the swarms of species of Draba and Viola. Last but not least important it affords material for a complete systematic and morphologic study of the newly arisen group of forms.
The physiologic laws, however, which govern this process are only very imperfectly revealed by such a study. The instances are too few. Moreover the seeds from which the mutants spring, escape observation. It is simply impossible to tell from which individual plants they have been derived. The laevifolia and the brevistylis have been found almost every year, the first always recurring on the same spot, the second on various parts of the original field. It is therefore allowable to assume a common [548] origin for all the observed individuals of either strain. But whether, besides this, similar strains are produced anew by the old lamarckiana group, it is impossible to decide on the sole ground of these field-observations.
The same holds good with the other novelties. Even if one of them should germinate repeatedly, without ever opening its flowers, the possibility could not be excluded that the seeds might have come originally from the same capsule but lain dormant in the earth during periods of unequal length.
Other objections might be cited that can only be met by direct and fully controlled experiments. Next to the native locality comes the experimental garden. Here the rule prevails that every plant must be fertilized with pollen of its own, or with pollen of other individuals of known and recorded origin. The visits of insects must be guarded against, and no seeds should be saved from flowers which have been allowed to open without this precaution. Then the seeds of each individual must be saved and sown separately, so as to admit of an appreciation, and if necessary, a numerical determination of the nature of its progeny. And last but not least the experiments should be conducted in a similar manner during a series of successive years.
[549] I have made four such experiments, each comprising the handling of many thousands of individual plants, and lasting through five to nine generations. At the beginning the plants were biennial, as in the native locality, but later I learned to cultivate them in annual generations. They have been started from different plants and seeds, introduced from the original field into my garden at Amsterdam.
It seems sufficient to describe here one of these pedigree-cultures, as the results of all four were similar. In the fall of 1886 I took nine large rosettes from the field, planted them together on an isolated spot in the garden, and harvested their seeds the next year. These nine original plants are therefore to be considered as constituting the first generation of my race. The second generation was sown in 1888 and flowered in 1889. It at once yielded the expected result. 15,000 seedlings were tested and examined, and among them 10 showed diverging characters. They were properly protected, and proved to belong to two new types. 5 of them were lata and 5 nanella. They flowered next year and displayed all the characters as described in our preceding lecture. Intermediates between them and the general type were not found, and no indication of their appearance was noted in their parents. [550] They came into existence at once, fully equipped, without preparation or intermediate steps. No series of generations, no selection, no struggle for existence was needed. It was a sudden leap into another type, a sport in the best acceptation of the word. It fulfilled my hopes, and at once gave proof of the possibility of the direct observation of the origin of species, and of the experimental control thereof.
The third generation was in the main a repetition of the second. I tried some 10,000 seedlings and found three lata and three nanella, or nearly the same proportion as in the first instance. But besides these a rubrinervis made its appearance and flowered the following year. This fact at once revealed the possibility that the instability of lamarckiana might not be restricted to the three new types now under observation. Hence the question arose how it would be possible to obtain other types or to find them if they were present. It was necessary to have better methods of cultivation and examination of the young plants. Accordingly I devoted the three succeeding years to working on this problem.
I found that it was not at all necessary to sow any larger quantities of seed, but that the young plants must have room enough to develop into full and free rosettes. Moreover I observed [551] that the attributes of lata and nanella, which I now studied in the offspring of my first mutants, were clearly discernible in extreme youth, while those of rubrinervis remained concealed some weeks longer. Hence I concluded that the young plants should be examined from time to time until they proved clearly to be only normal lamarckiana. Individuals exhibiting any deviation from the type, or even giving only a slight indication of it, were forthwith taken out of the beds and planted separately, under circumstances as favorable as possible. They were established in pots with well-manured soil and kept under glass, but fully exposed to sunshine. As a rule they grew very fast, and could be planted out early in June. Some of them, of course, proved to have been erroneously taken for mutants, but many exhibited new characters.
All in all I had 334 young plants which did not agree with the parental type. As I examined some 14,000 seedlings altogether, the result was estimated at about 2.5%. This proportion is much larger than in the yields of the two first generations and illustrates the value of improved methods. No doubt many good mutations had been overlooked in the earlier observations.
As was to be expected, lata and nanella [552] were repeated in this third generation (1895). I was sure to get nearly all of them, without any important exceptions, as I now knew how to detect them at almost any age. In fact, I found many of them; as many as 60 nanella and 73 lata, or nearly 5% of each. Rubrinervis also recurred, and was seen in 8 specimens. It was much more rare than the two first-named types.
But the most curious fact in that year was the appearance of oblonga. No doubt I had often seen it in former years, but had not attached any value to the very slight differences from the type, as they then seemed to me. I knew now that any divergence was to be esteemed as important, and should be isolated for further observation. This showed that among the selected specimens not less than 176, or more than 1% belonged to the oblonga type. This type was at that time quite new to me, and it had to be kept through the winter, to obtain stems and flowers. It proved to be as uniform as its three predecessors, and especially as sharply contrasted with lamarckiana. The opportunity for the discovery of any intermediates was as favorable as could be, because the distinguishing marks were hardly beyond doubt at the time of the selection and removal of the young plants. But no connecting links were found.
[553] The same holds good for albida, which appeared in 15 specimens, or in 0.1%, of the whole culture. By careful cultivation these plants proved not to be sickly, but to belong to a new, though weak type. It was evident that I had already seen them in former years, but having failed to recognize them had allowed them to be destroyed at an early age, not knowing how to protect them against adverse circumstances. Even this time I did not succeed in getting them strong enough to keep through the winter.
Besides these, two new types were observed, completing the range of all that have since been recorded to regularly occur in this family. They were scintillans and gigas. The first was obtained in the way just described. The other hardly escaped being destroyed, not having showed itself early enough, and being left in the bed after the end of the selection. But as it was necessary to keep some rosettes through the winter in order to have biennial flowering plants to furnish seeds, I selected in August about 30 of the most vigorous plants, planted them on another bed and gave them sufficient room for their stems and branches in the following summer. Most of them sent up robust shoots, but no difference was noted till the first flowers opened. One plant had a much larger crown of bright blossoms than any of the others. [554] As soon as these flowers faded away, and the young fruits grew out, it became clear that a new type was showing itself. On that indication I removed all the already fertilized flowers and young fruits, and protected the buds from the visits of insects. Thus the isolated flowers were fertilized with their own pollen only, and I could rely upon the purity of the seed saved. This lot of seeds was sown in the spring of 1897 and yielded a uniform crop of nearly 300 young gigas plants.
Having found how much depends upon the treatment, I could gradually decrease the size of my cultures. Evidently the chance of discovering new types would be lessened thereby, but the question as to the repeated production of the same new forms could more easily and more clearly be answered in this way. In the following year (1896) I sowed half as many seeds as formerly, and the result proved quite the same. With the exception of gigas all the described forms sprang anew from the purely fertilized ancestry of normal lamarckianas. It was now the fifth generation of my pedigree, and thus I was absolutely sure that the descendants of the mutants of this year had been pure and without deviation for at least four successive generations.
Owing partly to improved methods of selection, [555] partly no doubt to chance, even more mutants were found this year than in the former. Out of some 8,000 seedlings I counted 377 deviating ones, or nearly 5%, which is a high proportion. Most of them were oblonga and lata, the same types that had constituted the majority in the former year.
Albida, nanella and rubrinervis appeared in large numbers, and even scintillans, of which I had but a single plant in the previous generation, was repeated sixfold.
New forms did not arise, and the capacity of my strain seemed exhausted. This conclusion was strengthened by the results of the next three generations, which were made on a much smaller scale and yielded the same, or at least the mutants most commonly seen in previous years.
Instead of giving the figures for these last two years separately, I will now summarize my whole experiment in the form of a pedigree. In this the normal lamarckiana was the main line, and seeds were only sown from plants after sufficient isolation either of the plants themselves, or in the latter years by means of paper bags enclosing the inflorescences. I have given the number of seedlings of lamarckiana which were examined each year in the table below. Of course by far the largest number of them were [556] thrown away as soon as they showed their differentiating characters in order to make room for the remaining ones. At last only a few plants were left to blossom in order to perpetuate the race. I have indicated for each generation the number of mutants of each of the observed forms, placing them in vertical columns underneath their respective heads. The three first generations were biennial, but the five last annual.
PEDIGREE OF A MUTATING FAMILY
OF OENOTHERA LAMARCKIANA IN THE
EXPERIMENTAL GARDEN AT AMSTERDAM
| Gener: | O. gig. | albida | obl. | rubrin. | Lam. | nanella | lata. | scint. |
| VIII. | 5 | 1 | 0 | 1700 | 21 | 1 | ||
| VII. | 9 | 0 | 3000 | 11 | ||||
| VI. | 11 | 29 | 3 | 1800 | 9 | 5 | 1 | |
| V. | 25 | 135 | 20 | 8000 | 49 | 142 | 6 | |
| IV. | 1 | 15 | 176 | 8 | 14000 | 60 | 73 | 1 |
| III. | 1 | 10000 | 3 | 3 | ||||
| II. | 15000 | 5 | 5 | |||||
| I. | 9 |
It is most striking that the various mutations of the evening-primrose display a great degree of regularity. There is no chaos of forms, no indefinite varying in all degrees and in all directions. Quite on the contrary, it is at once evident that very simple rules govern the whole phenomenon.
I shall now attempt to deduce these laws from [557] my experiment. Obviously they apply not only to our evening-primroses, but may be expected to be of general validity. This is at once manifest, if we compare the group of new mutants with the swarms of elementary forms which compose some of the youngest systematic species, and which, as we have seen before, are to be considered as the results of previous mutations. The difference lies in the fact that the evening-primroses have been seen to spring from their ancestors and that the drabas have not. Hence the conclusion that in comparing the two we must leave out the pedigree of the evening-primroses and consider only the group of forms as they finally show themselves. If in doing so we find sufficient similarity, we are justified in the conclusion that the drabas and others have probably originated in the same way as the evening-primroses. Minor points of course will differ, but the main lines cannot have complied with wholly different laws. All so-called swarms of elementary species obviously pertain to a single type, and this type includes our evening-primroses as the only controlled case.
Formulating the laws of mutability for the evening-primroses we therefore assume that they hold good for numerous other corresponding cases.
[558] I. The first law is, that new elementary species appear suddenly, without intermediate steps. This is a striking point, and the one that is in the most immediate contradiction to current scientific belief. The ordinary conception assumes very slow changes, in fact so slow that centuries are supposed to be required to make the differences appreciable. If this were true, all chance of ever seeing a new species arise would be hopelessly small. Fortunately the evening-primroses exhibit contrary tendencies. One of the great points of pedigree-culture is the fact that the ancestors of every mutant have been controlled and recorded. Those of the last year have seven generations of known lamarckiana parents preceding them. If there had been any visible preparation towards the coming mutation, it could not have escaped observation. Moreover, if visible preparation were the rule, it could hardly go on at the same time and in the same individuals in five or six diverging directions, producing from one parent, gigas and nanella, lata and rubrinervis, oblonga and albida and even scintillans. On the other hand the mutants, that constitute the first representatives of their race, exhibit all the attributes of the new type in full display at once. No series of generations, no selection, [559] no struggle for existence are needed to reach this end. In previous lectures I have mentioned that I have saved the seeds of the mutants whenever possible, and have always obtained repetitions of the prototype only. Reversions are as absolutely lacking as is also a further development of the new type. Even in the case of the inconstant forms, where part of the progeny yearly return to the stature of lamarckiana, intermediates are not found. So it is also with lata, which is pistillate and can only be propagated by cross-fertilization. But though the current belief would expect intermediates at least in this case, they do not occur. I made a pedigree-culture of lata during eight successive generations, pollinating them in different ways, and always obtained cultures which were partly constituted of lata and partly of lamarckiana specimens. But the latas remained lata in all the various and most noticeable characters, never showing any tendency to gradually revert into the original form. Intermediate forms, if not occurring in the direct line from one species to another, might be expected to appear perhaps on lateral branches. In this case the mutants of one type, appearing in the same year, would not be a pure type, but would exhibit different degrees of deviation from the parent. The best would then have to [560] be chosen in order to get the new type in its pure condition. Nothing of the kind, however, was observed. All the oblonga-mutants were pure oblongas. The pedigree shows hundreds of them in the succeeding years, but no difference was seen and no material for selection was afforded. All were as nearly equal as the individuals of old elementary species.
II. New forms spring laterally from the main stem. The current conception concerning the origin of species assumes that species are slowly converted into others. The conversion is assumed to affect all the individuals in the same direction and in the same degree. The whole group changes its character, acquiring new attributes. By inter-crossing they maintain a common line of progress, one individual never being able to proceed much ahead of the others.
The birth of the new species necessarily seemed to involve the death of the old one. This last conclusion, however, is hard to understand. It may be justifiable to assume that all the individuals of one locality are ordinarily intercrossed, and are moreover subjected to the same external conditions. They might be supposed to vary in the same direction if these conditions were changed slowly. But this could of course have no possible influence on the plants of the [561] same species growing in distant localities, and it would be improbable they should be affected in the same way. Hence we should conclude that when a species is converted into a new type in one locality this is only to be considered as one of numerous possible ones, and its alteration would not in the least change the aspect of the remainder of the species.
But even with this restriction the general belief is not supported by the evidence of the evening-primroses. There is neither a slow nor a sudden change of all the individuals. On the contrary, the vast majority remain unchanged; thousands are seen exactly repeating the original prototype yearly, both in the native field and in my garden. There is no danger that lamarckiana might die out from the act of mutating, nor that the mutating strain itself would be exposed to ultimate destruction from this cause.
In older swarms, such as Draba or Helianthemum, no such center, around which the various forms are grouped, is known. Are we to conclude therefore that the main strain has died out? Or is it perhaps concealed among the throng, being distinguished by no peculiar character? If our gigas and rubrinervis were growing in equal numbers with the lamarckiana in the native field, would it be possible to decide [562] which of them was the progenitor of the others? Of course this could be done by long and tedious crossing experiments, showing atavism in the progeny, and thereby indicating the common ancestor. But even this capacity seems to be doubtful and connected only with the state of mutability and to be lost afterwards. Therefore if this period of mutation were ended, probably there would be no way to decide concerning the mutual relationship of the single species.
Hence the lack of a recognizable main stem in swarms of elementary species makes it impossible to answer the question concerning their common origin.
Another phase of the opposition between the prevailing view and my own results seems far more important. According to the current belief the conversion of a group of plants growing in any locality and flowering simultaneously would be restricted to one type. In my own experiments several new species arose from the parental form at once, giving a wide range of new forms at the same time and under the same conditions.
III. New elementary species attain their full constancy at once.
Constancy is not the result of selection or of improvement. It is a quality of its own. It can neither be constrained by selection if it is absent [563] from the beginning, nor does it need any natural or artificial aid if it is present. Most of my new species have proved constant from the first. Whenever possible, the original mutants have been isolated during the flowering period and artificially self-fertilized. Such plants have always given a uniform progeny, all children exhibiting the type of the parent. No atavism was observed and therefore no selection was needed or even practicable.
Briefly considering the different forms, we may state that the full experimental proof has been given for the origin of gigas and rubrinervis, for albida and oblonga, and even for nanella, which is to be considered as of a varietal nature; with lata the decisive experiment is excluded by its unisexuality. laevifolia and brevistylis were found originally in the field, and never appeared in my cultures. No observations were made as to their origin, and seeds have only been sown from later generations. But these have yielded uniform crops, thereby showing that there is no ground for the assumption that these two older varieties might behave otherwise than the more recent derivatives.
Scintillans and elliptica constitute exceptions to the rule given. They repeat their character, from pure seed, only in part of the offspring. I have tried to deliver the scintillans from this [564] incompleteness of heredity, but in vain. The succeeding generations, if produced from true representatives of the new type, and with pure fertilization, have repeated the splitting in the same numerical proportions. The instability seems to be here as permanent a quality as the stability in other instances. Even here no selection has been adequate to change the original form.
IV. Some of the new strains are evidently elementary species, while others are to be considered as retrograde varieties.
It is often difficult to decide whether a given form belongs to one or another of these two groups. I have tried to show that the best and strictest conception of varieties limits them to those forms that have probably originated by retrograde or degressive steps. Elementary species are assumed to have been produced in a progressive way, adding one new element to the store. Varieties differ from their species clearly in one point, and this is either a distinct loss, or the assumption of a character, which may be met with in other species and genera. laevifolia is distinguished by the loss of the crinkling of the leaves, brevistylis by the partial loss of the epigynous qualities of the flowers, and nanella is a dwarf. These three new forms are therefore [565] considered to constitute only retrograde steps, and no advance. This conclusion has been fully justified by some crossing experiments with brevistylis, which wholly complies with Mendel’s law, and in one instance with nanella, which behaves in the same manner, if crossed with rubrinervis.
On the other hand, gigas and rubrinervis, oblonga and albida obviously bear the characters of progressive elementary species. They are not differentiated from lamarckiana by one or two main features. They diverge from it in nearly all organs, and in all in a definite though small degree. They may be recognized as soon as they have developed their first leaves and remain discernible throughout life. Their characters refer chiefly to the foliage, but no less to the stature, and even the seeds have peculiarities. There can be little doubt but that all the attributes of every new species are derived from one principal change. But why this should affect the foliage in one manner, the flowers in another and the fruits in a third direction, remains obscure. To gain ever so little an insight into the nature of these changes, we may best compare the differences of our evening-primroses with those between the two hundred elementary species of Draba and other similar instances. In doing so we find the same main [566] feature, the minute differences in nearly all points.
V. The same new species are produced in a large number of individuals.
This is a very curious fact. It embraces two minor points, viz: the multitude of similar mutants in the same year, and the repetition thereof in succeeding generations. Obviously there must be some common cause. This cause must be assumed to lie dormant in the Lamarckianas of my strain, and probably in all of them, as no single parent-plant proved ever to be wholly destitute of mutability. Furthermore the different causes for the sundry mutations must lie latent together in the same parent-plant. They obey the same general laws, become active under similar conditions, some of them being more easily awakened than others. The germs of the oblonga, lata and nanella are especially irritable, and are ready to spring into activity at the least summons, while those of gigas, rubrinervis and scintillans are far more difficult to arouse.
These germs must be assumed to lie dormant during many successive generations. This is especially evident in the case of lata and nanella, which appeared in the first year of the pedigree culture and which since have been repeated yearly, and have been seen to arise by mutation [567] also during the last season (1903). Only gigas appeared but once, but then there is every reason to assume that in larger sowings or by a prolongation of the experiments it might have made a second appearance.
Is the number of such germs to be supposed to be limited or unlimited? My experiment has produced about a dozen new forms. Without doubt I could easily have succeeded in getting more, if I had had any definite reason to search for them. But such figures are far from favoring the assumption of indefinite mutability. The group of possible new forms is no doubt sharply circumscribed. Partly so by the morphologic peculiarities of lamarckiana, which seem to exclude red flowers, composite leaves, etc. No doubt there are more direct reasons for these limits, some changes having taken place initially and others later, while the present mutations are only repetitions of previous ones, and do not contribute new lines of development to those already existing. This leads us to the supposition of some common original cause, which produced a number of changes, but which itself is no longer at work, but has left the affected qualities, and only these, in the state of mutability.
In nature, repeated mutations must be of far greater significance than isolated ones. How [568] great is the chance for a single individual to be destroyed in the struggle for life? Hundreds of thousands of seeds are produced by lamarckiana annually in the field, and only some slow increase of the number of specimens can be observed. Many seeds do not find the proper circumstances for germination, or the young seedlings are destroyed by lack of water, of air, or of space. Thousands of them are so crowded when becoming rosettes that only a few succeed in producing stems. Any weakness would have destroyed them. As a matter of fact they are much oftener produced in the seed than seen in the field with the usual unfavorable conditions; the careful sowing of collected seeds has given proof of this fact many times.
The experimental proof of this frequency in the origin of new types, seems to overcome many difficulties offered by the current theories on the probable origin of species at large.
VI. The relation between mutability and fluctuating variability has always been one of the chief difficulties of the followers of Darwin. The majority assumed that species arise by the slow accumulation of slight fluctuating deviations, and the mutations were only to be considered as extreme fluctuations, obtained, in the main, by a continuous selection of small differences in a constant direction.
[569] My cultures show that quite the opposite is to be regarded as fact. All organs and all qualities of lamarckiana fluctuate and vary in a more or less evident manner, and those which I had the opportunity of examining more closely were found to comply with the general laws of fluctuation. But such oscillating changes have nothing in common with the mutations. Their essential character is the heaping up of slight deviations around a mean, and the occurrence of continuous lines of increasing deviations, linking the extremes with this group. Nothing of the kind is observed in the case of mutations. There is no mean for them to be grouped around and the extreme only is to be seen, and it is wholly unconnected with the original type. It might be supposed that on closer inspection each mutation might be brought into connection with some feature of the fluctuating variability. But this is not the case. The dwarfs are not at all the extreme variants of structure, as the fluctuation of the height of the lamarckiana never decreases or even approaches that of the dwarfs. There is always a gap. The smallest specimens of the tall type are commonly the weakest, according to the general rule of the relationship between nourishment and variation, but the tallest dwarfs are of course the most robust specimens of their group. [570] Fluctuating variability, as a rule, is subject to reversion. The seeds of the extremes do not produce an offspring which fluctuates around their parents as a center, but around some point on the line which combines their attributes with the corresponding characteristic of their ancestors, as Vilmorin has put it. No reversion accompanies mutation, and this fact is perhaps the completest contrast in which these two great types of variability are opposed to each other.
The offspring of my mutants are, of course, subject to the general laws of fluctuating variability. They vary, however, around their own mean, and this mean is simply the type of the new elementary species.
VII. The mutations take place in nearly all directions.
Many authors assume that the origin of species is directed by unknown causes. These causes are assumed to work in each single case for the improvement of the animals and plants, changing them in a manner corresponding in a useful way to the changes that take place in their environment. It is not easy to imagine the nature of these influences nor how they would bring about the desired effect.
This difficulty was strongly felt by Darwin, and one of the chief purposes of his selection theory may be said to have been the attempt [571] to surmount it. Darwin tried to replace the unknown cause by natural agencies, which lie under our immediate observation. On this point Darwin was superior to his predecessors, and it is chiefly due to the clear conception of this point that his theory has gained its deserved general acceptance. According to Darwin, changes occur in all directions, quite independently of the prevailing circumstances. Some may be favorable, others detrimental, many of them without significance, neither useful nor injurious. Some of them will sooner or later be destroyed, while others will survive, but which of them will survive, is obviously dependent upon whether their particular changes agree with the existing environic conditions or not. This is what Darwin has called the struggle for life. It is a large sieve, and it only acts as such. Some fall through and are annihilated, others remain above and are selected, as the phrase goes. Many are selected, but more are destroyed; daily observation does not leave any doubt upon this point.
How the differences originate is quite another question. It has nothing to do with the theory of natural selection nor with the struggle for life. These have an active part only in the accumulation of useful qualities, and only in so [572] far as they protect the bearers of such characters against being crowded out by their more poorly constituted competitors.
However, the differentiating characteristics of elementary species are only very small. How widely distant they are from the beautiful adaptative organizations of orchids, of insectivorous plants and of so many others! Here the difference lies in the accumulation of numerous elementary characters, which all contribute to the same end. Chance must have produced them, and this would seem absolutely improbable, even impossible, were it not for Darwin’s ingenious theory. Chance there is, but no more than anywhere else. It is not by mere chance that the variations move in the required direction. They do go, according to Darwin’s view, in all directions, or at least in many. If these include the useful ones, and if this is repeated a number of times, cumulation is possible; if not, there is simply no progression, and the type remains stable through the ages. Natural selection is continually acting as a sieve, throwing out the useless changes and retaining the real improvements. Hence the accumulation in apparently predisposed directions, and hence the increasing adaptations to the more specialized conditions of life. It must be obvious to any one who can free himself from the current ideas, [573] that this theory of natural selection leaves the question as to how the changes themselves are brought about, quite undecided. There are two possibilities, and both have been propounded by Darwin. One is the accumulation of the slight deviations of fluctuating variability, the other consists of successive sports or leaps taking place in the same direction.
In further lectures a critical comparison of the two views will be given. Today I have only to show that the mutations of the evening-primroses, though sudden, comply with the demands made by Darwin as to the form of variability which is to be accepted as the cause of evolution and as the origin of species.
Some of my new types are stouter and others weaker than their parents, as shown by gigas and albida. Some have broader leaves and some narrower, lata and oblonga. Some have larger flowers (gigas) or deeper yellow ones (rubrinervis), or smaller blossoms (scintillans), or of a paler hue (albida). In some the capsules are longer (rubrinervis), or thicker (gigas), or more rounded (lata), or small (oblonga), and nearly destitute of seeds (brevistylis). The unevenness of the surface of the leaves may increase as in lata, or decrease as in laevifolia. The tendency to become annual prevails in rubrinervis, but gigas tends to become [574] biennial. Some are rich in pollen, while scintillans is poor. Some have large seeds, others small. Lata has become pistillate, while brevistylis has nearly lost the faculty to produce seeds. Some undescribed forms were quite sterile, and some I observed which produced no flowers at all. From this statement it may be seen that nearly all qualities vary in opposite directions and that our group of mutants affords wide material for the sifting process of natural selection. On the original field the laevifolia and brevistylis have held their own during sixteen years and probably more, without, however, being able to increase their numbers to any noticeable extent. Others perish as soon as they make their appearance, or a few individuals are allowed to bloom, but probably leave no progeny.
But perhaps the circumstances may change, or the whole strain may be dispersed and spread to new localities with different conditions. Some of the latter might be found to be favorable to the robust gigas, or to rubrinervis, which requires a drier air, with rainfall in the springtime and sunshine during the summer. It would be worth while to see whether the climate of California, where neither O. lamarckiana nor O. biennis are found wild, would not exactly [575] suit the requirements of the new species rubrinervis and gigas.
NOTE. Oenotheras are native to America and all of the species growing in Europe have escaped from gardens directly, or may have arisen by mutation, or by hybridization of introduced species. A fixed hybrid between O. cruciata and O. biennis constituting a species has been in cultivation for many years. The form known as O. biennis in Europe, and used by de Vries in all of the experiments described in these lectures, has not yet been found growing wild in America and is not identical with the species bearing that name among American botanists. Concerning this matter Professor de Vries writes under date of Sept. 12, 1905: “The ‘biennis’ which I collected in America has proved to be a motley collection of forms, which at that time I had no means of distinguishing. No one of them, so far as they are now growing in my garden is identical with our biennis of the sand dunes.” The same appears to be the case with O. muricata. Plants from the Northeastern American seaboard, identifiable with the species do not entirely agree with those raised from seed received from Holland.
O. lamarckiana has not been found growing wild in America in recent years although the evidence at hand seems to favor the conclusion that it was seen and collected in the southern states in the last century. (See MacDougal, Vail, Shull, and Small: Mutants and Hybrids of the Oenotheras. Publication 24. Carnegie Institution. Washington, D.C., 1905.) EDITOR.
[576]
LECTURE XX
THE ORIGIN OF WILD SPECIES AND VARIETIES
New species and varieties occur from time to time in the wild state. Setting aside all theoretical conceptions as to the common origin of species at large, the undoubted fact remains that new forms are sometimes met with. In the case of the peloric toad-flax the mutations are so numerous that they seem to be quite regular. The production of new species of evening-primroses was observed on the field and afterwards duplicated in the garden. There is no reason to think that these cases are isolated instances. Quite on the contrary they seem to be the prototypes of repeated occurrences in nature.
If this conception is granted, the question at once arises, how are we to deal with analogous cases, when fortune offers them, and what can we expect to learn from them?
A critical study of the existing evidence seems to be of great importance in order to ascertain the best way of dealing with new facts, and of estimating the value of the factors concerned. [577] It is manifest that we must be very careful and conservative in dealing with new facts that are brought to our attention, and every effort should be made to bring additional evidence to light. Many vegetable anomalies are so rare that they are met with only by the purest chance, and are then believed to be wholly new. When a white variety of some common plant is met with for the first time we generally assume that it originated on that very spot and only a short time previously. The discovery of a second locality for the same variety at once raises the question as to a common origin in the two instances. Could not the plants of the second locality have arisen from seeds transported from the first?
White varieties of many species of blue-bells and gentians are found not rarely, white-flowering plants of heather, both of Erica Tetralix and Calluna vulgaris occur on European heaths; white flowers of Brunella vulgaris, Ononis repens, Thymus vulgaris and others may be seen in many localities in the habitats of the colored species. Pelories of labiates seem to occur often in Austria, but are rare in Holland; white bilberries (Vaccinium Myrtillus) have many known localities throughout Europe, and nearly all the berry-bearing species in the large heath family are recorded as having white varieties.
[578] Are we to assume a single origin for all the representatives of such a variety, as we have done customarily for all the representatives of a wild species? Or can the same mutation have been repeated at different times and in distant localities? If a distinct mutation from a given species is once possible, why should it not occur twice or thrice?
A variety which seems to be new to us may only appear so, because the spot where it grows had hitherto escaped observation. Lychnis preslii is a smooth variety of Lychnis diurna and was observed for the first time in the year 1842 by Sekera. It grew abundantly in a grove near Munchengratz in southern Hungary. It was accompanied by the ordinary hairy type of the species. Since then it has been observed to be quite constant in the same locality, and some specimens have been collected for me there lately by Dr. Nemec, of Prague. No other native localities of this variety have been discovered, and there can be no doubt that it must have arisen from the ordinary campion near the spot where it still grows. But this change may have taken place some years before the first discovery, or perhaps one or more centuries ago. This could only be known if it could be proved that the locality had been satisfactorily investigated previously, and that the variety had not [579] been met with. Even in this case only something would be discovered about the time of the change, but nothing about its real nature.
So it is in many cases. If a variety is observed in a number of specimens at the time of its first discovery, and at a locality not studied previously, it takes the aspect of an old form of limited distribution, and little can be learned as to the circumstances under which it arose. If on the contrary it occurs in very small numbers or perhaps even in a single individual, and if the spot where it is found is located so that it could hardly have escaped previous observation, then the presumption of a recent origin seems justified.
What has to be ascertained on such occasions to give them scientific value? Three points strike me as being of the highest importance. First, the constancy of the new type; secondly, the occurrence or lack of intermediates, and last, but not least, the direct observation of a repeated production.
The first two points are easily ascertained. Whether the new type is linked with its more common supposed ancestor by intermediate steps is a query which at once strikes the botanist. It is usually recorded in such cases, and we may state at once that the general result is, that such intermediates do not occur. This is [580] of the highest importance and admits of only two explanations. One is that intermediates may be assumed to have preceded the existent developed form, and to have died out afterwards. But why should they have done so, especially in cases of recent changes? On the other hand the intermediates may be lacking because they have never existed, the change having taken place by a sudden leap, such as the mutations described in our former lectures. It is manifest that the assumption of hypothetical intermediates could only gain some probability if they had been found in some instance. Since they do not occur, the hypothesis seems wholly unsupported.
The second point is the constancy of the new type. Seeds should be saved and sown. If the plant fertilizes itself without the aid of insects, as do some evening-primroses, the seed saved from the native locality may prove wholly pure, and if it does give rise to a uniform progeny the constancy of the race may be assumed to be proved, provided that repeated trials do not bring to light any exceptions. If the offspring shows more than one type, cross-fertilization is always to be looked to as the most probable cause, and should be excluded, in order to sow pure seeds. Garden-experiments of this kind, and repeated trials, should always be combined [581] with the discovery of a presumed mutation. In many instances the authors have realized the importance of this point, and new types have been found constant from the very beginning. Many cases are known which show no reversions and even no partial reversions. This fact throws a distinct light on our first point, as it makes the hypothesis of a slow and gradual development still more improbable.
My third point is of quite another nature and has not as yet been dealt with. But as it appeals to me as the very soul of the problem, it seems necessary to describe it in some detail. It does not refer to the new type itself, nor to any of its morphologic or hereditary attributes, but directly concerns the presumed ancestors themselves.
The peloric toad-flax in my experiment was seen to arise thrice from the same strain. Three different individuals of my original race showed a tendency to produce peloric mutations, and they did so in a number of their seeds, exactly as the mutations of the evening-primroses were repeated nearly every year. Hence the inference, that whenever we find a novelty which is really of very recent date, the parent-strain which has produced it might still be in existence on the same spot. In the case of shrubs or perennials the very parents might yet be found. [582] But it seems probable, and is especially proved in the case of the evening-primroses, that all or the majority of the representatives of the whole strain have the same tendency to mutate. If this were a general rule, it would suffice to take some pure seeds from specimens of the presumed parents and to sow and multiply the individuals to such an extent that the mutation might have a chance to be repeated.
Unfortunately, this has not as yet been done, but in my opinion it should be the first effort of any one who has the good luck to discover a new wild mutation. Specimens of the parents should be transplanted into a garden and fertilized under isolated conditions. Seeds saved from the wild plant would have little worth, as they might have been partly fertilized by the new type itself.
After this somewhat lengthy discussion of the value of observations surrounding the discovery of new wild mutations, we now come to the description of some of the more interesting cases. As a first example, I will take the globular fruited shepherd’s purse, described by Solms Laubach as Capsella heegeri. Professor Heeger discovered one plant with deviating fruits, in a group of common shepherd’s purses in the market-place near Landau in Germany, in the fall of 1897. They were nearly spherical, [583] instead of flat and purse-shaped. Their valves were thick and fleshy, while those of the ordinary form are membranaceous and dry. The capsules hardly opened and therefore differed in this point from the shepherd’s purse, which readily loosens both its valves as soon as it is ripe.
Only one plant was observed; whence it came could not be determined, nor whether it had arisen from the neighboring stock of Capsella or not. The discoverer took some seed to his garden and sent some to the botanical garden at Strassburg, of which Solms-Laubach is the director. The majority of the seeds of course were sowed naturally on the original spot. The following year some of the seeds germinated and repeated the novelty. The leaves, stems and flowers were those of the common shepherd’s purse, but no decision could be reached concerning the type of this generation before the first flowers had faded and the rounded capsules had developed. Then it was seen that the heegeri came true from seed. It did so both in the gardens and on the market-place, where it was observed to have multiplied and spread in some small measure. The same was noted the following year, but then the place was covered with gravel and all the plants destroyed. It is not recorded to have been seen wild since.
[584] Intermediate forms have not been met with. Some slight reversions may occur in the autumn on the smallest and weakest lateral branches. Such reversions, however, seem to be very rare, as I have tried in vain to produce them on large and richly branched individuals, by applying all possible inducements in the form of manure and of cutting, to stimulate the production of successive generations of weaker side branches.
This constancy was proved by the experiments of Solms-Laubach, which I have repeated in my own garden during several years with seed received from him. No atavists or deviating specimens have been found among many hundreds of flowering plants.
It is important to note that within the family of the crucifers the form of the capsule and the attributes of the valves and seeds are usually considered to furnish the characteristics of genera, and this point has been elucidated at some length by Solms-Laubach. There is, however, no sufficient reason to construe a new genus on the ground of Heeger’s globular fruited shepherd’s purse; but as a true elementary species, and even as a good systematic species it has proved itself, and as such it is described by Solms-Laubach, who named it in honor of its discoverer.
Exactly analogous discoveries have been [586] instead of displaying a bright yellow cup. O. cruciata grows in the Adirondack Mountains, in the states of New York and Vermont, and seems to be abundant there. It has been introduced into botanical gardens and yielded a number of hybrids, especially with O. biennis and O. lamarckiana, and the narrow petals of the parent-species may be met with in combination with the stature and vegetative characteristics of these last named species. O. cruciata has a purple foliage, while biennis and lamarckiana are green, and many of the hybrids may instantly be recognized by their purple color.
The curious attribute of the petals is not to be considered simply as a reduction in size. On anatomical inquiry it has been found that these narrow petals bear some characteristics which, on the normal plants, are limited to the calyx. Stomata and hairs, and the whole structure of the surface and inner tissues on some parts of these petals are exactly similar to those of the calyx, while on others they have retained the characteristics of petals. Sometimes there may even be seen by the naked eye green longitudinal stripes of calyx-like structure alternating with bright yellow petaloid parts. For these reasons the cruciata character may be considered as a case of sepalody of the petals, or of the petals being partly converted into sepals.
[587] It is worth while to note that as a monstrosity this occurrence is extremely rare throughout the whole vegetable kingdom, and only very few instances have been recorded.
Two cases of sudden mutations have come to my knowledge, producing this same anomaly in allied species. One has been already alluded to; it pertains to the common evening-primrose or Oenothera biennis, and one is a species belonging to another genus of the same family, the great hairy willow-herb or Epilobium hirsutum. I propose to designate both new forms by the varietal name of cruciata, or cruciatum.
Oenothera biennis cruciata was found in a native locality of the O. biennis itself. It consisted of only one plant, showing in all its flowers the cruciata marks. In all other respects it resembled wholly the biennis, especially in the pure green color of its foliage, which at once excluded all suspicion of hybrid origin with the purple O. cruciata. Moreover in our country this last occurs only in the cultivated state in botanical gardens.
Intermediates were not seen, and as the plant bore some pods, it was possible to test its constancy. I raised about 500 plants from its seeds, out of which more than 100 flowered in the first year. The others were partly kept through the winter and flowered next year. Seeds saved in [588] both seasons were sown on a large scale. Both the first and the succeeding generations of the offspring of the original plant came true without any exception. Intermediates are often found in hybrid cultures, and in them the character is a very variable one, but as yet they were not met with in progeny of this mutant. All these plants were exactly like O. biennis, with the single exception of their petals.
Epilobium hirsutum cruciatum was discovered by John Rasor near Woolpit, Bury St. Edmunds, in England. It flowered in one spot, producing about a dozen stems, among large quantities of the parent-species, which is very common there, as it is elsewhere in Europe. This species is a perennial, multiplying itself by underground runners, and the stems of the new variety were observed to stand so close to each other that they might be considered as the shoots of one individual. In this case this specimen might probably be the original mutant, as the variety had not been seen on that spot in previous years, even as it has not been found elsewhere in the vicinity.
Intermediates were not observed, though the difference is a very striking one. In the cruciate flowers the broad and bright purple petals seem at first sight to be wholly wanting. They are too weak to expand and to reflex the calyx [589] as in the normal flowers of the species. The sepals adhere to one another, and are only opened at their summit by the protruding pistils. Even the stamens hardly come to light. At the period of full bloom the flowers convey only the idea of closed buds crowned by the conspicuous white cross of the stigma. Any intermediate form would have at once betrayed itself by larger colored petals, coming out of the calyx-sheath. The cruciate petals are small and linear and greenish, recalling thereby the color of the sepals.
Mr. Rasor having sent me some flowers and some ripe capsules of his novelty, I sowed the latter in my experimental garden, where the plant flowered in large numbers and with many thousands of flowers both in 1902 and 1903. All of these plants and all of these flowers repeated the cruciate type exactly, and not the slightest impurity or tendency to partial reversion has been observed.
Thus true and constant cruciate varieties have been produced from accidentally observed initial plants, and because of their very curious characters they will no doubt be kept in botanical gardens, even if they should eventually become lost in their native localities.
At this point I might note another observation made on the wild species of Oenothera cruciata [590] from the Adirondacks. Through the kindness of Dr. MacDougal, of the New York Botanical Garden, I received seeds from Sandy Hill near Lake George. When the plants, grown from these seeds, flowered, they were not a uniform lot, but exhibited two distinct types. Some had linear petals and thin flower-buds, and in others the petals were a little broader and the buds more swollen. The difference was small, but constant on all the flowers, each single plant clearly belonging to one or the other of the two types. Probably two elementary species were intermixed here, but whether one is the systematic type and the other a mutation, remains to be seen.
Nor seem these two types to exhaust the range of variability of Oenothera cruciata. Dr. B.L. Robinson of Cambridge, Mass., had the kindness to send me seeds from another locality in the same region. The seeds were collected in New Hampshire and in my garden produced a true and constant cruciata, but with quite different secondary characters from both the aforesaid varieties. The stems and flower-spikes and even the whole foliage were much more slender, and the calyx-tubes of the flowers were noticeably more elongated. It seems not improbable that Oenothera cruciata includes a group of lesser unities, and may prove to comprise a [591] swarm of elementary species, while the original strain might even now be still in a condition of mutability. A close scrutiny in the native region is likely to reveal many unexpected features.
A very interesting novelty has already been described in a former lecture. It is the Xanthium wootoni, discovered in the region about Las Vegas, New Mexico, by T.D.A. Cockerell. It is similar in all respects to X. commune, but the burrs are more slender and the prickles much less numerous, and mostly stouter at their base. It grows in the same localities as the X. commune, and is not recorded to occur elsewhere. Whether it is an old variety or a recent mutation it is of course impossible to decide. In a culture made in my garden from the seed sent me by Mr. Cockerell, I observed (1903) that both forms had a subvariety with brownish foliage, and, besides this, one of a pure green. Possibly this species, too, is still in a mutable condition.
Perhaps the same may be asserted concerning the beautiful shrub, Hibiscus Moscheutos, observed in quite a number of divergent types by John W. Harshberger. They grew in a small meadow at Seaside Park, New Jersey, in a locality which had been undisturbed for years. They differed from each other in nearly all the [592] organs, in size, in the diameter of the stems, which were woody in some and more fleshy in others, in the shape of the foliage and in the flowers. More than twenty types could be distinguished and seeds were saved from a number of them, in order to ascertain whether they are constant, or whether perhaps a main stem in a mutating condition might be found among them. If this should prove to be the case, the relations between the observed forms would probably be analogous to those between the O. lamarckiana and its derivatives.
Many other varieties have sprung from the type-species under similar conditions from time to time. A fern-leaved mercury, Mercurialis annua laciniata, was discovered in the year 1719 by Marchant. The type was quite new at the time and maintained itself during a series of years. The yellow deadly nightshade or Atropa Belladonna lutea was found about 1850 in the Black Forest in Germany in a single spot, and has since been multiplied by seeds. It is now dispersed in botanical gardens, and seems to be quite constant. A dwarf variety of a bean, Phaseolus lunatus, was observed to spring from the ordinary type by a sudden leap about 1895 by W.W. Tracy, and many similar cases could be given.
The annual habit is not very favorable for [593] the discovery of new forms in the wild state. New varieties may appear, but may be crowded out the first year. The chances are much greater with perennials, and still greater with shrubs or trees. A single aberrant specimen may live for years and even for centuries, and under such conditions is pretty sure to be discovered sooner or later. Hence it is no wonder that many such cases are on record. They have this in common that the original plant of the variety has been found among a vast majority of representatives of the corresponding species. Nothing of course is directly known about its origin. Intermediate links have as a rule been wanting, and the seeds, which have often been sown, have not yielded reliable results, as no care was taken to preserve the blossoms from intercrossing with their parent-forms.
Stress should be laid upon one feature of these curious occurrences. Relatively often the same novelty has been found twice or thrice, or even more frequently, and under conditions which make it very improbable that any relation between such occurrences might exist. The same mutation must have taken place more than once from the same main stem.
The most interesting of these facts are connected with the origin of the purple beech, which [594] is now so universally cultivated. I take the following statements from an interesting historical essay of Prof. Jaggi. He describes three original localities. One is near the Swiss village, Buch am Irchel, and is located on the Stammberg. During the 17th century five purple beeches are recorded to have grown on this spot. Four of them have died, but one is still alive. Seedlings have germinated around this little group, and have been mostly dug up and transplanted into neighboring gardens. Nothing is known about the real origin of these plants, but according to an old document, it seems that about the year 1190 the purple beeches of Buch were already enjoying some renown, and attracting large numbers of pilgrims, owing to some old legend. The church of Embrach is said to have been built in connection with this legend, and was a goal for pilgrimages during many centuries.
A second native locality of the purple beech is found in a forest near Sondershausen in Thuringen, Germany, where a fine group of these trees is to be seen. They were mentioned for the first time in the latter half of the eighteenth century, but must have been old specimens long before that time. The third locality seems to be of much later origin. It is a forest near Roveredo in South Tyrol, where a new [595] university is being erected. It is only a century ago that the first specimens of the purple beech were discovered there.
As it is very improbable that the two last named localities should have received their purple beeches from the first named forest, it seems reasonable to assume that the variety must have been produced at least thrice.
The purple beech is now exceedingly common in cultivation. But Jaggi succeeded in showing that all the plants owe their origin to the original trees mentioned above, and are, including nearly all cultivated specimens with the sole exception of the vicinity of Buch, probably derived from the trees in Thuringen. They are easily multiplied by grafting, and come true from seed, at least often, and in a high proportion. Whether the original trees would yield a pure progeny if fertilized by their own pollen has as yet not been tested. The young seedlings have purple seed-leaves, and may easily be selected by this character, but they seem to be always subjected in a large measure to vicinism.
Many other instances of trees and shrubs, found in accidental specimens constituting a new variety in the wild state, might be given. The oak-leaved beech has been found in a forest of Lippe-Detmold in Germany and near Versailles, [596] whence it was introduced into horticulture by Carriere. Similarly divided and cleft leaves seem to have occurred more often in the wild state, and cut-leaved hazels are recorded from Rouen in France, birches and alders from Sweden and Lapland, where both are said to have been met with in several forests. The purple barberry was found about 1830 by Bertin, near Versailles. Weeping varieties of ashes were found wild in England and in Germany, and broom-like oaks, Quercus pedunculata fastigiata, are recorded from Hessen-Darmstadt, Calabria, the Pyrenees and other localities. About the real origin of all these varieties nothing is definitely known.
The “single-leaved” strawberry is a variety often seen in botanical gardens, as it is easily propagated by its runners. It was discovered wild in Lapland at the time of Linnaeus, and appeared afterwards unexpectedly in a nursery near Versailles. This happened about the year 1760 and Duchesne tested it from seeds and found it constant. This strain, however, seems to have died out before the end of the 18th century. In a picture painted by Holbein (1495-1543), strawberry leaves can be seen agreeing exactly with the monophyllous type. The variety may thus be assumed to have arisen independently [597] at least thrice, at different periods and in distant localities.
From all these statements and a good many others which can be found in horticultural and botanical literature, it may be inferred that mutations are not so very rare in nature as is often supposed. Moreover we may conclude that it is a general rule that they are neither preceded nor accompanied by intermediate steps, and that they are ordinarily constant from seed from the first.
Why then are they not met with more often? In my opinion it is the struggle for life which is the cause of this apparent rarity; which is nothing else than the premature death of all the individuals that so vary from the common type of their species as to be incapable of development under prevailing circumstances. It is obviously without consequence whether these deviations are of a fluctuating or of a mutating nature. Hence we may conclude that useless mutations will soon die out and will disappear without leaving any progeny. Even if they are produced again and again by the same strain, but under the same unfavorable conditions, there will be no appreciable result.
Thousands of mutations may perhaps take place yearly among the plants of our immediate vicinity without any chance of being discovered. [598] We are trained to the appreciation of the differentiating marks of systematic species. When we have succeeded in discerning these as given by our local flora lists, we rest content. Meeting them again we are in the habit of greeting them with their proper names. Such is the satisfaction ensuing from this knowledge that we do not feel any inclination for further inquiry. Striking deviations, such as many varietal characters, may be remarked, but then they are considered as being of only secondary interest. Our minds are turned from the delicately shaded features which differentiate elementary species.
Even in the native field of the evening-primroses, no botanist would have discovered the rosettes with smaller or paler leaves, constituting the first signs of the new species. Only by the guidance of a distinct theoretical idea were they discovered, and having once been pointed out a closer inspection soon disclosed their number.
Variability seems to us to be very general, but very limited. The limits however, are distinctly drawn by the struggle for existence. Of course the chance for useful mutations is a very small one. We have seen that the same mutations are as a rule repeated from time to time by the same species. Now, if a useful mutation, [599] or even a wholly indifferent one, might easily be produced, it would have been so, long ago, and would at the present time simply exist as a systematic variety. If produced anew somewhere the botanist, would take it for the old variety and would omit to make any inquiry as to its local origin.
Thousands of seeds with perhaps wide circles of variability are ripened each year, but only those that belong to the existing old narrow circles survive. How different would Nature appear to us if she were free to evolve all her potentialities!
Darwin himself was struck with this lack of harmony between common observations and the probable real state of things. He discussed it in connection with the cranesbill of the Pyrenees (Geranium pyrenaicum). He described how this fine little plant, which has never been extensively cultivated, had escaped from a garden in Staffordshire and had succeeded in multiplying itself so as to occupy a large area.
In doing so it had evidently found place for an uncommonly large number of plantlets from its seeds and correspondingly it had commenced to vary in almost all organs and qualities and nearly in all imaginable directions. It displayed under these exceptional circumstances a capacity which never had been exceeded and [600] which of course would have remained concealed if its multiplication had been checked in the ordinary way.
Many species have had occasion to invade new regions and cover them with hundreds of thousands of individuals. First are to be cited those species which have been introduced from America into Europe since the time of Columbus, or from Europe into this country. Some of them have become very common. In my own country the evening-primroses and Canada fleabane or are examples, and many others could be given. They should be expected to vary under these circumstances in a larger degree. Have they done so? Manifestly they have not struck out useful new characters that would enable their bearers to found new elementary species. At least none have been observed. But poor types might have been produced, and periods of mutability might have been gone through similar to that which is now under observation for Lamarck’s evening primrose in Holland.
From this discussion we may infer that the chances of discovering new mutating species are great enough to justify the utmost efforts to secure them. It is only necessary to observe large numbers of plants, grown under circumstances which allow the best opportunities for [601] all the seeds. And as nature affords such opportunities only at rare intervals, we should make use of artificial methods. Large quantities of seed should be gathered from wild plants and sowed under very favorable conditions, giving all the nourishment and space required to the young seedlings. It is recommended that they be sown under glass, either in a glass-house or protected against cold and rain by glass-frames. The same lot of seed will be seen to yield twice or thrice as many seedlings if thus protected, compared with what it would have produced when sown in the field or in the garden. I have nearly wholly given up sowing seeds in my garden, as circumstances can be controlled and determined with greater exactitude when the sowing is done in a glasshouse.
The best proof perhaps, of the unfavorable influence of external conditions for slightly deteriorated deviations is afforded by variegated leaves. Many beautiful varieties are seen in our gardens and parks, and even corn has a variety with striped leaves. They are easily reproduced, both by buds and by seeds, and they are the most ordinary of all varietal deviations. They may be expected to occur wild also. But no real variegated species, nor even good varieties with this attribute occurs in nature. [602] On the other hand occasional specimens with a single variegated leaf, or with some few of them, are actually met with, and if attention is once drawn to this question, perhaps a dozen or so instances might be brought together in a summer. But they never seem to be capable of further evolution, or of reproducing themselves sufficiently and of repeating their peculiarity in their progeny. They make their appearance, are seen during a season, and then disappear. Even this slight incompleteness of some spots on one or two leaves may be enough to be their doom.
It is a common belief that new varieties owe their origin to the direct action of external conditions and moreover it is often assumed that similar deviations must have similar causes, and that these causes may act repeatedly in the same species, or in allied, or even systematically distant genera. No doubt in the end all things must have their causes, and the same causes will lead under the same circumstances to the same results. But we are not justified in deducing a direct relation between the external conditions and the internal changes of plants. These relations may be of so remote a nature that they cannot as yet be guessed at. Therefore only direct experience may be our guide. Summing up the result of our facts and discussions [603] we may state that wild new elementary species and varieties are recorded to have appeared from time to time. Invariably this happened by sudden leaps and without intermediates. The mutants are constant when propagated by seed, and at once constitute a new race. In rare instances this may be of sufficient superiority to win a place for itself in nature, but more often it has qualities which have led to its introduction into gardens as an ornamental plant or into botanical gardens by reason of the interest afforded by their novelty, or by their anomaly.
Many more mutations may be supposed to be taking place all around us, but artificial sowings on a large scale, combined with a close examination of the seedlings and a keen appreciation of the slightest indications of deviation seem required to bring them to light.
[604]
LECTURE XXI
MUTATIONS IN HORTICULTURE
It is well known that Darwin based his theory of natural selection to a large extent upon the experience of breeders. Natural and artificial selection exhibit the same general features, yet it was impossible in Darwin’s time to make a critical and comparative analysis of the two processes.
In accordance with our present conception there is selection of species and selection within the species. The struggle for life determines which of a group of elementary species shall survive and which shall disappear. In agricultural practice the corresponding process is usually designated by the name of variety-testing. Within the species, or within the variety, the sieve of natural selection is constantly eliminating poor specimens and preserving those that are best adapted to live under the given conditions. Some amelioration and some local races are the result, but this does not appear to be of much importance. On the contrary, the selection [605] within the race holds a prominent place in agriculture, where it is known by the imposing term, race-breeding.
Experience and methods in horticulture differ from those in agriculture in many points. Garden varieties have been tested and separated for a long time, but neither vegetables nor flowers are known to exhibit such motley groups of types as may be seen in large forage crops.
New varieties which appear from time to time may be ornamental or otherwise in flowers, and more or less profitable than their parents in vegetables and fruits. In either case the difference is usually striking, or if not, its culture would be unprofitable.
The recognition of useful new varieties being thus made easy, the whole attention of the breeder is reduced to isolating the seeds of the mutants that are to be saved and sown separately, and this process must be repeated during a few years, in order to produce the quantity of seed that is needed for a profitable introduction of the variety into commerce. In proportion to the abundance of the harvest of each year this period is shorter for some and longer for other species.
Isolation in practice is not so simple nor so easy an affair as it is in the experimental garden. Hence we have constant and nearly unavoidable [606] cross-fertilizations with the parent form or with neighboring varieties, and consequent impurity of the new strain. This impurity we have called vicinism, and in a previous lecture have shown its effects upon the horticultural races on one hand, and on the other, on the scientific value that can be ascribed to the experience of the breeder. We have established the general rule that stability is seldom met with, but that the observed instability is always open to the objection of being the result of vicinism. Often this last agency is its sole cause; or it may be complicated with other factors without our being able to discern them.
Though our assertion that the practice of the horticulturist in producing new varieties is limited to isolation, whenever chance affords them, is theoretically valid, it is not always so. We may discern between the two chief groups of varieties. The retrograde varieties are constant, the individuals not differing more from one another than those of any ordinary species. The highly variable varieties play an important part in horticulture. Double flowers, striped flowers, variegated leaves and some others yield the most striking instances. Such forms have been included in previous lectures among the ever-sporting varieties, because their peculiar characters oscillate between two extremes, viz: [607] the new one of the variety and the corresponding character of the original species.
In such cases isolation is usually accompanied by selection: rarely has the first of a double, striped or variegated race well filled or richly striped flowers or highly spotted leaves. Usually minor degrees of the anomaly are seen first, and the breeder expects the novelty to develop its features more completely and more beautifully in subsequent generations. Some varieties need selection only in the beginning, in others the most perfect specimens must be chosen every year as seed-bearers. For striped flowers, it has been prescribed by Vilmorin, that seeds should be taken only from those with the smallest stripes, because there is always reversion. Mixed seed or seed from medium types would soon yield plants with too broad stripes, and therefore less diversified flowers.
In horticulture, new varieties, both retrograde and ever-sporting, are known to occur almost yearly. Nevertheless, not every novelty of the gardener is to be considered as a mutation in the scientific sense of the word. First of all, the novelties of perennial and woody species are to be excluded. Any extreme case of fluctuating variability may be preserved and multiplied in the vegetative way. Such types are designated [608] in horticulture as varieties, though obviously they are of quite another nature than the varieties reproduced by seed. Secondly, a large number, no doubt the greater number of novelties, are of hybrid origin. Here we may discern two cases. Hybrids may be produced by the crossing of old types, either of two old cultivated forms or newly introduced species, or ordinarily between an old and an introduced variety. Such novelties are excluded from our present discussion. Secondly, hybrids may be produced between a true, new mutation and some of the already existing varieties of the same species. Examples of this obvious and usual practice will be given further on, but it must be pointed out now that by such crosses a single mutation may produce as many novelties as there are available varieties of the same species.
Summarizing these introductory remarks we must lay stress on the fact that only a small part of the horticultural novelties are real mutations, although they do occur from time to time. If useful, they are as a rule isolated and multiplied, and if necessary, improved by selection. They are in many instances, as constant from seed as the unavoidable influence of vicinism allows them to be. Exact observations on the origin, or on the degree of constancy, are usually lacking, [609] the notes being ordinarily made for commercial purposes, and often only at the date of introduction into trade, when the preceding stages of the novelty may have been partly forgotten.
With this necessary prelude I will now give a condensed survey of the historical facts relating to the origin of new horticultural varieties. An ample description has been given recently by Korshinsky, a Russian writer, who has brought together considerable historical material as evidence of the sudden appearance of novelties throughout the whole realm of garden plants.
The oldest known, and at the same time one of the most accurately described mutations is the origin of the cut-leaved variety of the greater celandine or Chelidonium majus. This variety has been described either as such, or as a distinct species, called Chelidonium laciniatum Miller.
It is distinguished from the ordinary species, by the leaves being cut into narrow lobes, with almost linear tips, a character which is, as we have seen on a previous occasion, repeated in the petals. It is at present nearly as commonly cultivated in botanical gardens as the C. majus, and has escaped in many localities and is observed to thrive as readily as the native wild [610] plants. It was not known until a few years before the close of the 16th century. Its history has been described by the French botanist, Rose. It was seen for the first time in the garden of Sprenger, an apothecary of Heidelberg, where the C. majus had been cultivated for many years. Sprenger discovered it in the year 1590, and was struck by its peculiar and sharply deviating characters. He was anxious to know whether it was a new plant and sent specimens to Clusius and to Plater, the last of whom transmitted them to Caspar Bauhin. These botanists recognized the type as quite new and Bauhin described it some years afterwards in his Phytopinax under the name of Chelidonium majus foliis quernis, or oak-leaved celandine. The new variety soon provoked general interest and was introduced into most of the botanical gardens of Europe. It was recognized as quite new, and repeated search has been made for it in a wild state, but in vain. No other origin has been discovered than that of Sprenger’s garden. Afterwards it became naturalized in England and elsewhere, but there is not the least doubt as to its derivation in all the observed cases.
Hence its origin at Heidelberg is to be considered as historically proven, and it is of course only legitimate to assume that it originated in [611] the year 1590 from the seeds of the C. majus. Nevertheless, this was not ascertained by Sprenger, and some doubt as to a possible introduction from elsewhere might arise. If not, then the mutation must have been sudden, occurring without visible preparation and without the appearance of intermediates.
From the very first, the cut-leaved celandine has been constant from seed. Or at least it has been propagated by seed largely and without difficulty. Nothing, however, is known about it in the first few years of its existence. Later careful tests were made by Miller, Rose and others and later by myself, which have shown its stability to be absolute and without reversion, and it has probably been so from the beginning. The fact of its constancy has led to its specific distinction by Miller, as varieties were in his time universally, and up to the present time not rarely, though erroneously, believed to be less stable than true species.
Before leaving the laciniate celandine it is to be noted that in crosses with C. majus it follows the law of Mendel, and for this reason should be considered as a retrograde variety, the more so, as it is also treated as such from a morphological point of view by Stahl and others.
We now come to an enumeration of those cases in which the date of the first appearance [612] of a new horticultural variety has been recorded, and I must apologize for the necessity of again quoting many variations, which have previously been dealt with from another point of view. In such cases I shall limit myself as closely as possible to historical facts. They have been recorded chiefly by Verlot and Carriere, who wrote in Paris shortly after the middle of the past century, and afterwards by Darwin, Korshinsky, and others. It is from their writings and from horticultural literature at large that the following evidence is brought together.
A very well-known instance is that of the dwarf variety of Tagetes signata, which arose in the nursery of Vilmorin in the year 1860. It was observed for the first time in a single individual among a lot of the ordinary Tagetes signata. It was found impossible to isolate it, but the seeds were saved separately. The majority of the offspring returned to the parental type, but two plants were true dwarfs. From these the requisite degree of purity for commercial purposes was reached, the vicinists not being more numerous than 10% of the entire number. The same mutation had been observed a year earlier in the same nursery in a lot of Saponaria calabrica. The seeds of this dwarf repeated the variety in the next generation, but in the third none were observed. Then the variety was [613] thought to be lost, and the culture was given up, as the Mendelian law of the splitting of varietal hybrids was not known. According to our present knowledge we might expect the atavistic descendants of the first dwarf to be hybrids, and to be liable to split in their progeny into one-fourth dwarfs and three-fourths normal specimens. From this it is obvious that the dwarfs would have appeared a second time if the strain had been continued by means of the seeds of the vicinistic progeny.
In order to avoid a return to this phase of the question, another use of the vicinists should at once be pointed out. It is the possibility of increasing the yield of the new variety. If space admits of sowing the seeds of the vicinists, a quarter of the progeny may be expected to come true to the new type, and if they were partly pollinated by the dwarfs, even a larger number would do so. Hence it should be made a rule to sow these seeds also, at least when those of the true representatives of the novelty do not give seed enough for a rapid multiplication.
Other dwarfs are recorded to have sprung from species in the same sudden and unexpected manner, as for instance Ageratum coeruleum of the same nursery, further Clematis Viticella nana and Acer campestre nanum. Prunus Mahaleb nana was discovered in 1828 in one [614] specimen near Orleans by Mme. LeBrun in a large culture of Mahaleb. Lonicera tatarica nana appeared in 1825 at Fontenay-aux Roses. A tall variety of the strawberry is called “Giant of Zuidwijk” and originated at Boskoop in Holland in the nursery of Mr. van de Water, in a lot of seedlings of the ordinary strawberry. It was very large, but produced few runners, and was propagated with much difficulty, for after six years only 15 plants were available. It proved to be a late variety with abundant large fruit, and was sold at a high price. For a long time it was prominent in cultures in Holland only.
Varieties without prickles are known to have originated all of a sudden in sundry cases. Gleditschia sinensis, introduced in 1774 from China, gave two seedlings without spines in the year 1823, in the nursery of Caumzet. It is curious in being one of the rare instances where a simultaneous mutation in two specimens is acknowledged, because as a rule, such records comply with the prevailing, though inexact, belief that horticultural mutations always appear in single individuals.
From Korshinsky’s survey of varieties with cut leaves or laciniate forms the following cases may be quoted. In the year 1830 a nurseryman named Jacques had sown a large lot of elms, [615] Ulmus pedunculata. One of the seedlings had cut leaves. He multiplied it by grafting and gave it to the trade under the name of U. pedunculata urticaefolia. It has since been lost.
Laciniate alders seem to have been produced by mutation at sundry times. Mirbel says that the Alnus glutinosa laciniata is found wild in Normandy and in the forests of Montmorency near Paris. A similar variety has been met with in a nursery near Orleans in the year 1855. In connection with this discovery some discussion has arisen concerning the question whether it was probable that the Orleans strain was a new mutation, or derived in some way from the trees cited by Mirbel. Of course, as always in such cases, any doubt, once pronounced, affects the importance of the observation for all time, since it is impossible to gather sufficient historical evidence to fully decide the point. The same variety had appeared under similar circumstances in a nursery at Lyons previously (1812).
Laciniated maples are said to be of relatively frequent occurrence in nurseries, among seedlings of the typical species. Loudon says that once 100 laciniated seedlings were seen to originate from seed of some normal trees. But in this case it is rather probable that the presumed [616] normal parents were in reality hybrids between the type and the laciniated form, and simply split according to Mendel’s law. This hypothesis is partly founded on general considerations and partly on experiments made by myself with the cut-leaved celandine, previously alluded to, which I crossed with the type. The hybrids repeated the features of the species and showed no signs of their internal hybrid constitution. But the following year one-fourth of their progeny returned to the cut-leaved form. If the same thing has taken place in the case of Loudon’s maples, but without their hybrid origin being known, the result would have been precisely what he observed.
Broussonetia papyriffera dissecta originated about 1830 at Lyons, and a second time in 1866 at Fontenay-aux-Roses. The cut-leaved hazelnuts, birches, beeches and others have mostly been found in the wild state, as I have already pointed out in a previous lecture. A similar variety of the elder, Sambucus nigra laciniata, and its near ally, Sambucus racemosa laciniata, are often to be seen in our gardens. They have been on record since 1886 and come true from seed, but their exact origin seems to have been forgotten. Cut-leaved walnuts have been known since 1812; they come true from seed, but are extremely liable to vicinism, a nuisance which is [617] ascribed by some authors to the fact that often on the same tree the male catkins flower and fall off several weeks before the ripening of the pistils of the other form of flowers.
Weeping varieties afford similar instances. Sophora japonica pendula originated about 1850, and Gleditschia triacanthos pendula some time later in a nursery at Chateau-Thierry (Aisne, France). In the year 1821 the bird’s cherry, or Prunus Padus, produced a weeping variety, and in 1847 the same mutation was observed for the allied Prunus Mahaleb. Numerous other instances of the sudden origin of weeping trees, both of conifers and of others, have been brought together in Korshinsky’s paper. This striking type of variation includes perhaps the best examples of the whole historical evidence. As a rule they appear in large sowings, only one, or only a few at a time. Many of them have not been observed during their youth, but only after having been planted out in parks and forests, since the weeping characters show only after several years.
The monophyllous bastard-acacia originated in the same way. Its peculiarities will be dealt with on another occasion, but the circumstances of its birth may as well be given here. In 1855 in the nursery of Deniau, at Brain-sur-l’Authion (Maine et Loire), it appeared in a lot of [618] seedlings of the typical species in a single individual. This was transplanted into the Jardin des Plantes at Paris, where it flowered and bore seeds in 1865. It must have been partly pollinated by the surrounding normal representatives of the species, since the seeds yielded only one-fourth of true offspring. This proportion, however, has varied in succeeding years. Briot remarks that the monophyllous bastard acacia is liable to petaloid alterations of its stamens, which deficiency may encroach upon its fertility and accordingly upon the purity of its offspring.
Broom-like varieties often occur among trees, and some are known for their very striking reversions by buds, as we have seen on a previous occasion. They are ordinarily called pyramidal or fastigiate forms, and as far as their history goes, they arise suddenly in large sowings of the normal species. The fastigiate birch was produced in this way by Baumann, the Abies concolor fastigiata by Thibault and Keteleer at Paris, the pyramidal cedar by Paillat, the analogous form of Wellingtonia by Otin. Other instances could easily be added, though of course some of the most highly prized broom-like trees are so old that nothing is known about their origin. This, for instance, is the case with the pyramidal yew-tree, Taxus baccata fastigiata. [619] Others have been found wild, as already mentioned in a former lecture.
An analogous case is afforded by the purpleleaved plums, of which the most known form is Prunus Pissardi. It is said to be a purple variety of Prunus cerasifera, and was introduced at the close of the seventies from Persia, where it is said to have been found in Tabris. A similar variety arose independently and unexpectedly in the nursery of Spath, near Berlin, about 1880, but it seems to differ in some minor points from the Persian prototype.
A white variety of Cyclamen vernum made its appearance in the year 1836 in Holland. A single individual was observed for the first time among a large lot of seedlings, in a nursery near Haarlem. It yielded a satisfactory amount of seed, and the progeny was true to the new type. Such plants propagate slowly, and it was only twenty-seven years later (1863) that the bulbs were offered for sale by the Haarlem firm of Krelage & Son. The price of each bulb was $5.00 in that year, but soon afterwards was reduced to $1.00 each, which was about thrice the ordinary price of the red variety.
The firm of Messrs. Krelage & Son has brought into commerce a wide range of new bulb-varieties, all due to occasional mutations, some by seed and others by buds, or to the accidental [620] transference of new qualities into the already existing varieties by cross-pollination through the agency of insects. Instead of giving long lists of these novelties, I may cite the black tulips, which cost during the first few years of their introduction about $25.00 apiece.
Horticultural mutations are as a rule very rare, especially in genera or species which have not yet been brought to a high degree of variability. In these the wide range of varieties and the large scale in which they are multiplied of course give a greater chance for new varieties. But then the possibilities of crossing are likewise much larger, and apparent changes due to this cause may easily be taken for original mutations.
The rarity of the mutations is often proved by the lapse of time between the introduction of a species and its first sport. Some instances may be given. They afford a proof of the length of the period during which the species remained unaltered, although some of these alterations may be due to a cross with an allied form. Erythrina Crista-galli was introduced about 1770, and produced its first sport in 1884, after more than a century of cultivation. Begonia semperflorens has been cultivated since 1829, and for half a century before it commenced sporting. The same length of time has elapsed [621] between the first culture and the first variation of Crambe maritima. Other cases are on record in which the variability exhibited itself much sooner, perhaps within a few years after the original discovery of the species. But such instances seem, as a rule, to be subject to doubt as to the concurrence of hybridization. So for instance the Iris lortetii, introduced in the year 1895 from the Lebanon, which produced a white variety from its very first seeds. If by chance the introduced plants were natural hybrids between the species and the white variety, this apparent and rather improbable mutation would find a very simple explanation. The length of the period preceding the first signs of variability is largely, of course, due to divergent methods of culture. Such species as Erythrina, which are perennial and only sown on a small scale, should not be expected to show varieties very soon. Annual species, which are cultivated yearly in thousands or even hundreds of thousands of individuals, have a much better chance. Perhaps the observed differences are largely due to this cause.
Monstrosities have, from time to time, given rise to cultivated races. The cockscomb or Celosia is one of the most notorious instances. Cauliflowers, turnips and varieties of cabbages are recorded by De Candolle to have arisen in [622] culture, more than a century ago, as isolated monstrous individuals. They come true from seed, but show deviations from time to time which seem to be intimately linked with their abnormal characters. Apetalous flowers may be considered as another form of monstrosity, and in Salpiglossis sinuata such a variety without a corolla made its appearance in the year 1892 in the nursery of Vilmorin. It appeared suddenly, yielded a good crop of seed and was constant from the outset, without any sign of vicinism or impurity.
In several cases the origin of a variety is obscure, while the subsequent historical evidence is such as to make an original sudden appearance quite probable. Although these instances offer but indirect evidence, and will sooner or later lose their importance, it seems desirable to lay some stress on them here, because most of these cases are very obvious and more striking than purely historical facts. Sterile varieties belong to this heading. Sometimes they bear fruit without kernels, sometimes flowers without sexual organs, or even no flowers at all. Instances have been given in the lecture on retrograde varieties; they are ordinarily assumed to have originated by a leap, because it is not quite clear how a loss of the capacity for the formation of seeds could have been slowly accumulated [623] in preceding generations. An interesting case is afforded by a sterile variety of corn, which originated some time ago in my own pedigree-cultures made for another purpose, and which had begun with an ear of 1886. The first generation from the original seeds showed nothing particular, but the second at once produced quite a number of sterile plants. The sterility was caused by the total lack of branches, including those bearing the pistillate flowers. The terminal spikes themselves were reduced to naked spindles, without branches, without flowers and even almost without bracts.
In some individuals, however, this negative character was seen to give way at the tip, showing a few small naked branches. Of course it was impossible to propagate this curious form, but my observations showed that it sprang into existence from known ancestors by a single step or sudden leap. This leap, however, was not confined to a single specimen; on the contrary it affected 40 plants out of a culture of 340 individuals. The same phenomenon was repeated from the seeds of the normal plants in the following year, but afterwards the monstrosity disappeared.
The Italian poplar affords another instance. It is considered by some authors as a distinct species, Populus italica, and by others as a [624] broom-like variety of the Populus nigra, from which it is distinguished by its erect branches and other characters of minor importance. It is often called the pyramidal or fastigiate poplar. Its origin is absolutely unknown and it occurs only in the cultivated state. In Italy it seems to have been cultivated from the earliest historical times, but it was not introduced into other countries till the eighteenth century. In 1749 it was brought into France, and in 1758 into England, and to day it may be seen along roads throughout central Europe and in a large part of Asia. But the most curious fact is that it is only observed in staminate specimens; pistillate trees have not been found, although often sought for. This circumstance makes it very probable that the origin of the broom-like poplar was a sudden mutation, producing only one individual. This being staminate, it has been propagated exclusively by cuttings. It is to be admitted, however, that no material evidence is at hand to prove that it is not an original wild species, the pistillate form of which has been lost by vegetative multiplication. One form only of many dioecious plants is to be found in cultivation, as, for instance some South American species of Ribes.
Total lack of historical evidence concerning [625] the origin of a variety has sometimes been considered as sufficient proof of a sudden origin. The best known instance is that of the renowned cactus-dahlia with its recurved instead of incurved ray-florets. It was introduced from Mexico into the Netherlands by Van den Berg of Jutphaas, under the following remarkable circumstances. In the autumn of 1872 one of his friends had sent him a small case, containing seeds, bulbs and roots from Mexico. From one of these roots a Dahlia shoot developed. It was cultivated with great care and bloomed next year. It surprised all who saw it by the unexpected peculiarity of its large rich crimson flowers, the rays of which were reversed tubular. The margins of the narrow rays were curved backwards, showing the bright color of the upper surface. It was a very showy novelty, rapidly multiplied by cuttings, and was soon introduced into commerce. It has since been crossed with nearly all other available varieties of the Dahlia, giving a large and rich group of forms, bound together by the curious curling of the petals. It has never been observed to grow in Mexico, either wild or in gardens, and thus the introduced individual has come to be considered as the first of its race.
I have already mentioned that the rapid production of large numbers of new varieties, by [626] means of the crossing of the offspring of a single mutant with previously existing sorts, is a very common feature in horticultural practice. It warns us that only a small part of the novelties introduced yearly are due to real mutations. Further instances of novelties with such a common origin are the purple-leaved dahlias, the gooseberries without prickles, the double petunias, erect gloxinias and many others. Accumulation of characters, acquired in different races of a species, may easily be effected in this way; in fact it is one of the important factors in the breeding of horticultural novelties.
I have alluded more than once in this lecture to the question, whether it is probable that mutations occur in one individual or in more. The common belief among horticulturists is that, as a rule, they appear in a single plant. This belief is so widespread that whenever a novelty is seen for the first time in two or more specimens it is at once suggested that it might have originated and been overlooked in a previous generation. Not caring to confess a lack of close observation, the number of mutants in such cases is usually kept secret. At least this statement has been made to me by some of the horticulturists at Erfurt, whom I visited some years ago in order to learn as much as [627] possible about the methods of production of their novelties. Hence it is simply impossible to decide the question on the basis of the experience of the breeders. Even in the case of the same novelty arising in sundry varieties of the same species, the question as to common origin, by means of crossing, is often hard to decide, as for instance in moss-roses and nectarines. On the other hand, instances are on record where the same novelty has appeared at different times, often at long intervals. Such is the case with the butterfly-cyclamen, a form with wide-spreading petals which originated in Martin’s nursery in England. The first time it was seen it was thought to be of no value, and was thrown away, but when appearing for a second time it was multiplied and eventually placed on the market. Other varieties of Cyclamen, as for instance the crested forms, are also known to have originated repeatedly.
In concluding this series of examples of horticultural mutations, I might mention two cases, which have occurred in my own experimental garden. The first refers to a tubular Dahlia. It has ray-florets, the ligules of which have their margins grown together so as to form tubes, with the outer surface corresponding to the pale under-surface of the corolla.
This novelty originated in a single plant in a [628] culture from the seed of the dwarf variety “Jules Chretien.” The seeds were taken from introduced plants in my garden, and as the sport has no ornamental value it is uncertain whether this was the first instance or whether it had previously occurred in the nursery at Lyons, from whence the bulbs were secured. Afterwards it proved true from seed, but was very variable, exhibiting rather the features of an ever-sporting variety.
Another novelty was seen the first time in several individuals. It was a pink sport of the European cranesbill, Geranium pratense. It arose quite unexpectedly in the summer of 1902 from a striped variety of the blue species. It was seen in seven specimens out of a lot of about a hundred plants. This strain was introduced into my garden in 1897, when I bought two plants under the name of Geranium pratense album, which however proved to belong to the striped variety. From their seeds I sowed in 1898 a first generation, of which a hundred plants flowered the next year, and from their seeds I sowed in 1900 the lot which produced the sport. Neither the introduced plants nor their offspring had exhibited the least sign of a color-variation, besides the blue and white stripes. Hence it is very probable that my novelty was a true first mutation, the more probably [629] so since a pink variety would without doubt have a certain horticultural value and would have been preserved if it had occurred. But as far as I have been able to ascertain, it is as yet unknown, nor has it been described until today.
Summing up the results of this long, though very incomplete, list of horticultural novelties with a more or less well-known origin, we see that sudden appearances are the rule. Having once sprung into existence the new varieties are ordinarily constant, except as affected by vicinism. Details concerning the process are mostly unavailable or at least are of very doubtful value. And to this it should be added that really progressive mutations have hardly been observed in horticulture. Hence the theoretical value of the facts is far less than might have been expected.
[630]
LECTURE XXII
SYSTEMATIC ATAVISM
The steady cooperation of progression and retrogression is one of the important principles of organic evolution. I have dwelt upon this point more than once in previous lectures. I have tried to show that both in the more important lines of the general pedigree of the vegetable kingdom, and in the numerous lateral branches ending in the genera and species within the families, progression and retrogression are nearly always at work together. Your attention has been directed to the monocotyledons as an example, where retrogression is everywhere so active that it can almost be said to be the prevailing movement. Reduction in the vegetative and generative organs, in the anatomical structure and growth of the stems, and in sundry other ways is the method by which the monocotyledons have originated as a group from their supposed ancestors among the lower dicotyledonous families. Retrogression is the leading idea in the larger families of the group, [631] as for instance in the aroids and the grasses. Retrograde evolution is also typical in the highest and most highly differentiated family of the monocotyledons, the orchids, which have but one or two stamens. In the second place I have had occasion more than once to assert that retrogression, though seemingly consisting in the disappearance of some quality, need not, as a rule, be considered as a complete loss. Quite on the contrary, it is very probable that real losses are extremely rare, if not wholly lacking. Ordinarily the loss is only apparent, the capacity becomes inactive only, but is not destroyed. The character has become latent, as it is commonly stated, and therefore may return to activity and to the full display of its peculiarity, whenever occasion offers.
Such a return to activity was formerly called atavism. But as we have seen, when dealing with the phenomena of latency at large, sundry cases of latency are to be distinguished, in order to get a clear insight into these difficult processes.
So it is with atavism, too. If any plant reverts to a known ancestor, we have a positive and simple case. But ancestors with alternate specific marks are as a rule neither historically nor experimentally manifest. They are only reputed to be such, and the presumption rests [632] upon the systematic affinity between the derivative species and its nearest probable allies. Such reversions are now to be examined at some length and may be adequately treated under the head of systematic atavism. To this form of atavism pertain, on the basis of our definition, those phenomena by which species assume one or more characters of allies, from which they are understood to have descended by the loss of the character under discussion. The phenomena themselves consist in the production of anomalies and varieties, and as the genetic relation of the latter is often hardly beyond doubt, the anomalies seem to afford the best instances for the study of systematic atavism. This study has for its chief aim the demonstration of the presence of the latent characters, and to show that they return to activity suddenly and not by a slow and gradual recovery of the former features. It supports the assertion that the visible elementary characters are essentially an external display of qualities carried by the bearers of heredity, and that these bearers are separate entities, which may be mingled together, but are not fused into a chaotic primitive life-substance. Systematic atavism by this means leads us to a closer examination of the internal and concealed causes, which rule the affinities and divergencies of [633] allied species. It brings before us and emphasizes the importance of the conception of the so-called unit-characters.
The primrose will serve as an example. In the second lecture we have seen that the old species of Linnaeus, the Primula veris, was split up by Jacquin into three smaller ones, which are called P. officinalis, P. elatior and P. acaulis. From this systematic treatment we can infer that these three forms are assumed to be derived from a common ancestor. Now two of them bear their flowers in bracted whorls, condensed into umbels at the summits of a scape. The scapes themselves are inserted in the axils of the basal leaves, and produce the flowers above them. In the third species, Primula acaulis, this scape is lacking and the flowers are inserted singly in the axils on long slender stalks. For this reason the species is called acaulescent, indicating that it has no other stem than the subterranean rootstock. But on closer inspection we observe that the flower stalks are combined into little groups, each group occupying the aril of one of the basal leaves. This fact at once points to an analogy with the umbellate allies, and induces us to examine the insertion of the flowers more critically. In doing so we find that they are united at their base so as to constitute a sessile umbel. [634] The scapes are not absolutely lacking, but only reduced to almost invisible rudiments.
Relying upon this conclusion we infer that all of the three elementary species have umbels, some pedunculate and the others not. On this point they agree with the majority of the allied species in the genus and in other genera, as for instance in Androsace. Hence the conclusion that the common ancestors were perennial plants with a rootstock bearing their flowers in umbels or whorls on scapes. Lacking in the Primula veris, these scapes must obviously have been lost at the time of the evolution of this form.
Proceeding on this line of speculation we at once see that a very adequate opportunity for systematic atavism is offered here. According to our general conception the apparent loss of a scape is no proof of a corresponding internal loss, but might as well be caused simply by the reduction of the scape-growing capacity to a latent or inactive state. It might be awakened afterwards by some unknown agency, and return to activity.
Now this is exactly what happens from time to time. In Holland the acaulescent primrose is quite a common plant, filling the woods in the spring with thousands of clusters of bright yellow flowers. It is a very uniform type, but in [635] some years it is seen to return to atavistic conditions in some rare individuals. More than once I have observed such cases myself, and found that the variation is only a partial one, producing one or rarely two umbels on the same plant, and liable to fail of repetition when the varying specimens are transplanted into the garden for further observation. But the fact remains that scapes occur. The scapes themselves are of varying length, often very short, and seldom long, and their umbels display the involucre of bracts in a manner quite analogous to that of the Primula officinalis and P. elatior. To my mind this curious anomaly strongly supports the view of the latent condition of the scape in the acaulescent species, and that such a dormant character must be due to a descent from ancestors with active scapes, seems to be in no need of further reiteration. Returning to activity the scapes at once show a full development, in no way inferior to that of the allied forms, and only unstable in respect to their length.
A second example is afforded by the bracts of the crucifers. This group is easily distinguished by its cruciform petals and the grouping of the flowers into long racemes. In other families each flower of such an inflorescence would be subtended by a bract, according to the [636] general rule that in the higher plants side branches are situated in the arils of leaves. Bracts are reduced leaves, but the spikes of the cruciferous plants are generally devoid of them. The flower-stalks, with naked bases, seem to arise from the common axis at indefinite points.
Hence the inference that crucifers are an exception to a general rule, and that they must have originated from other types which did comply with this rule, and accordingly were in the possession of floral bracts. Or, in other words, that the bracts must have been lost during the original evolution of the whole family. This conclusion being accepted, the accidental re-apparition of bracts within the family must be considered as a case of systematic atavism, quite analogous to the re-appearance of the scapes in the acaulescent primrose. The systematic importance of this phenomenon, however, is far greater than in the first case, in which we had only to deal with a specific character, while the abolition of the bracts has become a feature of a whole family.
This reversion is observed to take place according to two widely different principles. On one hand, bracts may be met with in a few stray species, assuming the rank of a specific character. On the other hand they may be seen [637] to occur as an anomaly, incompletely developed, often very rare and with all the appearance of an accidental variation, but sometimes so common as to seem nearly normal.
Coming now to particular instances, we may turn our attention in the first place to the genus Sisymbrium. This is a group of about 50 species, of wide geographic distribution, among which the hedge mustard (S. officinalis) is perhaps the most common of weeds. Two species are reputed to have bracts, Sisymbrium hirsutum and S. supinum. Each flower-stalk of their long racemes is situated in the aril of such a bract, and the peculiarity is quite a natural one, corresponding exactly to what is seen in the inflorescence of other families. Besides the Sisymbrium some six other genera afford similar structures.
Erucastrum pollichii has been already alluded to in a former lecture when dealing with the same problem from another point of view. As previously stated, it is one of the most manifest and most easily accessible examples of a latent character becoming active through systematic atavism. In fact, its bracts are found so often as to be considered by some authors as of quite normal occurrence. Contrasted with those of the above mentioned species of Sisymbrium, they are not seen at the base of all the flower [638] stalks, but are limited to the lowermost part of the raceme, adorning a few, often ten or twelve, and rarely more flower-stalks. Moreover they exhibit a feature which is indicative of the presence of an abnormality. They are not all of the same size, but decrease in length from the base of the raceme upward, and finally slowly disappear.
Besides these rare cases there are quite a number of cruciferous species on record, which have been observed to bear bracts. Penzig in his valuable work on teratology gives a list of 33 such genera, many of them repeating the anomaly in more than one species. Ordinary cabbages are perhaps the best known instance, and any unusual abundance of nourishment, or anomalous cause of growth seems to be liable to incite the development of bracts. The hedge garlic or garlic mustard (Alliaria), the shepherd’s purse, the wormseed or Erysimum cheiranthoides and many others afford instances. In my cultures of Heeger’s shepherd’s purse, the new species derived at Landau in Germany from the common shepherd’s purse, the anomaly was observed to occur more than once, showing that the mutation, which changed the fruits, had not in the least affected this subordinate anomalous peculiarity. In all these cases the bracts behave as with the Erucastrum, [639] being limited to the base of the spike, and decreasing in size from the lower flowers upward. Connected with these atavistic bracts is a feature of minor importance, which however, by its almost universal accompaniment of the bracts, deserves our attention, as it is indicative of another latent character. As a rule, the bracts are grown together with their axillary flower-stalk. This cohesion is not complete, nor is it always developed in the same degree. Sometimes it extends over a large part of the two organs, leaving only their tips free, but on other occasions it is limited to a small part of the base. But it is very interesting that this same cohesion is to be seen in the shepherd’s purse, in the wormseed and in the cabbage, as well as in the case of the Erucastrum and most of the other observed cases of atavistic bracts. This fact suggests the idea of a common origin for these anomalies, and would lead to the hypothesis that the original ancestors of the whole family, before losing the bracts, exhibited this peculiar mode of cohesion.
Bracts and analogous organs afford similar cases of systematic atavism in quite a number of other families. Aroids sometimes produce long bracts from various places on their spadix, as may be seen in the cultivated greenhouse species, Anthurium scherzerianum. [640] Poppies have been recorded to bear bracts analogous to the little scales on the flower-stalks of the pansies, on the middle of their flower stalks. A similar case is shown by the yellow foxglove or Digitalis parviflora. The foxgloves as a rule have naked flower-stalks, without the two little opposite leafy organs seen in so many other instances. The yellow species, however, has been seen to produce such scales from time to time. The honeysuckle genus is, as a rule, devoid of the stipules at the base of the petiole, but Lonicera etrusca has been observed to develop such organs, which were seen to be free in some, but in other specimens were adnate to the base of the leaf, and even connate with those of the opposite leaf.
Other instances could be given proving that bracts and stipules, when systematically lacking, are liable to reappear as anomalies. In doing so, they generally assume the peculiar characters that would be expected of them by comparison with allied genera in which they are of normal occurrence. There can be no doubt that their absence is due to an apparent loss, resulting from the reduction of a formerly active quality to inactivity. Resuming this effective state, the case attains the value and significance accorded to systematic atavism.
A very curious instance of reduced bracts, developing [641] to unusual size, is afforded by a variety of corn, which is called Zea Mays cryptosperma, or Zea Mays tunicata. In ordinary corn the kernels are surrounded by small and thin, inconspicuous and membranaceous scales. Invisible on the integrate spikes, when ripe, they are easily detected by pulling the kernels out. In cryptosperma they are so strongly developed as to completely hide the kernels. Obviously they constitute a case of reversion to the characters of some unknown ancestor, since the corn is the only member of the grass-family with naked kernels. The var. tunicata, for this same reason, has been considered to be the original wild form, from which the other varieties of corn have originated. But as no historical evidence on this point is at hand, we must leave it as it is, notwithstanding the high degree of attractiveness attached to the suggestion.
The horsetail-family may be taken as a further support of our assertion. Some species have stems of two kinds, the fertile being brownish and appearing in early spring before the green or sterile ones. In others the stems are all alike, green and crowned with a conelike spike of sporangia-bearing scales. Manifestly the dimorphous cases are to be considered as the younger ones, partly because they are obvious exceptions to the common rule, and [642] partly because the division of labor is indicative of a higher degree of evolution. But sometimes these dimorphic species are seen to revert to the primary condition, developing a fertile cone at the summit of the green summer-stem. I have had the opportunity of collecting an instance of this anomaly on the tall Equisetum telmateja in Switzerland, and other cases are on record in teratological literature. It is an obvious example of systematic atavism, occurring suddenly and with the full development of all the qualities needed for the normal production of sporangia and spores. All of these must be concealed in a latent condition within the young tissues of the green stems.
More than once I have had occasion to deal with the phenomenon of torsions, as exhibited by the teasels and some other plants. This anomaly has been shown to be analogous to the cases described as double adaptations. The capacity of evolving antagonistic characters is prominent in both. The antagonists are assumed to lie quietly together while inactive. But as soon as evolution calls them into activity they become mutually exclusive, because only one of them can come to full display in the same organ. External influences decide which of the two becomes dominant and which remains dormant. This decision must take place separately [643] for each stem and each branch, but as a rule, the stronger ages are more liable to furnish anomalies than the weaker.
Exactly the same thing is true of double adaptations. Every bud of the water-persicaria may develop either into an erect or into a floating stem, according as it is surrounded by water or by relatively dry soil. In other cases utility is often less manifest, but some use may either be proved, or shown to be very probable. At all events the term adaptation includes the idea of utility, and obviously useless contrivances could hardly be brought under the same head.
We have also dealt with the question of heredity. It is obvious that from the flowers of the floating and erect stems of the water-persicaria seeds will result, each capable of yielding both forms. Quite the same thing was the case with the teasels. Some 40% of the progeny produce beautifully twisted stems, but whether the seed was saved from the most completely twisted specimens or from the straight plants of the race was of no importance.
This phenomenon of twisting may now be considered from quite another point of view. It is a case of systematic atavism, or of the reacquirement of some ancient and long-lost quality. This quality is the alternate position of [644] the leaves, which has been replaced in the teasel family by a grouping in pairs. In order to prove the validity of this assertion, it will be necessary to discuss two points separately, viz.: relative positions of the leaves, and the manner in which the alternate position causes the stems to become twisted.
Leaves are affixed to their stems and branches in various ways. Among them one is of wide occurrence throughout the whole realm of the higher plants, while all the others are more rare. Moreover these subordinate arrangements are, as a rule, confined to definite systematic groups. Such groups may be large, as for instance, the monocotyledons, that have their leaves arranged in two opposite rows in many families, or small, as genera or subdivisions of genera. Apart from these special cases the main stem and the greater part of the branches of the pedigree of the higher plants exhibit a spiral condition or a screw arrangement, all leaves being inserted at different points and on different sides of the stem. This condition is assumed to be the original one, from which the more specialized types have been derived. As is usual with characters in general, it is seen to vary around an average, the spiral becoming narrower and looser. A narrow spiral condenses the leaves, while a [645] loose one disperses them. According to such fluctuating deviations the number of leaves, inserted upon a given number of spiral circuits, is different in different species. In a vast majority of cases 13 leaves are found on 5 circuits, and as we have only to deal with this proportion in the teasels we will not consider others.
In the teasels this screw-arrangement has disappeared, and has been replaced by a decussate grouping. The leaves are combined into pairs, each pair occupying the opposite sides of one node. The succeeding pairs alternate with one another, so as to place their leaves at right angles. The leaves are thus arranged on the whole stem in four equidistant rows.
On the normal stem of a teasel the two members of a pair are tied to one another in a comparatively complicated way. The leaves are broadly sessile and their bases are united so as to constitute a sort of cup. The margins of these cups are bent upward, thereby enabling them to hold water, and after a rainfall they may be seen filled to the brim. It is believed that these little reservoirs are useful to the plant during the flowering period, because they keep the ants away from the honey. Considering the internal structure of the stem at the base of these cups we find that the vascular bundles of the two opposite leaves are strongly connected [646] with one another, constituting a ring which narrowly surrounds the stem, and which would impede an increase in thickness, if such were in the nature of the plant. But since the stems end their existence during the summer of their development, this structure is of no real harm.
The grouping of the leaves in alternate pairs may be seen within the bud as well as on the adult stems. In order to do this, it is necessary to make transverse sections through the heart of the rosette of the leaves of the first year. If cut through the base, the pair exhibit connate wings, corresponding to the water-cups; if cut above these, the leaves seem to be free from one another.
In order to compare the position of leaves of the twisted plants with this normal arrangement, the best way is to make a corresponding section through the heart of the rosette of the first year. It is not necessary to make a microscopic preparation. In the fall the changed disposition may at once be seen to affect the central leaves of the group. All the rosettes of the whole race commence with opposite leaves; those that are to produce straight stems remain in this condition, but the preparation for twisting begins at the end of the first year as shown by a special arrangement of the leaves. This [647] disposition may then be seen to extend to the very center of the rosette, by use of microscopical sections. Examining sections made in the spring, the original arrangement of the leaves of the stem is observed to continue until the beginning of the growth of the shoot. It is easy to estimate the number of leaves corresponding to a given number of spiral circuits in these sections and the proportion is found to indicate 13 leaves on 5 turns. These figures are the same as those given above for the ordinary arrangement of alternate leaves in the main lines of the pedigree of the vegetable kingdom.
Leaving aside for the moment the subsequent changes of this spiral arrangement, it becomes at once clear that here we have a case of systematic atavism. The twisted teasels lose their decussation, but in doing so the leaves are not left in a disorderly dispersion, but a distinct new arrangement takes its place, which is to be assumed as the normal one for the ancestors of the teasel family. The case is to be considered as one of atavism. Obviously no other explanation is possible, than the supposition that the 5-13 spiral is still latent, though not displayed by the teasels. But in the very moment when the faculty of decussation disappears, it resumes its place, and becomes [648] as prominent as it must once have been in the ancestors, and is still in that part of their offspring, which has not become changed in this respect. Thus the proof of our assertion of systematic atavism is, in this case, not obtained by the inspection of the adult, but by the investigation of the conditions in an early stage. It remains to be explained how the twisting may finally be caused by this incipient grouping of the leaves. Before doing so, it may be as well to state that the case of the teasel is not an isolated one, and that the same conclusions are supported by the valerian, and a large number of other examples. In early spring some rosettes show a special condition of the leaves, indicating thereby at once their atavism and their tendency to become twisted as soon as they begin to expand. The Sweet William or Dianthus barbatus affords another instance; it is very interesting because a twisted race is available, which may produce thousands of instances developed in all imaginable degrees, in a single lot of plants. Viscaria oculata is another instance belonging to the same family.
The bedstraw (Galium) also includes many species which from time to time produce twisted stems. I have found them myself in Holland on Galium verum and G. Aparine. Both seem [649] to be of rare occurrence, as I have not succeeded in getting any repetition by prolonged culture.
Species, which generally bear their leaves in whorls, are also subjected to casual atavisms of this kind, as for instance the tall European horsetail, Equisetum Telmateja, which occasionally bears cones on its green summer stems. Its whorls are changed on the twisted parts into clearly visible spirals. The ironwood or Casuarina quadrivalvis is sometimes observed to produce the same anomaly on its smaller lateral branches.
Coming now to the discussion of the way in which the twisting is the result of the spiral disposition of the leaves, we may consider this arrangement on stems in the adult state. These at once show the spiral line and it is easy to follow this line from the base up to the apex. In the most marked cases it continues without interruption, not rarely however, ending in a whorl of three leaves and a subsequent straight internode, of which there may even be two or three. The spiral exhibits the basal parts of the leaves, with the axillary lateral branches. The direction of the screw is opposed to that of the twisting, and the spiral ribs are seen to cross the line of insertion of the leaves at nearly right angles. On this line the leaves are nearer [650] to one another than would correspond to the original proportion of 5 turns for 13 leaves. In fact, 10 or even 13 leaves may not rarely be counted on a single turn. Or the twist may become so strong locally as to change the spiral into a longitudinal line. On this line all inserted leaves extend themselves in the same direction, resembling an extended flag.
The spiral on the stem is simply the continuation of the spiral line from within the rosettes of the first year. Accordingly it is seen to become gradually less steep at the base. For this reason it must be one and the same with this line, and in extreme youth it must have produced its leaves at the same mutual distances as this line. Transverse sections of the growing summits of the stems support this conclusion.
From these several facts we may infer that the steepness of the spiral line increases on the stem, as it is gradually changed into a screw. Originally 5 turns were needed for 13 leaves, but this number diminishes and 4 or 3 or even 2 turns may take the same number of foliar organs, until the screw itself is changed into a straight line.
This change consists in an unwinding of the whole spiral, and in order to effect this the stem must become wound up in the opposite direction. The winding of the foliar screw must [651] curve the longitudinal ribs. The straighter and steeper the screw becomes, the more the ribs will become twisted. That this happens in the opposite direction is obvious, without further discussion. The twisting is the inevitable consequence of the reversal of the screw.
Two points remain to be dealt with. One is the direct proof of the reversal of the screw, the other the discussion of its cause. The first may be observed by a simple experiment. Of course it proceeds only slowly, but all that is necessary is to mark the position of one of the younger leaves of a growing stem of a twisting individual and to observe the change in its position in a few hours. It will be seen to have turned some way around the stem, and finally may be seen to make a complete revolution in the direction opposite to the screw, and thereby demonstrating the fact of its uncurling.
The cause of this phenomenon is to be sought in the intimate connection of the basal parts of the leaves, which we have detailed above. The fibrovascular strands constitute a strong rope, which is twisted around the stem along the line on which the leaves are inserted. The strengthening of the internodes may stretch this rope to some extent, but it is too strong to be rent asunder. Hence it opposes the normal growth, and the only manner in which the internodes [652] may adjust themselves to the forces which tend to cause their expansion is by straightening the rope. In doing so they may find the required space, by growing out in an unusual direction, bending their axes and twisting the ribs.
To prove the validity of this explanation, a simple experiment may be given. If the fibrovascular rope is the mechanical impediment which hinders the normal growth, we may try the effect of cutting through this rope. By this means the hindrance may at least locally be removed. Now, of course, the operation must be made in an early stage before, or at the beginning of the period of growth, in every case before the uncurling of the rope begins. Wounds made at this time are apt to give rise to malformations, but notwithstanding this difficulty I have succeeded in giving the necessary proof. Stems operated upon become straight where the rope is cut through, though above and under the wounded part they go on twisting in the usual way.
Sometimes the plants themselves succeed in tearing the rope asunder, and long straight internodes divide the twisted stems in two or more parts in a very striking manner. A line of torn leaf-bases connects the two parts of the screw and gives testimony of what has passed within [653] the tissues. At other times the straightening may have taken place directly internal to a leaf, and it is torn and may be seen to be attached to the stem by two distinct bases.
Summing up this description of the hereditary qualities of our twisted teasels and of their mechanical consequences, we may say that the loss of the normal decussation is the cause of all the observed changes. This special adaptation, which places the leaves in alternating pairs, replaced and concealed the old and universal arrangement on a screw line. In disappearing, it leaves the latter free, and according to the rule of systematic atavism, this now becomes active and takes its place. If the fibrovascular connection of the leaf-bases were lost at the same time the stems would grow and become straight and tall. This change however, does not occur, and the bases of the leaves now constitute a continuous rope instead of separate rings, and thereby impede the stretching of the internodes. These in their turn avoid the difficulty by twisting themselves in a direction opposite to that of the spiral of the leaves.
As a last example of systematic atavism I will refer to the reversionary changes, afforded by the tomatoes. Though the culture of this plant is a recent one, it seems to be at present in a state of mutability, producing new strains, or [654] assuming the features of their presumable ancestors. In his work “The Survival of the Unlike,” Bailey has given a detailed description of these various types. Moreover, he has closely studied the causes of the changes, and shown the great tendency of the tomatoes to vicinism. By far the larger part of the observed cases of running out of varieties are caused by accidental crosses through the agency of insects. Even improvements are not rarely due to this cause. Besides these common and often unavoidable changes, others of greater importance occur from time to time. Two of them deserve to be mentioned. They are called the “Upright” and the “Mikado” types, and differ as much or even more from their parents than the latter do from any one of their wild congeners. Their characters come true from seed. The “Mikado” race or the Lycopersicum grandifolium (L. latifolium) has larger and fewer leaflets than the slender and somewhat flimsy foliage of the common form. Flat or plane blades with decurrent margins constitute another character. This variety, however, does not concern our present discussion. The upright type has stiff and self-sustaining stems and branches, resembling rather a potato-plant than a tomato. Hence the name Lycopersicum solanopsis or L. validum, under which it is usually described. [655] The foliage of the plant is so distinct as to yield botanical characters of sufficient importance to justify this specific designation. The leaflets are reduced in numbers and greatly modified, and the flowers in the inflorescence are reduced to two or three. This curious race came in suddenly, without any premonition, and the locality and date of its mutation are still on record. Until some years ago it had not made its appearance for a second time. Obviously it is to be considered as a reversionary form. The limp stems of the common tomatoes are in all respects indicative of the cultivated condition. They cannot hold themselves erect, but must be tied up to supports. The color of the leaves is a paler green than should be expected from a wild plant. Considering other species of the genus Solanum, of which the Lycopersicum is a subdivision, the stems are as a rule erect and self-supporting, with some few exceptions. These, however, are special adaptations as shown by the winding stems of the bitter-sweet.
From this discussion we seem justified in concluding that the original appearance of the upright type was of the nature of systematic atavism. It differs however, from the already detailed cases in that it is not a monstrosity, nor an ever-sporting race, but is as constant a form [656] as the best variety or species. Even on this ground it must be considered as a representative of a separate group of instances of the universal rule of systematic reversions.
Of late the same mutation has occurred in the garden of C.A. White at Washington. The parent form in this case was the “Acme,” of the ordinary weak and spreading habit of growth. It is known as one of the best and most stable of the varieties and was grown by Mr. White for many years, and had not given any sign of a tendency towards change. Seeds from some of the best plants in 1899 were sown the following spring, and the young seedlings unexpectedly exhibited a marked difference from their parents. From the very outset they were more strong and erect, more compact and of a darker green than the “Acme.” When they reached the fruiting stage they had developed into typical representatives of the Lycopersicum solanopsis or upright division. The whole lot of plants comprised only some 30 specimens, and this number, of course, is too small to base far-reaching conclusions upon. But all of the lot showed this type, no true “Acme” being seen among them. The fruit differed in flavor, consistency and color from that of the parent, and it also ripened earlier than the latter. No seed was saved from [657] these plants, but the following year the “Acme” was sown again and found true to its type. Seeds saved from this generation in 1900 have, however, repeated the mutation, giving rise to exactly the same new upright form in 1901. This was called by its originator “The Washington.” Seeds from this second mutation were kindly sent to me by Mr. White, and proved true to their type when sown in my garden.
Obviously it is to be assumed in the case of the tomatoes as well as in instances from other genera cited, that characters of ancestors, which are not displayed in their progeny, have not been entirely lost, but are still present, though in a latent condition. They may resume their activity unexpectedly, and at once develop all the features which they formerly had borne.
Latency, from this point of view, must be one of the most common things in nature. All organisms are to be considered as internally formed of a host of units, partly active and partly inactive. Extremely minute and almost inconceivably numerous, these units must have their material representatives within the most intimate parts of the cells.
[658]
LECTURE XXIII
TAXONOMIC ANOMALIES
The theory of descent is founded mainly on comparative studies, which have the advantage of affording a broad base and the convincing effect of concurrent evidence brought together from widely different sources. The theory of mutation on the other hand rests directly upon experimental investigations, and facts concerning the actual descent of one form from another are as yet exceedingly rare. It is always difficult to estimate the validity of conclusions drawn from isolated instances selected from the whole range of contingent phenomena, and this is especially true of the present case. Systematic and physiologic facts seem to indicate the existence of universal laws, and it is not probable that the process of production of new species would be different in the various parts of the animal and vegetable kingdoms. Moreover the principle of unit-characters, the preeminent significance of which has come to be more fully recognized of late, is in full harmony [659] with the theory of sudden mutations. Together these two conceptions go to strengthen the probability of the sudden origin of all specific characters.
Experimental researches are limited in their extent, and the number of cases of direct observation of the process of mutation will probably never become large enough to cover the whole field of the theory of descent. Therefore it will always be necessary to show that the similarity between observed and other cases is such as to lift above all doubt the assertion of their resulting from the same causes.
Besides the direct comparison of the mutations described in our former lectures, with the analogous cases of the horticultural and natural production of species and varieties at large, another way is open to obtain the required proof. It is the study of the phenomena, designated by Casimir de Candolle by the name of taxonomic anomalies. It is the assertion that characters, which are specific in one case, may be observed to arise as anomalies or as varieties in other instances. If they can be shown to be identical or nearly so in both, it is obviously allowable to assume the same origin for the specific character and for the anomaly. In other terms, the specific marks may be considered as having originated according to the laws [660] that govern the production of anomalies, and we may assume them to lie within reach of our experiments. The experimental treatment of the origin of species may also be looked upon as a method within our grasp.
The validity and the significance of these considerations will at once become clear, if we choose a definite example. The broadest and most convincing one appears to me to be afforded by the cohesion of the petals in gamopetalous flowers. According to the current views the families with the petals of their flowers united are regarded as one or two main branches of the whole pedigree of the vegetable kingdom. Eichler and others assume them to constitute one branch, and therefore one large subdivision of the system. Bessey, on the other hand, has shown the probability of a separate origin for those groups which have inferior ovaries. Apart from such divergencies the connation of the petals is universally recognized as one of the most important systematic characters.
How may this character have originated? The heath-family or the Ericaceae and their nearest allies are usually considered to be the lowest of the gamopetalous plants. In them the cohesion of the petals is still subject to reversionary exceptions. Such cases of atavism may [661] be observed either as specific marks, or in the way of anomalies. Ledum, Monotropa and Pyrola, or the Labrador tea, the Indian pipe and wintergreen are instances of reversionary gamopetalism with free petals. In heaths (Erica Tetralix) and in rhododendrons the same deviation is observed to occur from time to time as an anomaly, and even the common Rhododendron ponticum of our gardens has a variety in which the corolla is more or less split. Sometimes it exhibits five free petals, while at other times only one or two are entirely free, the remaining four being incompletely loosened.
Such cases of atavism make it probable that the coherence of the petals has originally arisen by the same method, but by action in the opposite direction. The direct proof of this conclusion is afforded by a curious observation, made by Vilmorin upon the bright and large-flowered garden-poppy, Papaver bracteatum. Like all poppies it has four petals, which are free from one another. In the fields of Messrs. Vilmorin, where it is largely cultivated for its seeds, individuals occur from time to time which are anomalous in this respect. They exhibit a tendency to produce connate petals. Their flowers become monopetalous, and the whole strain is designated by the name of Papaver [662] bracteatum monopetalum. Henry de Vilmorin had the kindness to send me some of these plants, and they have flowered in my garden during several years. The anomaly is highly variable. Some flowers are quite normal, exhibiting no sign of connation; others are wholly gamopetalous, the four petals being united from their base to the very margin of the cup formed. In consequence of the broadness of the petals however, this cup is so wide as to be very shallow.
Intermediate states occur, and not infrequently. Sometimes only two or three petals are united, or the connation does not extend the entire length of the petals. These cases are quite analogous to the imperfect splitting of the corolla of the rhododendron. Giving free rein to our imagination, we may for a moment assume the possibility of a new subdivision of the vegetable kingdom, arising from Vilmorin’s poppy and having gamopetalous flowers for its chief character. If the character became fixed, so as to lose its present state of variability, such a group of supposititious gamopetalous plants might be quite analogous to the corresponding real gamopetalous families. Hence there can be no objection to the view, that the heaths have arisen in an analogous manner from their polypetalous ancestors. Other species of [663] the same genus have also been recorded to produce gamopetalous flowers, as for instance, Papaver hybridum, by Hoffmann. Poppies are not the sole example of accidental gamopetaly. Linnaeus observed the same deviation long ago for Saponaria officinalis, and since, it has been seen in Clematis Vitalba by Jaeger, in Peltaria alliacea by Schimper, in Silene annulata by Boreau and in other instances. No doubt it is not at all of rare occurrence, and the origin of the present gamopetalous families is to be considered as nothing extraordinary. It is, as a matter of fact, remarkable that it has not taken place in more numerous instances, and the mallows show that such opportunities have been available at least more than once.
Other instances of taxonomic anomalies are afforded by leaves. Many genera, the species of which mainly bear pinnate or palmate leaves, have stray types with undivided leaves. Among the brambles, Rubus odoratus and R. flexuosus may be cited, among the aralias, Aralia crassifolia and A. papyrifera, and among the jasmines, the deliciously scented sambac (Jasminum Sambac). But the most curious instance is that of the telegraph-plant, or Desmodium gyrans, each complete leaf of which consists of a large terminal leaflet and two little lateral ones. These latter keep up, [664] night and day, an irregular jerking movement, which has been compared to the movements of a semaphore. Desmodium is a papilionaceous plant and closely allied to the genus Hedysarum, which has pinnate leaves with numerous pairs of leaflets. Its place in the system leaves no doubt concerning its origin from pinnate-leaved ancestors. At the time of its origination its leaves must have become reduced as to the number of the blades, while the size of the terminal leaflet was correspondingly increased.
It might seem difficult to imagine this great change taking place suddenly. However, we are compelled to familiarize ourselves with such hypothetical assumptions. Strange as they may seem to those who are accustomed to the conception of continuous slow improvements, they are nevertheless in complete agreement with what really occurs. Fortunately the direct proof of this assertion can be given, and in a case which is narrowly related, and quite parallel to that of the Desmodium, since it affects a plant of the same family. It is the case of the monophyllous variety of the bastard-acacia or Robinia Pseud-Acacia. In a previous lecture we have seen that it originated suddenly in a French nursery in the year 1855. It can be propagated by seed, and exhibits a curious degree [665] of variability of its leaves. In some instances these are one-bladed, the blade reaching a length of 15 cm., and hardly resembling those of the common bastard-acacia. Other leaves produce one or two small leaflets at the base of the large terminal one, and by this contrivance are seen to be very similar to those of the Desmodium, repeating its chief characters nearly exactly, and only differing somewhat in the relative size of the various parts. Lastly real intermediates are seen between the monophyllous and the pinnate types. As far as I have been able to ascertain, these are produced on weak twigs under unfavorable conditions; the size of the terminal leaflet decreases and the number of the lateral blades increases, showing thereby the presence of the original pinnate type in a latent condition.
The sudden origin of this “one-leaved” acacia in a nursery may be taken as a prototype of the ancient origin of Desmodium. Of course the comparison only relates to a single character, and the movements of the leaflets are not affected by it. But the monophylly, or rather the size of the terminal blade and the reduction of the lateral ones, may be held to be sufficiently illustrated by the bastard-acacia. It is worth while to state, that analogous varieties have also arisen in other genera. The “one-leaved” [666] strawberry has already been referred to. It originated from the ordinary type in Norway and at Paris. The walnut likewise, has its monophyllous variety. It was mentioned for the first time as a cultivated tree about 1864, but its origin is unknown. A similar variety of the walnut, with “one-bladed” leaves but of varying shapes, was found wild in a forest near Dieppe in France some years ago, and appeared to be due to a sudden mutation.
Something more is known concerning the “one-bladed” ashes, varieties of which are often seen in our parks and gardens. The common form has broad and deeply serrate leaves, which are far more rounded than the leaflets of the ordinary ash. The majority of the leaves are simple, but some produce one or two smaller leaflets at their base, closely corresponding in this respect to the variations of the “one-bladed” bastard-acacia, and evidently indicating the same latent and atavistic character. In some instances this analogy goes still further, and incompletely pinnate leaves are produced with two or more pairs of leaflets. Besides this variable type another has been described by Willdenow. It has single leaves exclusively, never producing smaller lateral leaflets, and it is said to be absolutely constant from seed, while the more variable types [667] seem to be also more inconstant when propagated sexually. The difference is so striking and affords such a reliable feature that Koch proposed to make two distinct varieties of them, calling the pure type Fraxinus excelsior monophylla, and the varying trees F. excelsior exheterophylla. Some writers, and among them Willdenow, have preferred to separate the “one-leaved” forms from the species, and to call them Fraxinus simplicifolia.
According to Smith and to Loudon, the “one-leaved” ashes are found wild in different districts in-England. Intermediate forms have not been recorded from these localities. This mode of origin is that already detailed for the laciniate varieties of alders and so many other trees. Hence it may be assumed that the “one-leaved” ashes have sprung suddenly but frequently from the original pinnate species. The pure type of Willdenow should, in this case, be considered as due to a slightly different mutation, perhaps as a pure retrograde variety, while the varying strains may only be eversporting forms. This would likewise explain part of their observed inconstancy.
In this respect the historic dates, as collected by Korshinsky, are not very convincing. Vicinism has of course, almost never been excluded, and part of the multiformity of the offspring [668] must obviously be due to this most universal agency. Indirect vicinism also plays some part, and probably affords the explanation of some reputed mutative productions of the variety. So, for instance, in the case of Sinning, who after sowing the seeds of the common ash, got as large a proportion as 2% of monophyllous trees in a culture of some thousand plants. It is probable that his seeds were taken partly from normal plants, and partly from hybrids between the normal and the “one-bladed” type, assuming that these hybrids have pinnate leaves like their specific parent, and bear the characters of the other parent only in a latent condition.
Our third example relates to peltate leaves. They have the stalk inserted in the middle of the blade, a contrivance produced by the connation of the two basal lobes. The water-lilies are a well known instance, exhibiting sagittate leaves in the juvenile stage and changing in many species, into nearly circular peltate forms, of which Victoria regia is a very good example, although its younger stages do not always excite all the interest they deserve. The Indian cress (Tropaeolum), the marsh pennywort or Hydrocotyle, and many other instances could be quoted. Sometimes the peltate leaves are not at all orbicular, but are elongated, oblong or elliptic, and with only the lobes [669] at the base united. The lemon-scented Eucalyptus citriodora is one of the most widely known cases. In other instances the peltate leaves become more or less hollow, constituting broad ascidia as in the case of the crassulaceous genus Umbilicus.
This connation of the basal lobes is universally considered as a good and normal specific character. Nevertheless it has its manifest analogy in the realm of the anomalies. This is the pitcher or ascidium. On some trees it is of quite common occurrence, as on the lime-tree (Tilia parvifolia) and the magnolia (Magnolia obovata and its hybrids). It is probable that both these forms have varieties with, and others without, ascidia. Of the lime-tree, instances are known of single trees which produce hundreds of such anomalous leaves yearly, and one such a tree is growing in the neighborhood of Amsterdam at Lage Vuursche. I have alluded to these cases more than once, but on this occasion a closer inspection of the structure of the ascidium is required. For this purpose we may take the lime-tree as an example. Take the shape of the normal leaves in the first place. These are cordate at their base and mainly inequilateral, but the general shape varies to a considerable extent. This variation is closely related to the position of the leaves on the twigs, and shows [670] distinct indications of complying with the general law of periodicity. The first leaves are smaller, with more rounded lobes, the subsequent leaves attain a larger size, and their lobes slightly change their forms. In the first leaves the lobes are so broad as to touch one another along a large part of their margins, but in organs formed later this contact gradually diminishes and the typical leaves have the lobes widely separated. Now it is easily understood that the contact or the separation of the lobes must play a part in the construction of the ascidia, as soon as the margins grow together. Leaves which touch one-another, may be affected by the connation without any further malformation. They remain flat, become peltate and exhibit a shape which in some way holds a middle position between the pennyworts and the lemon-scented eucalyptus. Here we have the repetition of the specific characters of these plants by the anomaly of another. Whenever the margins are not in contact, and become connate, notwithstanding their separation, the blade must be folded together in some slight degree, in order to produce the required contact. This is the origin of the ascidium. It is quite superfluous to insist upon the fact that their width or narrowness must depend upon the corresponding normal form. The more distant the [671] lobes, the deeper the ascidium will become. It should be added that this explanation of the different shapes of ascidia is of general validity.
Ascidia of the snake-plantain or Plantago lanceolata are narrow tubes, because the leaves are oblong or lanceolate, while those of the broad leaved species of arrowhead, as for instance, the Sagittaria japonica, are of a conical shape.
From the evidence of the lime-tree we may conclude that normal peltate leaves may have originated in the same way. And from the fact that pitchers are one of the most frequent anomalies, we may conclude that the chance of producing peltate leaves must have been a very great one, and wholly sufficient to account for all observed cases. In every instance the previously existing shape of the leaf must have decided whether peltate or pitcher-like leaves would be formed. As far as we can judge peltate anomalies are quite uninjurious, while ascidia are forms which must impede the effect of the light on the leaf, as they conceal quite an important part of the upper surface. In this way it is easily conceivable that peltate leaves are a frequent specific character, while ascidia are not, as they only appear in the special cases of limited adaptation, as in the instances of the so called pitcher-plants. The genera Nepenthes, [672] Sarracenia and some others are very well known and perhaps even the bladderworts or Utricularia might be included here.
The reproduction of specific characters by anomalous ascidia is not at all limited to the general case as described above. More minute details may be seen to be duplicated in the same way. Proofs are afforded on one side by incomplete ascidia, and on the other by the double cups.
Incomplete ascidia are those of the Nepenthes. The leaf is divided into three parts, a blade, a tendril and the pitcher. Or in other words, the limb produces a tendril at its summit, by means of which the plant is enabled to fasten itself to surrounding shrubs and to climb between their branches. But the end of this tendril bears a well-formed urn, which however, is produced only after the revolving and grasping movements of the tendril have been made. Some species have more rounded and some more elongated ascidia and often the shape is seen to change with the development of the stem. The mouth of the urn is strengthened by a thick rim and covered with a lid. Numerous curious contrivances in these structures to catch ants and other insects have been described, but as they have no relation to our present discussion, we shall abstain from dealing with them. [673] Likewise we must refrain from a consideration of the physiologic qualities of the tendril, and confine our attention to the combination of a limb, a naked midvein and an ascidium. This combination is to be the basis of our discussion. It is liable to be produced all of a sudden. This assertion is proved by its occurrence as a varietal mark in one of our most ordinary cultivated plants. It is the group known as Croton, belonging to the genus Codiaeum. A variety is called interruptum and another appendiculatum, and these names both relate to the interruption of the leaves by a naked midvein. The leaves are seen to be built up of three parts. The lower half retains the aspect of a limb; it is crowned by a vein without lateral nerves or blade-like expansions, and this stalk in its turn bears a short limb on its summit. The base of this apical limb exhibits two connate lobes, forming together a wide cup or ascidium. It should be stated that these interruptum varieties are highly variable, especially in the relative size of the three principal parts of the leaf. Though it is of course conceded that the ascidium of Nepenthes has many secondary devices which are lacking in Croton, it seems hardly allowable to deny the possibility of an analogous origin for both. Those of the Croton, according to our knowledge regarding similar cases, must [674] have arisen at once, and hence the conclusion that the ascidia of Nepenthes are also originally due to a sudden mutation. Interrupted leaves, with an ascidium on a naked prolongation of the midvein, are by no means limited to the Croton varieties. As stray anomalies they have often been observed, and I myself had the opportunity of collecting them on magnolia, on clover and on some other species. They are additional evidence in support of the explanation given above.
In the same way double ascidia may be made use of to explain the foliar cups of the teasels and some other plants, as for instance, some European snakeroots (Eryngium maritimum and E. campestre), or the floral leaves of the honeysuckle. The leaves on the stems of the teasels are disposed in pairs, and the bases of the two leaves of each pair are connate so as to constitute large cups. We have already mentioned these cups, and recall them in the present connection to use them as a prototype of the double ascidia. These are constituted of two opposite leaves, accidentally connated at their base or along some part of their margins. If the leaves are sessile, the analogy with the teasels is complete, as shown, for instance, in a case of Cotyledon, a crassulaceous plant which is [675] known to produce such cups from time to time. They are narrower than those of the teasel, but this depends, as we have seen for the “one-leaved” ascidium, on the shape of the original leaf. In other respects they exactly imitate the teasel cups showing thereby how these cups may probably have originated.
In numerous cases of anomalies some accidental structures are parallel to specific characters, while others are not, being obviously injurious to their bearers. So it is also with the double ascidia. In the case of stalked leaves the two opposite stalks must, of course, constitute a long and very narrow tube, when growing together. This tube must bear at its summit the conical ascidium produced by the two connate limbs. At its base however, it includes the terminal bud of the stem, and frequently the tube is so narrow as to impede its further development. By this contrivance the double ascidium assumes a terminal position. Instances have been observed on magnolia, in Boehmeria and in other cases.
Flowers on leaves are of rare occurrence. Notwithstanding this, they constitute specific characters in some instances, accidental anomalies in others. Helwingia rusciflora flora is the most curious and best known instance. It is a little shrub, belonging to the Cornaceae, and [676] has broad elliptical undivided leaves. On the middle of the midvein these leaves are seen to bear small clusters of flowers; indeed this is the only place where flowers are produced. Each cluster has from 13-15 flowers, of which some are staminate and borne on stalks, while others are pistillate and nearly sessile. These flowers are small and of a pale greenish color and yield small stone-fruits, with a thin coating of pulpy tissue. As the name indicates, this mode of flowering is closely similar to that of Ruscus, which however, does not bear its flowers and berries on real leaves, but on leaflike expansions of the twigs. Phyllonoma ruscifolia, a saxifragaceous plant, bears the same specific name, indicating a similar origin of the flowers. Other instances have been collected by Casimir de Candolle, but their number is very small.
As a varietal mark, flowers on leaves likewise rarely occur. One instance however, is very remarkable, and we have already dealt with it, when treating of constant varieties, and of the lack of vicinism in the case of species with exclusive self-fertilization.
It is the “Nepaul-barley” or Hordeum trifurcatum. The leaves, which in this case bear the adventitious flowers, are the inner scales of the spikelets, and not on green leaves as in the [677] cases already alluded to. But this of course makes no real difference. The character is variable to a high degree, and this fact indicates its varietal nature, though it should be recalled that at least with the Helwingia, the majority of the leaves are destitute of flowers, and that in this way some degree of variability is present in this normal case too.
All in all there are three sorts of “Nepaul-barley.” They have the same varietal mark, but belong to different species of barley. These are differentiated according to the number of the rows in which the grains are seen on the spikes. These numbers may be two, four or six, giving rise to the specific names of Hordeum distichum, tetrastichum and hexastichum. Whether these three varieties are of independent, but parallel origin, or are to be considered as due to a single mutation and subsequent crosses is not known, all of them being of ancient origin. Historic evidence concerning their birth is wholly wanting. From analogy it would seem probable that the character had arisen by a mutation in one of the three named species, and had been transferred to others by means of accidental crosses, even as it has been artificially transmitted of late to quite a number of other sorts. But however admissible this conception may seem, there is of course no real objection [678] to the assumption of independent and parallel mutations.
For the purpose of a comparison with the Helwingia type we are however, not at all concerned with the species to which the trifurcatum variety belongs, but only with the varietal mark itself. The spikelets may be one-, two- or three-flowered, according to the species. If we choose for further consideration the hexastichum type, each spikelet produces three normal flowers and afterwards three normal grains. Morphologically however, the spikelet is not homologous to those parts of other grasses which have the same name. It is constituted of three real spikelets, and thus deserves the name of a triple construction. Each of these three little organs has its normal pair of outer scales or glumae. These are linear and short, ending in a long and narrow spine. Those of the middle-most spikelets stand on its outer side, while those of the lateral part are placed transversely. In this way they form a kind of involucre around the central parts. The latter consist of the inner and outer palets or scales, each two of which include one of the flowers. The outer palet is to be considered as the metamorphosed leaf, in the aril of which the flower is produced. In the common sorts of barley it bears a long awn, giving thereby its typical aspect to the [679] whole spike. The axillary flower is protected on the opposite side by a two-keeled inner palet. Each flower exhibits three stamens and an ovary. In the six-rowed barley all the three flowers of a triple spikelet are fertile, and each of them has a long awn on the top of the outer palet. But in the two-rowed species only the middle-most flower is normal and has an awn, the two remaining being sterile and more or less rudimentary and with only very short awns. From this description it is easily seen that the species of barley may be distinguished from one another, even at a casual glance, by the number of the rows of the awns, and therefore by the shape of the entire spikes. This striking feature, however, does not exist in the “Nepaul-barley.” The awns are replaced by curiously shaped appendices, which are three-lobed. The central lobe is oblong and hollow, and forms a kind of hood, which covers a small supernumerary floret. The two lateral lobes are narrower, often linear and extended into a smaller or longer awn. These awns are mostly turned away from the center of the spike. The central lobe may sometimes bear two small florets, but ordinarily only one is to be found, and this is often incomplete, having only one or two stamens, or is different in some other way. [680] These narrow lateral lobes heighten the abnormal aspect of the whole spike.
They are only produced at a somewhat advanced stage of the development of the palet, are united to one another and to the central part by strong veins, which form transversal anastomoses at their insertion. The length of these awns is very variable, and this quality is perhaps the most striking of the whole variety. Often they reach only 1-2 mm., or the majority may become longer and attain even 1 cm., while here and there, between them, longer ones are inserted, extending in some instances even as far as 3 cm. from the spike. Their transverse position in such cases is strikingly contrasted with the ordinary erect type of the awns.
These lateral lobes are to be regarded, from the morphologic point of view, as differentiated parts of the blade of the leaf. Before they are formed, or coincidently with the beginning of their development, the summit of the central lobe becomes hollow, and the development of the supernumerary flower commences. In different varieties, and especially in the most recent crosses of them, this development is excessively variable.
The accidental flower arises at some distance beneath the summit of the scale, on its middle [681] vein. The development begins with the protrusion of a little scale, and the flower itself is situated beneath this scale, and is to be protected by it and by the primary scale, but is turned upside down at the same time. Opposite to this organ, which represents the outer palet of the adventitious flower, two little swollen bodies are evolved. In the normal flowers of barley and other grains and grasses their function is to open the flowers by swelling, and afterwards collapse and allow them to close.
In the adventitious flowers of the “Nepaul-barley,” however, this function is quite superfluous. The stamens occur in varying numbers; typically there are three, but not rarely less, or more, are seen. In some instances the complete double whorl of six, corresponding to the ancestral monocotyledonous type, has been found. This is a very curious case of systematic atavism, quite analogous to the Iris pallida abavia, previously alluded to, which likewise has six stamens, and to the cases given in a previous lecture. But for our present discussion it is of no further interest. The ovary is situated in the middle of the flower, and in some instances two have been observed. This is also to be considered as a case of atavism.
All these parts of the adventitious flower are more or less subject to arrest of development, [682] in a later stage. They may even sometimes become abnormal. Stamens may unite into pairs, or carpels bear four stigmas. The pollen-sacs are as a rule barren, the mother-cells undergoing atrophy, while normal grains are seen but rarely. Likewise the ovaries are rudimentary, but Wittmack has observed the occasional production of ripe grains from these abnormal florets.
The scale is seldom seen to extend any farther upwards than the supernumerary flower. But in the rare instances where it does prolong its growth, it may repeat the abnormality and bear a second floret above the first. This of course is generally much weaker, and more rudimentary.
Raciborsky, who has lately given a full and very accurate description of this anomaly, lays great stress upon the fact that it is quite useless. It is perhaps the most obviously useless structure in the whole vegetable kingdom. Notwithstanding this, it has come to be as completely hereditary as any of the most beautiful adaptations in nature. Therefore it is one of the most serious objections to the hypothesis of slow and gradual improvements on the sole ground of their usefulness. The struggle for life and natural selection are manifestly inadequate to give even the slightest indication of [683] an explanation of this case. It is simply impossible to imagine the causes that might have produced such a character. The only way out of this difficulty is to assume that it has arisen at once, in its present apparently differentiated and very variable condition, and that, being quite uninjurious and since it does not decrease the fertility of the race, it has never been subjected to natural selection, and so has saved itself from destruction.
But if we once grant the probability of the origin of the “Nepaul-barley” by a sudden mutation, we obviously must assume the same in the case of the Helwingia and other normal instances. In this way we gain a further support for our assertion, that even the strangest specific characters may have arisen suddenly.
After having detailed at some length those proofs which seem to be the most striking, and which had not been previously described with sufficient detail, we may now take a hasty survey of other contingent cases. In the first place the cruciate flowers of some onagraceous plants should be remembered. Small linear petals occur as a specific character in Oenothera cruciata of the Adirondacks, but have been seen to arise as sudden mutations in the common evening-primrose (O. biennis) in Holland, and in the willow-herb (Epilobium hirsutum) in England. [684] Leaves placed in whorls of three are very rare. The oleander, juniper and some few other plants have ternate whorls as a specific character. As an anomaly, ternate whorls are far more common, and perhaps any plant with opposite leaves may from time to time produce them. Races rich in this abnormality are found in the wild state in the yellow loosestrife or Lysimachia vulgaris, in which it is a very variable specific character, the whorls varying from two to four leaves. In the cultivated state it is met with in the myrtle or Myrtus communis, where it has come to be of some importance in Israelitic ritual. Crisped leaves are known in a mallow, Malva crispa, and as a variety in cabbages, parsley, lettuce and others. The orbicular fruits of Heeger’s shepherd’s purse (Capsella heegeri) recall similar fruits of other cruciferous genera, as for instance, Camelina. Screw-like stems with wide spirals are specific in the flower-stalks of Cyclamen and Vallisneria, varietal in Juncus effusus spiralis and accidental in Scirpus lacustris. Dormant buds or small bulbs in inflorescences are normal for wild onions, Polygonum viviparum and others, varietal in Poa alpina vivipara and perhaps in Agave vivipara, and accidental in plantains (Plantago lanceolata), Saxifraga umbrosa and others. [685] Cleft leaves, one of the most general anomalies, are typical in Boehmeria biloba. The adnation of the peduncles of the inflorescences to the stem is typical in Solanum and accidental in many other cases.
It seems quite superfluous to add further proof. It is a very general phenomenon that specific characters occur in other genera as anomalies, and under such circumstances that the idea of a slow evolution on the ground of utility is absolutely excluded. No other explanation remains than that of a sudden mutation, and once granted for the abnormal cases, this explanation must obviously likewise be granted for the analogous specific characters.
Our whole discussion shows that mutations, once observed in definite instances, afford the most probable basis for the explanation of specific characters at large.
[686]
LECTURE XXIV
THE HYPOTHESIS OF PERIODIC MUTATIONS
The prevailing belief that slow and gradual, nearly invisible changes constitute the process of evolution in the animal and vegetable kingdom, did not offer a strong stimulus for experimental research. No appreciable response to any external agency was of course to be expected. Responses were supposed to be produced, but the corresponding outward changes would be too small to betray themselves to the investigator.
The direct observation of the mutations of the evening-primrose has changed the whole aspect of the problem at once. It is no longer a matter dealing with purely hypothetical conditions. Instead of the vague notions, uncertain hopes, and a priori conceptions, that have hitherto confused the investigator, methods of observation have been formulated, suitable for the attainment of definite results, the general nature of which is already known.
To my mind the real value of the discovery [687] of the mutability of the evening-primrose lies in its usefulness as a guide for further work. The view that it might be an isolated case, lying outside of the usual procedure of nature, can hardly be sustained. On such a supposition it would be far too rare to be disclosed by the investigation of a small number of plants from a limited area. Its appearance within the limited field of inquiry of a single man would have been almost a miracle.
The assumption seems justified that analogous cases will be met with, perhaps even in larger numbers, when similar methods of observation are used in the investigation of plants of other regions. The mutable condition may not be predicated of the evening-primroses alone. It must be a universal phenomenon, although affecting a small proportion of the inhabitants of any region at one time: perhaps not more than one in a hundred species, or perhaps not more than one in a thousand, or even fewer may be expected to exhibit it. The exact proportion is immaterial, because the number of mutable instances among the many thousands of species in existence must be far too large for all of them to be submitted to close scrutiny.
It is evident from the above discussion that next in importance to the discovery of the prototype of mutation is the formulation of methods [688] for bringing additional instances to light. These methods may direct effort toward two different modes of investigation. We may search for mutable plants in nature, or we may hope to induce species to become mutable by artificial methods. The first promises to yield results most quickly, but the scope of the second is much greater and it may yield results of far more importance. Indeed, if it should once become possible to bring plants to mutate at our will and perhaps even in arbitrarily chosen directions, there is no limit to the power we may finally hope to gain over nature.
What is to guide us in this new line of work? Is it the minute inspection of the features of the process in the case of the evening-primroses? Or are we to base our hopes and our methods on broader conceptions of nature’s laws? Is it the systematic study of species and varieties, and the biologic inquiry into their real hereditary units? Or is the theory of descent to be our starting-point? Are we to rest our conceptions on the experience of the breeder, or is perhaps the geologic pedigree of all organic life to open to us better prospects of success?
The answer to all such questions is a very simple one. All possibilities must be considered, and no line of investigation ignored. For myself I have based my field-researches and my [689] testing of native plants on the hypothesis of unit-characters as deduced from Darwin’s Pangenesis. This conception led to the expectation of two different kinds of variability, one slow and one sudden. The sudden ones known at the time were considered as sports, and seemed limited to retrograde changes, or to cases of minor importance. The idea that sudden steps might be taken as the principal method of evolution could be derived from the hypothesis of unit characters, but the evidence might be too remote for a starting point for experimental investigation.
The success of my test has given proof to the contrary. Hence the assertion that no evidence is to be considered as inadequate for the purpose under discussion. Sometime a method of discovering, or of producing, mutable plants may be found, but until this is done, all facts of whatever nature or direction must be made use of. A very slight indication may change forever the whole aspect of the problem.
The probabilities are now greatly in favor of our finding out the causes of evolution by a close scrutiny of what really happens in nature. A persistent study of the physiologic factors of this evolution is the chief condition of success. To this study field-observations may contribute as well as direct experiments, [690] microscopical investigations as well as extended pedigree-cultures. The cooperation of many workers is required to cover the field. Somewhere no doubt the desired principle lies hidden, but until it is discovered, all methods must be tried.
With this conception as the best starting point for further investigation, we may now make a brief survey of the other phase of the problem. We shall try to connect our observations on the evening-primroses with the theory of descent at large.
We start with two main facts. One is the mutability of Lamarck’s primrose, and the second is the immutable condition of quite a number of other species. Among them are some of its near allies, the common and the small flowered evening-primrose, or Oenothera biennis and O. muricata.
From these facts, a very important question arises in connection with the theory of descent. Is the mutability of our evening-primroses temporary, or is it a permanent condition? A discussion of this problem will give us the means of reaching a definite idea as to the scope of our inquiries.
Let us consider the present first. If mutability is a permanent condition, it has of course no beginning, and moreover is not due to the [691] agency of external circumstances. Should this be granted for the evening-primrose, it would have to be predicated for other species found in a mutable state. Then, of course, it would be useless to investigate the causes of mutability at large, and we should have to limit ourselves to the testing of large numbers of plants in order to ascertain which are mutable and which not.
If, on the other hand, mutability is not a permanent feature, it must once have had a beginning, and this beginning itself must have had an external cause. The amount of mutability and its possible directions may be assumed to be due to internal causes. The determination of the moment at which they will become active can never be the result of internal causes. It must be assigned to some external factor, and as soon as this is discovered the way for experimental investigation is open.
In the second place we must consider the past. On the supposition of permanency all the ancestors of the evening-primrose must have been mutable. By the alternative view mutability must have been a periodic phenomenon, producing at times new qualities, and at other times leaving the plants unchanged during long successions of generations. The present mutable state must then have been preceded by an immutable [692] condition, but of course thousands of mutations must have been required to produce the evening-primroses from their most remote ancestors.
If we take the species into consideration that are not mutable at present, we may ask how we are to harmonize them with each of the two theories proposed. If mutability is permanent, it is manifest that the whole pedigree of the animal and vegetable kingdom is to be considered as built up of main mutable lines, and that the thousands of constant species can only be taken to represent lateral branches of the genealogic tree.
These lateral branches would have lost the capacity of mutating, possessed by all their ancestors. And as the principle of the hypothesis under discussion does not allow a resumption of this habit, they would be doomed to eternal constancy until they finally die out. Loss of mutability, under this conception, means loss of the capacity for all further development. Only those lines of the main pedigree which have retained this capacity would have a future; all others would die out without any chance of progression.
If, on the other hand, mutability is not permanent, but a periodic condition, all lines of the genealogic tree must be assumed to show alternatively [693] mutating and constant species. Some lines may be mutating at the present moment; others may momentarily be constant. The mutating lines will probably sooner or later revert to the inactive state, while the powers of development now dormant may then become awakened on other branches.
The view of permanency represents life as being surrounded with unavoidable death, the principle of periodicity, on the contrary, follows the idea of resurrection, granting the possibility of future progression for all living beings. At the same time it yields a more hopeful prospect for experimental inquiry.
Experience must decide between the two main theories. It demonstrates the existence of polymorphous genera, such as Draba and Viola and hundreds of others. They clearly indicate a previous state of mutability. Their systematic relation is exactly what would be expected, if they were the result of such a period. Perhaps mutability has not wholly ceased in them, but, might be found to survive in some of their members. Such very rich genera however, are not the rule, but are exceptional cases, indicating the rarity of powerful mutative changes.
On the other hand, species may remain in a state of constancy during long, apparently during indefinite, ages.
[694] Many facts plead in favor of the constancy of species. This principle has always been recognized by systematists. Temporarily the current form of the theory of natural selection has assumed species to be inconstant, ever changing and continuously being improved and adapted to the requirements of the life-conditions. The followers of the theory of descent believed that this conclusion was unavoidable, and were induced to deny the manifest fact that species are constant entities. The mutation theory gives a clew to the final combination of the two contending ideas. Reducing the changeability of the species to distinct and probably short periods, it at once explains how the stability of species perfectly agrees with the principle of descent through modification.
On the other hand, the hypothesis of mutative periods is by no means irreconcilable with the observed facts of constancy. Such casual changes can be proved by observations such as those upon the evening-primrose, but it is obvious that a disproof can never be given. The principle grants the present constancy of the vast majority of living forms, and only claims the exceptional occurrence of definite changes.
Proofs of the constancy of species have been given in different ways. The high degree of similarity of the individuals of most of our [695] species has never been denied. It is observed throughout extended localities, and during long series of years. Other proofs are afforded by those plants which have been transported to distant localities some time since, but do not exhibit any change as a result of this migration. Widely dispersed plants remain the same throughout their range, provided that they belong to a single elementary species. Many species have been introduced from America into Europe and have spread rapidly and widely. The Canadian horsetail (Erigeron canadensis), the evening-primrose and many other instances could be given. They have not developed any special European features after their introduction. Though exposed to other environmental conditions and to competition with other species, they have not succeeded in developing a new character. Such species as proved adequate to the new environment have succeeded, while those which did not have succumbed.
Much farther back is the separation of the species which now live both in arctic regions and on the summits of our highest mountaintops. If we compare the alpine flora with the arctic plants, a high degree of similarity at once strikes us. Some forms are quite identical; others are slightly different, manifestly representing elementary species of the same systematic [696] type. Still others are more distant or even belong to different genera. The latter, and even the diverging, though nearly allied, elementary species, do not yield adequate evidence in any direction.
They may as well have lived together in the long ages before the separation of the now widely distant floras, or have sprung from a common ancestor living at that time, and subsequently have changed their habits. After excluding these unreliable instances, a good number of species remain, which are quite the same in the arctic and alpine regions and on the summits of distant mountain ranges. As no transportation over such large distances can have brought them from one locality to the other, no other explanation is left than that they have been wholly constant and unchanged ever since the glacial period which separated them. Obviously they must have been subjected to widely changing conditions. The fact of their stability through all these outward changes is the best proof that the ordinary external conditions do not necessarily have an influence on specific evolution. They may have such a result in some instances, in others they obviously have not. Many arctic forms bearing the specific name of alpinus justify this conclusion. Astragalus alpinus, Phleum alpinum, Hieracium alpinum and [697] others from the northern parts of Norway may be cited as examples.
Thus Primula imperialis has been found in the Himalayas, and many other plants of the high mountains of Java, Ceylon and northern India are identical forms. Some species from the Cameroons and from Abyssinia have been found on the mountains of Madagascar. Some peculiar Australian types are represented on the summit of Kini Balu in Borneo. None of these species, of course, are found in the intervening lowlands, and the only possible explanation of their identity is the conception of a common post-glacial origin, coupled with complete stability. This stability is all the more remarkable as nearly allied but slightly divergent forms have also been reported from almost all of these localities. Other evidence is obtained by the comparison of ancient plants with their living representatives. The remains in tombs of ancient Egypt have always afforded strong support of the views of the adherents of the theory of stability, and to my mind they still do so. The cereals and fruits and even the flowers and leaves in the funeral wreaths of Rameses and Amen-Hotep are the same that are still now cultivated in Egypt. Nearly a hundred or more species have been identified. Flowers of Acacia, leaves of Mimusops, [698] petals of Nymphaea may be cited as instances, and they are as perfectly preserved as the best herbarium-specimens of the present time. The petals and stamens retain their original colors, displaying them as brightly as is consistent with their dry state.
Paleontologic evidence points to the same conclusion. Of course the remains are incomplete, and rarely adequate for a close comparison. The range of fluctuating variability should be examined first, but the test of elementary species given by their constancy from seed cannot, of course, be applied. Apart from these difficulties, paleontologists agree in recognizing the very great age of large numbers of species. It would require a too close survey of geologic facts to go into details on this point. Suffice it to say that in more recent Tertiary deposits many species have been identified with living forms. In the Miocene period especially, the similarity of the types of phanerogamic plants with their present offspring, becomes so striking that in a large number of cases specific distinctions rest in greater part on theoretical conceptions rather than on real facts. For a long time the idea prevailed that the same species could not have existed through more than one geologic period. Many distinctions founded on this belief have since had to be abandoned. [699] Species of algae belonging to the well-preserved group of the diatoms, are said to have remained unchanged from the Carboniferous period up to the present time.
Summing up the results of this very hasty survey, we may assert that species remain unchanged for indefinite periods, while at times they are in the alternative condition. Then at once they produce new forms often in large numbers, giving rise to swarms of subspecies. All facts point to the conclusion that these periods of stability and mutability alternate more or less regularly with one another. Of course a direct proof of this view cannot, as yet, be given, but this conclusion is forced upon us by a consideration of known facts bearing on the principle of constancy and evolution.
If we are right in this general conception, we may ask further, what is to be the exact place of our group of new evening-primroses in this theory? In order to give an adequate answer, we must consider the whole range of the observations from a broader point of view. First of all it is evident that the real mutating period must be assumed to be much longer than the time covered by my observations. Neither the beginning nor the end have been seen. It is quite obvious that Oenothera lamarckiana was in a mutating condition when I first [700] saw it, seventeen years ago. How long had it been so? Had it commenced to mutate after its introduction into Europe, some time ago, or was it already previously in this state? It is as yet impossible to decide this point. Perhaps the mutable state is very old, and dates from the time of the first importation of the species into Europe.
Apart from all such considerations the period of the direct observations, and the possible duration of the mutability through even more than a century, would constitute only a moment, if compared with the whole geologic time. Starting from this conception the pedigree of our mutations must be considered as only one small group. Instead of figuring a fan of mutants for each year, we must condense all the succeeding swarms into one single fan, as might be done also for Draba verna and other polymorphous species. In Oenothera the main stem is prolonged upwards beyond the fan; in the others the main stem is lacking or at least undiscernable, but this feature manifestly is only of secondary importance. We might even prefer the image of a fan, adjusted laterally to a stem, which itself is not interrupted by this branch.
On this principle two further considerations are to be discussed. First the structure of the [701] fan itself, and secondly the combination of succeeding fans into a common genealogic tree.
The composition of the fan as a whole includes more than is directly indicated by the facts concerning the birth of new species. They arise in considerable quantities, and each of them in large numbers of individuals, either in the same or in succeeding years. This multiple origin must obviously have the effect of strengthening the new types, and of heightening their chances in the struggle for life. Arising in a single specimen they would have little chance of success, since in the field among thousands of seeds perhaps one only survives and attains complete development. Thousands or at least hundreds of mutated seeds are thus required to produce one mutated individual, and then, how small are its chances of surviving! The mutations proceed in all directions, as I have pointed out in a former lecture. Some are useful, others might become so if the circumstances were accidentally changed in definite directions, or if a migration from the original locality might take place. Many others are without any real worth, or even injurious. Harmless or even slightly useless ones have been seen to maintain themselves in the field during the seventeen years of my research, as proved by Oenothera laevifolia and Oenothera [702] brevistylis. Most of the others quickly disappear.
This failure of a large part of the productions of nature deserves to be considered at some length. It may be elevated to a principle, and may be made use of to explain many difficult points of the theory of descent. If, in order to secure one good novelty, nature must produce ten or twenty or perhaps more bad ones at the same time, the possibility of improvements coming by pure chance must be granted at once. All hypotheses concerning the direct causes of adaptation at once become superfluous, and the great principle enunciated by Darwin once more reigns supreme.
In this way too, the mutation-period of the evening-primrose is to be considered as a prototype. Assuming it as such provisionally, it may aid us in arranging the facts of descent so as to allow of a deeper insight and a closer scrutiny. All swarms of elementary species are the remains of far larger initial groups. All species containing only a few subspecies may be supposed to have thrown off at the outset far more numerous lateral branches, out of which however, the greater part have been lost, being unfit for the surrounding conditions. It is the principle of the struggle for life between elementary species, followed by the survival of the [703] fittest, the law of the selection of species, which we have already laid stress upon more than once.
Our second consideration is also based upon the frequent repetition of the several mutations. Obviously a common cause must prevail. The faculty of producing nanella or lata remains the same through all the years. This faculty must be one and the same for all the hundreds of mutative productions of the same form. When and how did it originate? At the outset it must have been produced in a latent condition, and even yet it must be assumed to be continuously present in this state, and only to become active at distant intervals. But it is manifest that the original production of the characters of Oenothera gigas was a phenomenon of far greater importance than the subsequent accidental transition of this quality into the active state. Hence the conclusion that at the beginning of each series of analogous mutations there must have been one greater and more intrinsic mutation, which opened the possibility to all its successors. This was the origination of the new character itself, and it is easily seen that this incipient change is to be considered as the real one. All others are only its visible expressions.
Considering the mutative period of our evening-primrose [704] as one unit-stride section in the great genealogic tree, this period includes two nearly related, but not identical changes. One is the production of new specific characters in the latent condition, and the other is the bringing of them to light and putting them into active existence. These two main factors are consequently to be assumed in all hypothetic conceptions of previous mutative periods.
Are all mutations to be considered as limited to such periods? Of course not. Stray mutations may occur as well. Our knowledge concerning this point is inadequate for any definite statement. Swarms of variable species are easily recognized, if the remnants are not too few. But if only one or two new species have survived, how can we tell whether they have originated-alone or together with others. This difficulty is still more pronounced in regard to paleontologic facts, as the remains of geologic swarms are often found, but the absence of numerous mutations can hardly be proved in any case.
I have more than once found occasion to lay stress on the importance of a distinction between progressive and retrograde mutations in previous lectures. All improvement is, of course, by the first of these modes of evolution, but apparent losses of organs or qualities are [705] perhaps of still more universal occurrence. Progression and regression are seen to go hand in hand everywhere. No large group and probably even no genus or large species has been evolved without the joint agency of these two great principles. In the mutation-period of the evening-primroses the observed facts give direct support to this conclusion, since some of the new species proved, on closer inspection, to be retrograde varieties, while others manifestly owe their origin to progressive steps. Such steps may be small and in a wrong direction; notwithstanding this they may be due to the acquisition of a wholly new character and therefore belong to the process of progression at large.
Between them however, there is a definite contrast, which possibly is in intimate connection with the question of periodic and stray mutations. Obviously each progressive change is dependent upon the production of a new character, for whenever this is lacking, no such mutation is possible. Retrograde changes, on the other hand, do not require such elaborate preliminary work. Each character may be converted into the latent condition, and for all we know, a special preparation for this purpose is not at all necessary. It is readily granted that such special preparation may occur, because the [706] great numbers in which our dwarf variety of the Oenothera are yearly produced are suggestive of such a condition. On the other hand, the laevifolia and brevistylis mutations have not been repeated, at least not in a visible way.
From this discussion we may infer that it is quite possible that a large part of the progressive changes, and a smaller part of the retrograde mutations, are combined into groups, owing their origin to common external agencies. The periods in which such groups occur would constitute the mutative periods. Besides them the majority of the retrograde changes and some progressive steps might occur separately, each being due to some special cause. Degressive mutations, or those which arise by the return of latent qualities to activity, would of course belong with the latter group.
This assumption of a stray and isolated production of varieties is to a large degree supported by experience in horticulture. Here there are no real swarms of mutations. Sudden leaps in variability are not rare, but then they are due to hybridization. Apart from this mixture of characters, varieties as a rule appear separately, often with intervals of dozens of years, and without the least suggestion of a common cause. It is quite superfluous to go into details, as we have dealt with the horticultural [707] mutations at sufficient length on a previous occasion. Only the instance of the peloric toadflax might be recalled here, because the historic and geographic evidence, combined with the results of our pedigree-experiment, plainly show that peloric mutations are quite independent of any periodic condition. They may occur anywhere in the wide range of the toad-flax, and the capacity of repeatedly producing them has lasted some centuries at least, and is perhaps even as old as the species itself.
Leaving aside such stray mutations, we may now consider the probable constitution of the great lines of the genealogic tree of the evening primroses, and of the whole vegetable and animal kingdom at large. The idea of drawing up a pedigree for the chief groups of living organisms is originally due to Haeckel, who used this graphic method to support the Darwinian theory of descent. Of course, Haeckel’s genealogic trees are of a purely hypothetic nature, and have no other purpose than to convey a clear conception of the notion of descent, and of the great lines of evolution at large. Obviously all details are subject to doubt, and many have accordingly been changed by his successors. These changes may be considered as partial improvements, and the somewhat picturesque form of Haeckel’s pedigree might well be replaced by [708] more simple plans. But the changes have by no means removed the doubts, nor have they been able to supplant the general impression of distinct groups, united by broad lines. This feature is very essential, and it is easily seen to correspond with the conception of swarms, as we have deduced it from the study of the lesser groups.
Genealogic trees are the result of comparative studies; they are far removed from the results of experimental inquiry concerning the origin of species. What are the links which bind them together? Obviously they must be sought in the mutative periods, which have immediately preceded the present one. In the case of the evening-primrose the systematic arrangement of the allied species readily guides us in the delimitations of such periods. For manifestly the species of the large genus of Oenothera are grouped in swarms, the youngest or most recent of which we have under observation. Its immediate predecessor must have been the subgenus Onagra, which is considered by some authors as consisting of a single systematic species, Oenothera biennis. Its multifarious forms point to a common origin, not only morphologically but also historically. Following this line backward or downward we reach another apparent mutation-period, which includes the origin of [709] the group called Oenothera, with a large number of species of the same general type as the Onagra-forms, Still farther downward comes the old genus Oenothera itself, with numerous subgenera diverging in sundry characters and directions.
Proceeding still farther we might easily construct a main stem with numerous succeeding fans of lateral branches, and thus reach, from our new empirical point of view, the theoretical conclusion already formulated.
Paleontologic facts readily agree with this conception. The swarms of species and varieties are found to succeed one another like so many stories. The same images are repeated, and the single stories seem to be connected by the main stems, which in each tier produce the whole number of allied forms. Only a few prevailing lines are prolonged through numerous geologic periods; the vast majority of the lateral branches are limited each to its own storey. It is simply the extension of the pedigree of the evening-primroses backward through ages, with the same construction and the same leading features. There can be no doubt that we are quite justified in assuming that evolution has followed the same general laws through the whole duration of life on earth. Only a moment of their lifetime is disclosed to us, but it [710] is quite sufficient to enable us to discern the laws and to conjecture the outlines of the whole scheme of evolution.
A grave objection which has, often, and from the very outset, been urged against Darwin’s conception of very slow and nearly imperceptible changes, is the enormously long time required. If evolution does not proceed any faster than what we can see at present, and if the process must be assumed to have gone on in the same slow manner always, thousands of millions of years would have been needed to develop the higher types of animals and plants from their earliest ancestors.
Now it is not at all probable that the duration of life on earth includes such an incredibly long time. Quite on the contrary the lifetime of the earth seems to be limited to a few millions of years. The researches of Lord Kelvin and other eminent physicists seem to leave no doubt on this point. Of course all estimates of this kind are only vague and approximate, but for our present purposes they may be considered as sufficiently exact.
In a paper published in 1862 Sir William Thomson (now Lord Kelvin) first endeavored to show that great limitation had to be put upon the enormous demand for time made by Lyell, Darwin and other biologists. From a consideration [711] of the secular cooling of the earth, as deduced from the increasing temperature in deep mines, he concluded that the entire age of the earth must have been more than twenty and less than forty millions of years, and probably much nearer twenty than forty. His views have been much criticised by other physicists, but in the main they have gained an ever-increasing support in the way of evidence. New mines of greater depth have been bored, and their temperatures have proved that the figures of Lord Kelvin are strikingly near the truth. George Darwin has calculated that the separation of the moon from the earth must have taken place some fifty-six millions of years ago. Geikie has estimated the existence of the solid crust of the earth at the most as a hundred million years. The first appearance of the crust must soon have been succeeded by the formation of the seas, and a long time does not seem to have been required to cool the seas to such a degree that life became possible. It is very probable that life originally commenced in the great seas, and that the forms which are now usually included in the plankton or floating-life included the very first living beings. According to Brooks, life must have existed in this floating condition during long primeval epochs, and evolved nearly all the main branches of the animal and vegetable kingdom [712] before sinking to the bottom of the sea, and later producing the vast number of diverse forms which now adorn the sea and land.
All these evolutions, however, must have been very rapid, especially at the beginning, and together cannot have taken more time than the figures given above.
The agency of the larger streams, and the deposits which they bring into the seas, afford further evidence. The amount of dissolved salts, especially of sodium chloride, has been made the subject of a calculation by Joly, and the amount of lime has been estimated by Eugene Dubois. Joly found fifty-five and Dubois thirty-six millions of years as the probable duration of the age of the rivers, and both figures correspond to the above dates as closely as might be expected from the discussion of evidence so very incomplete and limited.
All in all it seems evident that the duration of life does not comply with the demands of the conception of very slow and continuous evolution. Now it is easily seen, that the idea of successive mutations is quite independent of this difficulty. Even assuming that some thousands of characters must have been acquired in order to produce the higher animals and plants of the present time, no valid objection is raised. The demands of the biologists and the results of [713] the physicists are harmonized on the ground of the theory of mutation.
The steps may be surmised to have never been essentially larger than in the mutations now going on under our eyes, and some thousands of them may be estimated as sufficient to account for the entire organization of the higher forms. Granting between twenty and forty millions of years since the beginning of life, the intervals between two successive mutations may have been centuries and even thousands of years. As yet there has been no objection cited against this assumption, and hence we see that the lack of harmony between the demands of biologists and the results of the physicists disappears in the light of the theory of mutation.
Summing up the results of this discussion, we may justifiably assert that the conclusions derived from the observations and experiments made with evening-primroses and other plants in the main agree satisfactorily with the inferences drawn from paleontologic, geologic and systematic evidence. Obviously these experiments are wonderfully supported by the whole of our knowledge concerning evolution. For this reason the laws discovered in the experimental garden may be considered of great importance, and they may guide us in our further inquiries. Without doubt many minor [714] points are in need of correction and elaboration, but such improvements of our knowledge will gradually increase our means of discovering new instances and, new proofs.
The conception of mutation periods producing swarms of species from time to time, among which only a few have a chance of survival, promises to become the basis for speculative pedigree-diagrams, as well as for experimental investigations.
[715]
LECTURE XXV
GENERAL LAWS OF FLUCTUATION
The principle of unit-characters and of elementary species leads at once to the recognition of two kinds of variability. The changes of wider amplitude consist of the acquisition of new units, or the loss of already existing ones. The lesser variations are due to the degree of activity of the units themselves.
Facts illustrative of these distinctions were almost wholly lacking at the time of the first publication of Darwin’s theories. It was a bold conception to point out the necessity for such distinction on purely theoretical grounds. Of course some sports were well known and fluctuations were evident, but no exact analysis of the details was possible, a fact that was of great importance in the demonstration of the theory of descent. The lack of more definite knowledge upon this matter was keenly felt by Darwin, [716] and exercised much influence upon his views at various times.
Quetelet’s famous discovery of the law of fluctuating variability changed the entire situation and cleared up many difficulties. While a clear conception of fluctuations was thus gained, mutations were excluded from consideration, being considered as very rare, or non-existent. They seemed wholly superfluous for the theory of descent, and very little importance was attached to their study. Current scientific belief in the matter has changed only in recent years. Mendel’s law of varietal hybrids is based upon the principle of unit-characters, and the validity of this conception has thus been brought home to many investigators.
A study of fluctuating or individual variability, as it was formerly called, is now carried on chiefly by mathematical methods. It is not my purpose to go into details, as it would require a separate course of lectures. I shall consider the limits between fluctuation and mutation only, and attempt to set forth an adequate idea of the principles of the first as far as they touch these limits. The mathematical treatment of the facts is no doubt of very great value, but the violent discussions now going on between mathematicians such as Pearson, Kapteyn and others should warn biologists to abstain [717] from the use of methods which are not necessary for the furtherance of experimental work.
Fortunately, Quetelet’s law is a very clear and simple one, and quite sufficient for our considerations. It claims that for biologic phenomena the deviations from the average comply with the same laws as the deviations from the average in any other case, if ruled by chance only. The meaning of this assertion will become clear by a further discussion of the facts. First of all, fluctuating variability is an almost universal phenomenon. Every organ and every quality may exhibit it. Some are very variable, while others seem quite constant. Shape and size vary almost indefinitely, and the chemical composition is subject to the same law, as is well known for the amount of sugar in sugar-beets. Numbers are of course less liable to changes, but the numbers of the rays of umbels, or ray-florets in the composites, of pairs of blades in pinnate leaves, and even of stamens and carpels are known to be often exceedingly variable. The smaller numbers however, are more constant, and deviations from the quinate structure of flowers are rare. Complicated structures are generally capable of only slight deviations.
From a broad point of view, fluctuating variability [718] falls under two heads. They obey quite the same laws and are therefore easily confused, but with respect to questions of heredity they should be carefully separated. They are designated by the terms individual and partial fluctuation. Individual variability indicates the differences between individuals, while partial variability is limited to the deviations shown by the parts of one organism from the average structure. The same qualities in some cases vary individually and in others partially. Even stature, which is as markedly individual for annual and biennial plants as it is for man, becomes partially variant in the case of perennial herbs with numbers of stems. Often a character is only developed once in the whole course of evolution, as for instance, the degree of connation of the seed-leaves in tricotyls and in numerous cases it is impossible to tell whether a character is individual or partial. Consequently such minute details are generally considered to have no real importance for the hereditary transmission of the character under discussion.
Fluctuations are observed to take place only in two directions. The quality may increase or decrease, but is not seen to vary in any other way. This rule is now widely established by numerous investigations, and is fundamental to [719] the whole method of statistical investigation. It is equally important for the discussion of the contrast between fluctuations and mutations, and for the appreciation of their part in the general progress of organization. Mutations are going on in all directions, producing, if they are progressive, something quite new every time. Fluctuations are limited to increase and decrease of what is already available. They may produce plants with higher stems, more petals in the flowers, larger and more palatable fruits, but obviously the first petal and the first berry, cannot have originated by the simple increase of some older quality. Intermediates may be found, and they may mark the limit, but the demonstration of the absence of a limit is quite another question. It would require the two extremes to be shown to belong to one unit, complying with the simple law of Quetelet.
Nourishment is the potent factor of fluctuating variability. Of course in thousands of cases our knowledge is not sufficient to allow us to analyze this relation, and a number of phases of the phenomenon have been discovered only quite recently. But the fact itself is thoroughly manifest, and its appreciation is as old as horticultural science. Knight, who lived at the beginning of the last century, has laid great stress upon it, and it has since influenced practice in a [720] large measure. Moreover, Knight pointed out more than once that it is the amount of nourishment, not the quality of the various factors, that exercises the determinative influence. Nourishment is to be taken in the widest sense of the word, including all favorable and injurious elements. Light and temperature, soil and space, water and salts are equally active, and it is the harmonious cooperation of them all that rules growth.
We treated this important question at some length, when dealing with the anomalies of the opium-poppies, consisting of the conversion of stamens into supernumerary pistils. The dependency upon external influences which this change exhibited is quite the same as that shown by fluctuating variability at large. We inquired into the influence of good and bad soil, of sunlight and moisture and of other concurrent factors. Especial emphasis was laid upon the great differences to which the various individuals of the same lot may be exposed, if moisture and manure differ on different portions of the same bed in a way unavoidable even by the most careful preparation. Some seeds germinate on moist and rich spots, while their neighbors are impeded by local dryness, or by distance from manure. Some come to light on a sunny day, and increase their first leaves rapidly, while on [721] the following day the weather may be unfavorable and greatly retard growth. The individual differences seem to be due, at least in a very great measure, to such apparent trifles.
On the other hand partial differences are often manifestly due to similar causes. Considering the various stems of plants, which multiply themselves by runners or by buds on the roots, the assertion is in no need of further proof. The same holds good for all cases of artificial multiplication by cuttings, or by other vegetative methods. But even if we limit ourselves to the leaves of a single tree, or the branches of a shrub, or the flowers on a plant, the same rule prevails. The development of the leaves is dependent on their position, whether inserted on strong or weak branches, exposed to more or less light, or nourished by strong or weak roots. The vigor of the axillary buds and of the branches which they may produce is dependent upon the growth and activity of the leaves to which the buds are axillary.
This dependency on local nutrition leads to the general law of periodicity, which, broadly speaking, governs the occurrence of the fluctuating deviations of the organs. This law of periodicity involves the general principle that every axis, as a rule, increases in strength when [722] growing, but sooner or later reaches a maximum and may afterwards decrease.
This periodic augmentation and declination is often boldly manifest, though in other cases it may be hidden by the effect of alternate influences. Pinnate leaves generally have their lower blades smaller than the upper ones, the longest being seen sometimes near the apex and sometimes at a distance from it. Branches bearing their leaves in two rows often afford quite as obvious examples, and shoots in general comply with the same rule. Germinating plants are very easy of observation on this point. When they are very weak they produce only small leaves. But their strength gradually increases and the subsequent organs reach fuller dimensions until the maximum is attained. The phenomenon is so common that its importance is usually overlooked. It should be considered as only one instance of a rule, which holds good for all stems and all branches, and which is everywhere dependent on the relation of growth to nutrition.
The rule of periodicity not only affects the size of the organs, but also their number, whenever these are largely variable. Umbellate plants have numerous rays on the umbels of strong stems, but the number is seen to decrease and to become very small on the weakest lateral [723] branches. The same holds good for the number of ray-florets in the flower-heads of the composites, even for the number of stigmas on the ovaries of the poppies, which on weak branches may be reduced to as few as three or four. Many other instances could be given.
One of the best authenticated cases is the dependency of partial fluctuation on the season and on the weather. Flowers decline when the season comes to an end, become smaller and less brightly colored. The number of ray-florets in the flower-heads is seen to decrease towards the fall. Extremes become rarer, and often the deviations from the average seem nearly to disappear. Double flowers comply with this rule very closely, and many other cases will easily occur to any student of nature.
Of course, the relation to nourishment is different for individual and partial fluctuations. Concerning the first, the period of development of the germ within the seed is decisive. Even the sexual cells may be in widely different conditions at the moment of fusion, and perhaps this state of the sexual cells includes the whole matter of the decision for the average characters of the new individual. Partial fluctuation commences as soon as the leaves and buds begin to form, and all later changes in nutrition can only cause partial differences. All leaves, [724] buds, branches, and flowers must come under the influence of external conditions during the juvenile period, and so are liable to attain a development determined in part by the action of these factors.
Before leaving these general considerations, we must direct our attention to the question of utility. Obviously, fluctuating variability is a very useful contrivance, in many cases at least. It appears all the more so, as its relation to nutrition becomes manifest. Here two aspects are intimately combined. More nutrient matter produces larger leaves and these are in their turn more fit to profit by the abundance of nourishment. So it is with the number of flowers and flower-groups, and even with the numbers of their constituent organs. Better nourishment produces more of them, and thereby makes the plant adequate to make a fuller use of the available nutrient substances. Without fluctuation such an adjustment would hardly be possible, and from all our notions of usefulness in nature, we therefore must recognize the efficiency of this form of variability.
In other respects the fluctuations often strike us as quite useless or even as injurious. The numbers of stamens, or of carpels are dependent on nutrition, but their fluctuation is not known to have any attraction for the visiting insects.
[725] If the deviations become greater, they might even become detrimental. The flowers of the St. Johnswort, or Hypericum perforatum, usually have five petals, but the number varies from three to eight or more. Bees could hardly be misled by such deviations. The carpels of buttercups and columbines, the cells in the capsules of cotton and many other plants are variable in number. The number of seeds is thereby regulated in accordance with the available nourishment, but whether any other useful purpose is served, remains an open question. Variations in the honey-guides or in the pattern of color-designs might easily become injurious by deceiving insects, and such instances as the great variability of the spots on the corolla of some cultivated species of monkey-flowers, for instance, the Mimulus quinquevulnerus, could hardly be expected to occur in wild plants. For here the dark brown spots vary between nearly complete deficiency up to such predominancy as almost to hide the pale yellow ground-color.
After this hasty survey of the causes of fluctuating variability, we now come to a discussion of Quetelet’s law. It asserts that the deviations from the average obey the law of probability. They behave as if they were dependent on chance only.
Everyone knows that the law of Quetelet can [726] be demonstrated the most readily by placing a sufficient number of adult men in a row, arranging them according to their size. The line passing over their heads proves to be identical with that given by the law of probability. Quite in the same way, stems and branches, leaves and petals and even fruits can be arranged, and they will in the main exhibit the same line of variability. Such groups are very striking, and at the first glance show that the large majority of the specimens deviate from the mean only to a very small extent. Wider deviations are far more rare, and their number lessens, the greater the deviation, as is shown by the curvature of the line. It is almost straight and horizontal in the middle portion, while at the ends it rapidly declines, going sharply downward at one extreme and upward at the other.
It is obvious however, that in these groups the leaves and other organs could conveniently be replaced by simple lines, indicating their size. The result would be quite the same, and the lines could be placed at arbitrary, but equal distances. Or the sizes could be expressed by figures, the compliance of which with the general law could be demonstrated by simple methods of calculation. In this manner the variability of different organs can easily be compared. Another method of demonstration consists in [727] grouping the deviations into previously fixed divisions. For this purpose the variations are measured by standard units, and all the instances that fall between two limits are considered to constitute one group. Seeds and small fruits, berries and many other organs may conveniently be dealt with in this way. As an example we take ordinary beans and select them according to their size. This can be done in different ways. On a small piece of board a long wedge-shaped slit is made, into which seeds are pushed as far as possible. The margin of the wedge is calibrated in such a manner that the figures indicate the width of the wedge at the corresponding place. By this device the figure up to which a bean is pushed at once shows its length. Fractions of millimeters are neglected, and the beans, after having been measured, are thrown into cylindrical glasses of the same width, each glass receiving only beans of equal length. It is clear that by this method the height to which beans fill the glasses is approximately a measure of their number. If now the glasses are put in a row in the proper sequence, they at once exhibit the shape of a line which corresponds to the law of chance. In this case however, the line is drawn in a different manner from the first. It is to be pointed out that the glasses may be replaced by lines indicating [728] the height of their contents, and that, in order to reach a more easy and correct statement, the length of the lines may simply be made proportionate to the number of the beans in each glass. If such lines are erected on a common base and at equal distances, the line which unites their upper ends will be the expression of the fluctuating variability of the character under discussion.
The same inquiry may be made with other seeds, with fruits, or other organs. It is quite superfluous to arrange the objects themselves, and it is sufficient to arrange the figures indicating their value. In order to do this a basal line is divided into equal parts, the demarcations corresponding to the standard-units chosen for the test. The observed values are then written above this line, each finding its place between the two demarcations, which include its value. It is very interesting and stimulating to construct such a group. The first figures may fall here and there, but very soon the vertical rows on the middle part of the basal line begin to increase. Sometimes ten or twenty measurements will suffice to make the line of chance appear, but often indentations will remain. With the increasing number of the observations the irregularities gradually [729] disappear, and the line becomes smoother and more uniformly curved.
This method of arranging the figures directly on a basal line is very convenient, whenever observations are made in the field or garden. Very few instances need be recorded to obtain an appreciation of the mean value, and to show what may be expected from a continuance of the test. The method is so simple and so striking, and so wholly independent of any mathematical development that it should be applied in all cases in which it is desired to ascertain the average value of any organ, and the measure of the attendant deviations.
I cite an instance, secured by counting the ray-florets on the flower-heads of the corn-marigold or Chrysanthemum segetum. It was that, by which I was enabled to select the plant, which afterwards showed the first signs of a double head. I noted them in this way;
| 47 | ||||||
| 47 | 52 | |||||
| 41 | 54 | 68 | ||||
| 44 | 50 | 62 | 75 | |||
| 36 | 45 | 58 | 65 | 72 | 99 |
Of course the figures might be replaced in this work by equidistant dots or by lines, but experience teaches that the chance of making mistakes is noticeably lessened by writing down [730] the figures themselves. Whenever decimals are made use of it is obviously the best plan to keep the figures themselves. For afterwards it often becomes necessary to arrange them according to a somewhat different standard.
Uniting the heads of the vertical rows of figures by a line, the form corresponding to Quetelet’s law is easily seen. In the main it is always the same as the line shown by the measurements of beans and seeds. It proves a dense crowding of the single instances around the average, and on both sides of the mass of the observations, a few wide deviations. These become more rare in proportion to the amount of their divergency. On both sides of the average the line begins by falling very rapidly, but then bends slowly so as to assume a nearly horizontal direction. It reaches the basal line only beyond the extreme instances.
It is quite evident that all qualities, which can be expressed by figures, may be treated in this way. First, of all the organs occurring in varying numbers, as for instance the ray-florets of composites, the rays of umbels, the blades of pinnate and palmate leaves, the numbers of veins, etc., are easily shown to comply with the same general rule. Likewise the amount of chemical substances can be expressed in percentage numbers, as is done on a large [731] scale with sugar in beets and sugar-cane, with starch in potatoes and in other instances. These figures are also found to follow the same law.
All qualities which are seen to increase and to decrease may be dealt with in the same manner, if a standard unit for their measurement can be fixed. Even the colors of flowers may not escape our inquiry.
If we now compare the lines, compiled from the most divergent cases, they will be found to exhibit the same features in the main. Ordinarily the curve is symmetrical, the line sloping down on both sides after the same manner. But it is not at all rare that the inclination is steep on one side and gradual on the other. This is noticeably the case if the observations relate to numbers, the average of which is near zero. Here of course the allowance for variation is only small on one side, while it may increase with out distinct limits on the alternate slope. So it is for instance with the numbers of ray-florets in the example given on p. 729. Such divergent cases, however, are to be considered as exceptions to the rule, due to some unknown cause.
Heretofore we have discussed the empirical side of the problem only. For the purpose of experimental study of questions of heredity this is ordinarily quite sufficient. The inquiry [732] into the phenomenon of regression, or of the relation of the degree of deviation of the progeny to that of their parents, and the selection of extreme instances for multiplication are obviously independent of mathematical considerations. On the other hand an important inquiry lies in the statistical treatment of these phenomena, and such treatment requires the use of mathematical methods.
Statistics however, are not included in the object of these lectures, and therefore I shall refrain from an explanation of the method of their preparation and limit myself to a general comparison of the observed lines with the law of chance. Before going into the details, it should be repeated once more that the empirical result is quite the same for individual and for partial fluctuations. As a rule, the latter occur in far greater number, and are thus more easily investigated, but individual or personal averages have also been studied.
Newton discovered that the law of chance can be expressed by very simple mathematical calculations. Without going into details, we may at once state that these calculations are based upon his binomium. If the form (a + b) is calculated for some value of the exponent, and if the values of the coefficients after development are alone considered, they yield the basis [733] for the construction of what is called the line or curve of probability. For this construction the coefficients are used as ordinates, the length of which is to be made proportionate to their value. If this is done, and the ordinates are arranged at equal distances, the line which unites their summits is the desired curve. At first glance it exhibits a form quite analogous to the curves of fluctuating variability, obtained by the measurements of beans and in other instances. Both lines are symmetrical and slope rapidly down in the region of the average, while with increasing distance they gradually lose their steep inclination, becoming nearly parallel to the base at their termination.
This similarity between such empirical and theoretical lines is in itself an empirical fact. The causes of chance are assumed to be innumerable, and the whole calculation is based on this assumption. The causes of the fluctuations of biological phenomena have not as yet been critically examined to such an extent as to allow of definite conceptions. The term nourishment manifestly includes quite a number of separate factors, as light, space, temperature, moisture, the physical and chemical conditions of the soil and the changes of the weather. Without doubt the single factors are very numerous, but whether they are numerous enough to be treated [734] as innumerable, and thereby to explain the laws of fluctuations, remains uncertain. Of course the easiest way is to assume that they combine in the same manner as the causes of chance, and that this is the ground of the similarity of the curves. On the other hand, it is manifestly of the highest importance to inquire into the part the several factors play in the determination of the curves. It is not at all improbable that some of them have a larger influence on individual, and others on partial, fluctuations. If this were the case, their importance with respect to questions of heredity might be widely different. In the present state of our knowledge the fluctuation-curves do not contribute in any large measure to an elucidation of the causes. Where these are obvious, they are so without statistics, exactly as they were, previous to Quetelet’s discovery.
In behalf of a large number of questions concerning heredity and selection, it is very desirable to have a somewhat closer knowledge of these curves. Therefore I shall try to point out their more essential features, as far as this can be done without mathematical calculations.
At a first glance three points strike us, the average or the summit of the curve, and the extremes. If the general shape is once denoted by the results of observations or by the coefficients [735] of the binomium, all further details seem to depend upon them. In respect to the average this is no doubt the case; it is an empirical value without need of any further discussion. The more the number of the observations increases, the more assured and the more correct is this mean value, but generally it is the same for smaller and for larger groups of observations.
This however, is not the case with the extremes. It is quite evident that small groups have a chance of containing neither of them. The more the number of the observations increases, the larger is the chance of extremes. As a rule, and excluding exceptional cases, the extreme deviations will increase in proportion to the number of cases examined. In a hundred thousand beans the smallest one and the largest one may be expected to differ more widely from one another than in a few hundred beans of the same sample. Hence the conclusion that extremes are not a safe criterion for the discussion of the curves, and not at all adequate for calculations, which must be based upon more definite values.
A real standard is afforded by the steepness of the slope. This may be unequal on the two sides of one curve, and likewise it may differ for different cases. This steepness is usually measured by means of a point on the half curve and [736 ] for this purpose a point is chosen which lies exactly half way between the average and the extreme. Not however half way with respect to the amplitude of the extreme deviation, for on this ground it would partake of the uncertainty of the extreme itself. It is the point on the curve which is surpassed by half the number, and not reached by the other half of the number of the observations included in the half of the curve. This point corresponds to the important value called the probable error, and was designated by Galton as the quartile. For it is evident that the average and the two quartiles divide the whole of the observations into four equal parts.
Choosing the quartiles as the basis for calculations we are independent of all the secondary causes of error, which necessarily are inherent in the extremes. At a casual examination, or for demonstrative purposes, the extremes may be prominent, but for all further considerations the quartiles are the real values upon which to rest calculations.
Moreover if the agreement with the law of probability is once conceded, the whole curve is defined by the average and the quartiles, and the result of hundreds of measurements or countings may be summed up in three, or, in [737] the case of symmetrical curves, perhaps in two figures.
Also in comparing different curves with one another, the quartiles are of great importance. Whenever an empirical fluctuation-curve is to be compared with the theoretical form, or when two or more cases of variability are to be considered under one head, the lines are to be drawn on the same base. It is manifest that the averages must be brought upon the same ordinate, but as to the steepness of the line, much depends on the manner of plotting. Here we must remember that the mutual distance of the ordinates has been a wholly arbitrary one in all our previous considerations. And so it is, as long as only one curve is considered at a time. But as soon as two are to be compared, it is obvious that free choice is no longer allowed. The comparison must be made on a common basis, and to this effect the quartiles must be brought together. They are to lie on the same ordinates. If this is done, each division of the base corresponds to the same proportionate number of individuals, and a complete comparison is made possible.
On the ground of such a comparison we may thus assert that, fluctuations, however different the organs or qualities observed, are the same whenever their curves are seen to overlap one [738] another. Furthermore, whenever an empirical curve agrees in this manner with the theoretical one, the fluctuation complies with Quetelet’s law, and may be ascribed to quite ordinary and universal causes. But if it seems to diverge from this line, the cause of this divergence should be inquired into.
Such abnormal curves occur from time to time, but are rare. Unsymmetrical instances have already been alluded to, and seem to be quite frequent. Another deviation from the rule is the presence of more than one summit. This case falls under two headings. If the ray florets of a composite are counted, and the figures brought into a curve, a prominent summit usually corresponds to the average. But next to this, and on both sides, smaller summits are to be seen. On a close inspection these summits are observed to fall on the same ordinates, on which, in the case of allied species, the main apex lies. The specific character of one form is thus repeated as a secondary character on an allied species. Ludwig discovered that these secondary summits comply with the rule discovered by Braun and Schimper, stating the relation of the subsequent figures of the series. This series gives the terms of the disposition of leaves in general, and of the bracts and flowers on the composite flower [739] heads in our particular case. It is the series to which we have already alluded when dealing with the arrangement of the leaves on the twisted teasels. It commences with 1 and 2 and each following figure is equal to the sum of its two precedents. The most common figures are 3, 5, 8, 13, 18, 21, higher cases seldom coming under observation. Now the secondary summits of the ray-curves of the composites are seen to agree, as a rule, with these figures. Other instances could readily be given.
Our second heading includes those cases which exhibit two summits of equal or nearly equal height. Such cases occur when different races are mixed, each retaining its own average and its own curve-summit. We have already demonstrated such a case when dealing with the origin of our double corn-chrysanthemum. The wild species culminates with 13 rays, and the grandiflorum variety with 21. Often the latter is found to be impure, being mixed with the typical species to a varying extent. This is not easily ascertained by a casual inspection of the cultures, but the true condition will promptly betray itself, if curves are constructed. In this way curves may in many instances be made use of to discover mixed races. Double curves may also result from the investigation [740] of true double races, or ever-sporting varieties. The striped snapdragon shows a curve of its stripes with two summits, one corresponding to the average striped flowers, and the other to the pure red ones. Such cases may be discovered by means of curves, but the constituents cannot be separated by culture-experiments.
A curious peculiarity is afforded by half curves. The number of petals is often seen to vary only in one direction from what should be expected to be the mean condition. With buttercups and brambles and many others there is only an increase above the typical five; quaternate flowers are wanting or at least are very rare. With weigelias and many others the number of the tips of the corolla varies downwards, going from five to four and three. Hundreds of flowers show the typical five, and determine the summit of the curve. This drops down on one side only, indicating unilateral variability, which in many cases is due to a very intimate connection of a concealed secondary summit and the main one. In the case of the bulbous buttercup, Ranunculus bulbosus, I have succeeded in isolating this secondary summit, although not in a separate variety, but only in a form corresponding to the type of ever-sporting varieties.
[741] Recapitulating the results of this too condensed discussion, we may state that fluctuations are linear, being limited to an increase and to a decrease of the characters. These changes are mainly due to differences in nourishment, either of the whole organism or of its parts. In the first case, the deviations from the mean are called individual; they are of great importance for the hereditary characters of the offspring. In the second case the deviations are far more universal and far more striking, but of lesser importance. They are called partial fluctuations.
All these fluctuations comply, in the main, with the law of probability, and behave as if their causes were influenced only by chance.
[742]
LECTURE XXVI
ASEXUAL MULTIPLICATION OF EXTREMES
Fluctuating variability may be regarded from two different points of view. The multiformity of a bed of flowers is often a desirable feature, and all means which widen the range of fluctuation are therefore used to enhance this feature, and variability affords specimens, which surpass the average, by yielding a better or larger product.
In the case of fruits and other cultivated forms, it is of course profitable to propagate from the better specimens only, and if possible only from the very best. Obviously the best are the extremes of the whole range of diverging forms, and moreover the extremes on one side of the group. Almost always the best for practical purposes is that in which some quality is strengthened. Cases occur however, in which it is desirable to diminish an injurious peculiarity as far as possible, and in these instances the opposite extreme is the most profitable one.
These considerations lead us to a discussion [743] of the results of the choice of extremes, which it may be easily seen is a matter of the greatest practical importance. This choice is generally designated as selection, but as with most of the terms in the domain of variability, the word selection has come to have more than one meaning. Facts have accumulated enormously since the time of Darwin, a more thorough knowledge has brought about distinctions, and divisions at a rapidly increasing rate, with which terminology has not kept pace. Selection includes all kinds of choice. Darwin distinguished between natural and artificial selection, but proper subdivisions of these conceptions are needed.
In the fourth lecture we dealt with this same question, and saw that selection must, in the first place, make a choice between the elementary species of the same systematic form. This selection of species or species-selection was the work of Le Couteur and Patrick Shirreff, and is now in general use in practice where it has received the name of variety-testing. This clear and unequivocal term however, can hardly be included under the head of natural selection. The poetic terminology of selection by nature has already brought about many difficulties that should be avoided in the future. On the other hand, the designation of the process as a natural [744] selection of species complies as closely as possible with existing terminology, and does not seem liable to any misunderstanding.
It is a selection between species. Opposed to it is the selection within the species. Manifestly the first should precede the second, and if this sequence is not conscientiously followed it will result in confusion. This is evident when it is considered that fluctuations can only appear with their pure and normal type in pure strains, and that each admixture of other units is liable to be shown by the form of the curves. More over, selection chooses single individuals, and a single plant, if it is not a hybrid, can scarcely pertain to two different species. The first choice therefore is apt to make the strain pure.
In contrasting selection between species with that within the species, of course elementary species are meant, including varieties. The terms would be of no consequence if only rightly understood. For the sake of clearness we might designate the last named process with the term of intra-specific selection, and it is obvious that this term is applicable both to natural and to artificial selection.
Having previously dealt with species-selection at sufficient length, we may now confine ourselves to the consideration of the intra-specific [745] selection process. In practice it is of secondary importance, and in nature it takes a very subordinate position. For this reason it will be best to confine further discussions to the experience of the breeders.
Two different ways are open to make fluctuating variability profitable. Both consist in the multiplication of the chosen extremes, and this increase may be attained in a vegetative manner, or by the use of seeds. Asexual and sexual propagation are different in many respects, and so they are also in the domain of variability.
In order to obtain a clear comprehension of this difference, it is necessary to start from the distinction between individual and partial fluctuations, as given in the last lecture. This distinction may be discussed more understandingly if the causes of the variability are taken into consideration. We have dealt with them at some length, and are now aware that inner conditions only, determine averages, while some fluctuation around them is allowable, as influenced by external conditions. These outward influences act throughout life. At the very first they impress their stamp on the whole organism, and incite a lasting change in distinct directions. This is the period of the development of the germ within the seed; it begins with the fusion of the sexual cells, and each of them may be influenced [746] to a noticeable degree before this union. This is the period of the determination of individual variability. As soon as ramifications begin, the external conditions act separately on every part, influencing some to a greater and others to a lesser degree. Here we have the beginning of partial variability. At the outset all parts may be affected in the same way and in the same measure, but the chances of such an agreement, of course, rapidly diminish. This is partly due to differences in exposure, but mainly to alterations of the sensibility of the organs themselves.
It is difficult to gain a clear conception of the contrast between individual and partial variability, and neither is it easy to appreciate their cooperation rightly. Perhaps the best way is to consider their activity as a gradual narrowing of possibilities. At the outset the plant may develop its qualities in any measure, nothing being as yet fixed. Gradually however, the development takes a definite direction, for better or for worse. Is a direction once taken, then it becomes the average, around which the remaining possibilities are grouped. The plant or the organ goes on in this way, until finally it reaches maturity with one of the thousands of degrees of development, between which at the beginning it had a free choice.
[747] Putting this discussion in other terms, we find every individual and every organ in the adult state corresponding with a single ordinate of the curve. The curve indicates the range of possibilities, the ordinate shows the choice that has been made. Now it is clear at once that this choice has not been made suddenly but gradually. Halfway of the development, the choice is halfway determined, but the other half is still undefined. The first half is the same for all the organs of the plant, and is therefore termed individual; the second differs in the separate members, and consequently is known as partial. Which of the two halves is the greater and which the lesser, of course depends on the cases considered.
Finally we may describe a single example, the length of the capsules of the evening-primrose. This is highly variable, the longest reaching more than twice the length of the smallest. Many capsules are borne on the same spike, and they are easily seen to be of unequal size. They vary according to their position, the size diminishing in the main from the base upwards, especially on the higher parts. Likewise the fruits of weaker lateral branches are smaller. Curves are easily made by measuring a few hundred capsules from corresponding parts of different plants, or even by limiting the [748] inquiry to a single individual. These curves give the partial variability, and are found to comply with Quetelet’s law.
Besides this limited study, we may compare the numerous individuals of one locality or of a large plot of cultivated plants with one another. In doing so, we are struck with the fact that some plants have large and others small fruits. We now limit ourselves to the main spike of each plant, and perhaps to its lower parts, so as to avoid as far as possible the impression made by the partial fluctuations. The differences remain, and are sufficient to furnish an easy comparison with the general law. In order to do this, we take from each plant a definite number of capsules and measure their average length. In some experiments I took the twenty lowermost capsules of the main spikes. In this way one average was obtained for each plant, and combining these into a curve, it was found that these fluctuations also came under Quetelet’s law. Thus the individual averages, and the fluctuations around each of them, follow the same rule. The first are a measure for the whole plant, the second only for its parts. As a general resume we can assert that, as a rule, a quality is determined in some degree during the earlier stages of the organism, and that this determination is valid throughout its [749] life. Afterwards only the minor points remain to be regulated. This makes it at once clear that the range of individual and partial variability together must be wider than that of either of them, taken alone. Partial fluctuations cannot, of course, be excluded. Thus our comparison is limited to individual and partial variability on one side, and partial fluctuations alone on the other side.
Intra-specific selection is thus seen to fall under two heads: a selection between the individuals, and a choice within each of them. The first affords a wider and the latter a narrower field.
Individual variability, considered as the result of outward influences operative during extreme youth, can be excluded in a very simple manner. Obviously it suffices to exclude extreme youth, in other words, to exclude the use of seeds. Multiplication in a vegetative way, by grafting and budding, by runners or roots, or by simple division of rootstocks and bulbs is the way in which to limit variability to the partial half. This is all we may hope to attain, but experience shows that it is a very efficient means of limitation. Partial fluctuations are generally far smaller than individual and partial fluctuations together.
Individual variability in the vegetable kingdom [750] might be called seed-variation, as opposed to partial or bud-fluctuation. And perhaps these terms are more apt to convey a clear conception of the distinction than any other. The germ within the unripe seed is easily understood to be far more sensitive to external conditions than a bud.
Multiplication of extremes by seed is thus always counteracted by individual variability, which at once reopens all, or nearly all, the initial possibilities. Multiplication by buds is exempt from this danger and thus leads to a high degree of uniformity. And this uniformity is in many cases exactly what the breeder endeavors to obtain.
We will treat of this reopening of previous possibilities under the head of regression in the next lecture. It is not at all absolute, at least not in one generation. Part of the improvement remains, and favors the next generation. This part may be estimated approximately as being about one-third or one-half of the improvement attained. Hence the conclusion that vegetative multiplication gives rise to varieties which are as a rule twice or thrice as good as selected varieties of plants propagated by seeds. Hence, likewise the inference that breeders generally prefer vegetative multiplication of improved forms, and apply it in all possible cases. [751] Of course the application is limited, and forage crops and the greater number of vegetables will always necessarily be propagated by seed.
Nature ordinarily prefers the sexual way. Asexual multiplications, although very common with perennial plants, appear not to offer important material for selection. Hence it follows that in comparing the work of nature with that of man, the results of selection followed by vegetative propagation should always be carefully excluded. Our large bulb-flowers and delicious fruits have nothing in common with natural products, and do not yield a standard by which to judge nature’s work.
It is very difficult for a botanist to give a survey of what practice has attained by the asexual multiplication of extremes. Nearly all of the large and more palatable fruits are due to such efforts. Some flowers and garden-plants afford further instances. By far the greatest majority of improved asexual varieties, however, are not the result of pure intra-specific selection. They are due largely to the choice of the best existing elementary species, and to some extent to crosses between them, or between distinct systematic species. In practice selection and hybridization go hand in hand and it is often difficult to ascertain what part of [752] the result is due to the one, and what to the other factor.
The scientist, on the contrary, has nothing to do with the industrial product. His task is the analysis of the methods, in order to reach a clear appreciation of the influence of all the competing factors. This study of the working causes leads to a better understanding of the practical processes, and may become the basis of improvement in methods.
Starting from these considerations, we will now give some illustrative examples, and for the first, choose one in which hybridization is almost completely excluded.
Sugar-canes have long been considered to be plants without seed. Their numerous varieties are propagated only in a vegetative way. The stems are cut into pieces, each bearing one or two or more nodes with their buds. An entire variety, though it may be cultivated in large districts and even in various countries, behaves with respect to variability as a single individual. Its individual fluctuability has been limited to the earliest period of its life, when it arose from an unknown seed. The personal characters that have been stamped on this one seed, partly by its descent, and partly in the development of its germ during the period of ripening, have become the indelible characters [753] of the variety, and only the partial fluctuability, due to the effect of later influences, can now be studied statistically.
This study has for its main object the production of sugar in the stems, and the curves, which indicate the percentage of this important substance in different stems of the same variety, comply with Quetelet’s law. Each variety has its own average, and around this the data of the majority of the stems are densely crowded, while deviations on both sides are rare and become the rarer the wider they are. The “Cheribon” cane is the richest variety cultivated in Java, and has an average of 19% sugar, while it fluctuates between 11% and 28%. “Chunnic” averages 14%, “Black Manilla” 13% and “White Manilla” 10%; their highest and lowest extremes diverge in the same manner, being for the last named variety 1% and 15%.
This partial variability is of high practical interest, because on it a selection may be founded. According to the conceptions described in a previous lecture, fluctuating variability is the result of those outward factors that determine the strength of development of the plant or the organ. The inconstancy of the degree of sensibility, combined with the ever-varying weather conditions preclude any close proportionality, but apart from this difficulty there is, in the [754] main, a distinct relation between organic strength and the development of single qualities. This correlation has not escaped observation in the case of the sugar-cane, and it is known that the best grown stocks are generally the richest in sugar. Now it is evident that the best grown and richest stems will have the greater chance of transmitting these qualities to the lateral-buds. This at once gives, a basis for vegetative selection, upon which it is not necessary to choose a small number of very excellent stems, but simply to avoid the planting of all those that are below the average. By this means the yield of the cultures has often noticeably been enhanced.
As far as experience goes, this sort of selection, however profitable, does not conduce to the production of improved races. Only temporary ameliorations are obtained, and the selection must be made in the same manner every year. Moreover the improvement is very limited and does not give any promise of further increase. In order to reach this, one has to recur to the individual fluctuability, and therefore to seed.
Nearly half a century ago, Parris discovered, on the island of Barbados, that seeds might occasionally be gathered from the canes. These, however, yielded only grass-like plants of no real value. The same observation was made [755] shortly afterwards in Java and in other sugar producing countries. In the year 1885, Soltwedel, the director of one of the experiment stations for the culture of sugar-cane in Java, conceived the idea of making use of seedlings for the production of improved races. This idea is a very practical one, precisely because of the possibility of vegetative propagation. If individuals would show the same range as that of partial fluctuability, then the choice of the extremes would at once bring the average up to the richness of the best stocks. Once attained, this average would be fixed, without further efforts.
Unfortunately there is one great drawback. This is the infertility of the best variety, that of the “Cheribon” cane. It flowers abundantly in some years, but it has never been known to produce ripe seeds. For this reason Soltwedel had to start from the second best sort, and chose the “Hawaii” cane. This variety usually yields about 14% sugar, and Soltwedel found among his seedlings one that showed 15%. This fact was quite unexpected at that time, and excited widespread interest in the new method, and since then it has been applied to numerous varieties, and many thousands of seedlings have been raised and tested as to their sugar-production.
[756] From a scientific point of view the results are quite striking. From the practical standpoint, however, the question is, whether the “Hawaii” and other fertile varieties are adequate to yield seedlings, which will surpass the infertile “Cheribon” cane. Now “Hawaii” averages 14% and “Cheribon” 19%, and it is easily understood that a “Hawaii” seedling with more than 19% can be expected only from very large sowings. Hundreds of thousands of seedlings must be cultivated, and their juice tested, before this improvement can be reached. Even then, it may have no significance for practical purposes. Next to the amount of sugar comes the resistance to the disease called “Sereh,” and the new race requires to be ameliorated in this important direction, too. Other qualities must also be considered, and any casual deterioration in other characters would make all progress illusory. For these reasons much time is required to attain distinct improvements.
These great difficulties in the way of selecting extremes for vegetative propagation are of course met with everywhere. They impede the work of the breeder to such a degree, that but few men are able to surmount them. Breeding new varieties necessitates the bending of every effort to this purpose, and a clear conception of [757] the manifold aspects of this intricate problem. These fall under two heads, the exigencies of practice, and the physiologic laws of variability. Of course, only the latter heading comes within the limits of our discussion which includes two main points. First comes the general law of fluctuation that, though slight deviations from the average may be found by thousands, or rather in nearly every individual, larger and therefore important deviations are very rare. Thousands of seedlings must be examined carefully in order to find one or two from which it might be profitable to start a new race. This point is the same for practical and for scientific investigation. In the second place however, a digression is met with. The practical man must take into consideration all the varying qualities of his improved strains. Some of them must be increased and others be decreased, and their common dependency on external conditions often makes it very difficult to discover the desired combinations. It is obvious, however, that the neglect of one quality may make all improvement of other characters wholly useless. No augmentation of sugar-percentage, of size and flavor of fruits can counterbalance an increase in sensitiveness to disease, and so it is with other qualities also.
[758] Improved races for scientific investigation can be kept free from infection, and protected against numerous other injuries. In the experimental garden they may find conditions which cannot be realized elsewhere. They may show a luxuriant growth, and prove to be excellent material for research, but have features which, having been overlooked at the period of selection, would at once condemn them if left to ordinary conditions, or to the competition of other species.
Considering all these obstacles, it is only natural that breeders should use every means to reach their goal. Only in very rare instances do they follow methods analogous to scientific processes, which tend to simplify the questions as much as possible. As a rule, the practical way is the combination of as many causes of variability as possible. Now the three great sources of variability are, as has been pointed out on several occasions, the original multiformity of the species, fluctuating variability, and hybridization. Hence, in practical experiments, all three are combined. Together they yield results of the highest value, and Burbank’s improved fruits and flowers give testimony to the practical significance of this combination.
From a scientific point of view however, it is [759] ordinarily difficult, if not impossible, to discern the part which each of the three great branches of variability has taken in the origination of the product. A full analysis is rarely possible, and the treatment of one of the three factors must necessarily remain incomplete.
Notwithstanding these considerations, I will now give some examples in order to show that fluctuating variability plays a prominent part in these improvements. Of course it is the third in importance in the series. First comes the choice of the material from the assemblage of species, elementary species and varieties. Hybridization comes next in importance. But even the hybrids of the best parents may be improved, because they are no less subject to Quetelet’s law than any other strain. Any large number of hybrids of the same ancestry will prove this, and often the excellency of a hybrid variety depends chiefly, or at least definitely, on the selection of the best individuals. Being propagated only in a vegetative way, they retain their original good qualities through all further culture and multiplication.
As an illustrative example I will take the genus Canna. Originally cultivated for its large and bright foliage only, it has since become a flowering plant of value. Our garden strains have originated by the crossing of [760] a number of introduced wild species, among which the Canna indica is the oldest, now giving its name to the whole group. It has tall stems and spikes with rather inconspicuous flowers with narrow petals. It has been crossed with C. nepalensis and C. warczewiczii, and the available historic evidence points to the year 1846 as that of the first cross. This was made by Annee between the indica and the nepalensis; it took ten years to multiply them to the required degree for introduction into commerce. These first hybrids had bright foliage and were tall plants, but their flowers were by no means remarkable.
Once begun, hybridization was widely practiced. About the year 1889 Crozy exhibited at Paris the first beautifully flowering form, which he named for his wife, “Madame Crozy.” Since that time he and many others, have improved the flowers in the shape and size, as well as in color and its patterns. In the main, these ameliorations have been due to the discovery and introduction of new wild species possessing the required characters. This is illustrated by the following incident. In the year 1892 I visited Mr. Crozy at Lyons. He showed me his nursery and numerous acquisitions, those of former years as well as those that were quite new, and which were in the process of rapid [761] multiplication, previous to being given to the trade. I wondered, and asked, why no pure white variety was present. His answer was “Because no white species had been found up to the present time, and there is no other means of producing white varieties than by crossing the existing forms with a new white type.”
Comparing the varieties produced in successive periods, it is very easy to appreciate their gradual improvement. On most points this is not readily put into words, but the size of the petals can be measured, and the figures may convey at least some idea of the real state of things. Leaving aside the types with small flowers and cultivated exclusively for their foliage, the oldest flowers of Canna had petals of 45 mm. length and 13 mm. breadth. The ordinary types at the time of my visit had reached 61 by 21 mm., and the “Madame Crozy” showed 66 by 30 mm. It had however, already been surpassed by a few commercial varieties, which had the same length but a breadth of 35 mm. And the latest production, which required some years of propagation before being put on the market, measured 83 by 43 mm. Thus in the lapse of some thirty years the length had been doubled and the breadth tripled, giving flowers with broad corollas and with petals [762] joined all around, resembling the best types of lilies and Amaryllis.
Striking as this result unquestionably is, it remains doubtful as to what part of it is due to the discovery and introduction of new large flowered species, and what to the selection of the extremes of fluctuating variability. As far as I have been able to ascertain however, and according to the evidence given to me by Mr. Crozy, selection has had the largest part in regard to the size, while the color-patterns are introduced qualities.
The scientific analysis of other intricate examples is still more difficult. To the practical breeder they often seem very simple, but the student of heredity, who wishes to discern the different factors, is often quite puzzled by this apparent simplicity. So it is in the case of the double lilacs, a large number of varieties of which have recently been originated and introduced into commerce by Lemoine of Nancy. In the main they owe their origin to the crossing and recrossing of a single plant of the old double variety with the numerous existing single-flowered sorts.
This double variety seems to be as old as the culture of the lilacs. It was already known to Munting, who described it in the year 1671. Two centuries afterwards, in 1870, a new description [763] was given by Morren, and though more than one varietal name is recorded in his paper, it appears from the facts given that even at that time only one variety existed. It was commonly called Syringa vulgaris azurea plena, and seems to have been very rare and without real ornamental value.
Lemoine, however, conceived the desirability of a combination of the doubling with the bright colors and large flower-racemes of other lilacs, and performed a series of crosses. The “azurea plena” has no stamens, and therefore must be used in all crosses as the pistil-parent; its ovary is narrowly inclosed in the tube of the flower, and difficult to fertilize. On the other hand, new crosses could be made every year, and the total number of hybrids with different pollen-parents was rapidly increased. After five years the hybrids began to flower and could be used for new crosses, yielding a series of compound hybrids, which however, were not kept separate from the products of the first crosses.
Gradually the number of the flowering specimens increased, and the character of doubling was observed to be variable to a high degree. Sometimes only one supernumerary petal was produced, sometimes a whole new typical corolla was extruded from within the first. In the same [764] way the color and the number of the flowers on each raceme were seen to vary. Thousands of hybrids were produced, and only those which exhibited real advantages were selected for trade. These were multiplied by grafting, and each variety at present consists only of the buds of one original individual and their products. No constancy from seed is assumed, many varieties are even quite sterile.
Of course, no description was given of the rejected forms. It is only stated that many of them bore either single or poorly filled flowers, or were objectionable in some other way. The range of variability, from which the choices were made, is obscure and only the fact of the selection is prominent. What part is due to the combination of the parental features and what to the individual fluctuation of the hybrid itself cannot be ascertained.
So it is in numerous other instances. The dahlias have been derived from three or more original species, and been subjected to cultivation and hybridization in an ever-increasing scale for a century. The best varieties are only propagated in the vegetative way, by the roots and buds, or by grafting and cutting. Each of them is, with regard to its hereditary qualities, only one individual, and the individual characters were selected at the same time with the [765] varietal and hybrid characters. Most of them are very inconstant from seed and as a rule, only mixtures are offered for sale in seed-lists. Which of their ornamental features are due to fluctuating deviation from an average is of course unknown. Amaryllis and Gladiolus are surrounded with the same scientific uncertainties. Eight or ten, or even more, species have been combined into one large and multiform strain, each bringing its peculiar qualities into the mixed mass. Every hybrid variety is one individual, being propagated by bulbs only. Colors and color-patterns, shape of petals and other marks, have been derived from the wild ancestors, but the large size of many of the best varieties is probably due to the selection of the extremes of fluctuating variability. So it is with the begonias of our gardens, which are also composite hybrids, but are usually sown on a very large scale. Flowers of 15 cm. diameter are very showy, but there can be no doubt about the manner in which they are produced, as the wild species fall far short of this size.
Among vegetables the potatoes afford another instance. Originally quite a number of good species were in culture, most of them having small tubers. Our present varieties are due to hybridization and selection, each of them being propagated only in the vegetative way.
[766] Selection is founded upon different qualities, according to the use to be made of the new sort. Potatoes for the factory have even been selected for their amount of starch, and in this case at least, fluctuating variability has played a very important part in the improvement of the race.
Vegetative propagation has the great advantage of exempting the varieties from regression to mediocrity, which always follows multiplication by seeds. It affords the possibility of keeping the extremes constant, and this is not its only advantage. Another, likewise highly interesting, side of the question is the uniformity of the whole strain. This is especially important in the case of fruits, though ordinarily it is regarded as a matter of course, but there are some exceptions which give proof of the real importance of the usual condition. For example, the walnut-tree. Thousands of acres of walnut-orchards consist of seedling trees grown from nuts of unknown parentage. The result is a great diversity in the types of trees and in the size and shape of the nuts, and this diversity is an obvious disadvantage to the industry. The cause lies in the enormous difficulties attached to grafting or budding of these trees, which make this method very expensive and to a high degree uncertain and unsatisfactory.
[767] After this hasty survey of the more reliable facts of the practice of an asexual multiplication of the extremes of fluctuating variability, we may now return to the previously mentioned theoretical considerations. These are concerned with an estimation of the chances of the occurrence of deviations, large enough to exhibit commercial value. This chance may be calculated on the basis of Quetelet’s law, whenever the agreement of the fluctuation of the quality under consideration has been empirically determined. In the discussion of the methods of comparing two curves, we have pointed to the quartiles as the decisive points, and to the necessity of drawing the curves so that these points are made to overlie one another, on each side of the average. If now we calculate the binomium of Newton for different values of the exponent, the sum of the coefficients is doubled for each higher unit of the exponent, and at the same time the extreme limit of the curve is extended one step farther. Hence it is possible to calculate a relation between the value of the extreme and the number of cases required. It would take us too long to give this calculation in detail, but it is easily seen that for each succeeding step the number of individuals must be doubled, though the length of the steps, or the amount of increase of the quality [768] remains the same. The result is that many thousands of seedlings are required to go beyond the ordinary range of variations, and that every further improvement requires the doubling of the whole culture. If ten thousand do not give a profitable deviation, the next step requires twenty thousand, the following forty thousand, and so on. And all this work would be necessary for the improvement of a single quality, while practice requires the examination and amelioration of nearly all the variable characters of the strain.
Hence the rule that great results can only be obtained by the use of large numbers, but it is of no avail to state this conclusion from a scientific point of view. Scientific experimenters will rarely be able to sacrifice fifty thousand plants to a single selection. The problem is to introduce the principle into practice and to prove its direct usefulness and reliability. It is to Luther Burbank that we owe this great achievement. His principles are in full harmony with the teachings of science. His methods are hybridization and selection in the broadest sense and on the largest scale. One very illustrative example of his methods must suffice to convey an idea of the work necessary to produce a new race of superlative excellency. Forty thousand blackberry and raspberry [769] hybrids were produced and grown until the fruit matured. Then from the whole lot a single variety was chosen as the best. It is now known under the name of “Paradox.” All others were uprooted with their crop of ripening berries, heaped up into a pile twelve feet wide, fourteen feet high and twenty-two feet long, and burned. Nothing remained of that expensive and lengthy experiment, except the one parent-plant of the new variety. Similar selections and similar amount of work have produced the famous plums, the brambles and the blackberries, the Shasta daisy, the peach almond, the improved blueberries, the hybrid lilies, and the many other valuable fruits and garden-flowers that have made the fame of Burbank and the glory of horticultural California.
[770]
LECTURE XXVII
INCONSTANCY OF IMPROVED RACES
The greater advantages of the asexual multiplication of extremes are of course restricted to perennial and woody plants. Annual and biennial species cannot as a rule, be propagated in this way, and even with some perennials horticulturists prefer the sale of seeds to that of roots and bulbs. In all these cases it is clear that the exclusion of the individual variability, which was shown to be an important point in the last lecture, must be sacrificed.
Seed-propagation is subject to individual as well as to fluctuating variability. The first could perhaps be designated by another term, embryonic variability, since it indicates the fluctuations occurring during the period of development of the germ. This period begins with the fusion of the male and female elements and is largely dependent upon the vigor of these cells at the moment, and on the varying qualities they may have acquired. It comprises in the main the time of the ripening of the seed, and [771] might perhaps best be considered to end with the beginning of the resting stage of the ripe seed. Hence it is clear that the variability of seed-propagated annual races has a wider range than that of perennials, shrubs and trees. At present it is difficult to discern exactly the part each of these two main factors plays in the process. Many indications are found however, that make it probable that embryonic variability is wider, and perhaps of far greater importance than the subsequent partial fluctuations. The high degree of similarity between the single specimens of a vegetative variety, and the large amount of variability in seed-races strongly supports this view. The propagation and multiplication of the extremes of fluctuating variability by means of seeds requires a close consideration of the relation between seedling and parent. The easiest way to get a clear conception of this relation is to make use of the ideas concerning the dependency of variability upon nourishment. Assuming these to be correct in the main, and leaving aside all minor questions, we may conclude that the chosen extreme individual is one of the best nourished and intrinsically most vigorous of the whole culture. On account of these very qualities it is capable of nourishing all of its organs better and also its seeds. In other words, the seeds [772] of the extreme individuals have exceptional chances of becoming better nourished than the average of the seeds of the race. Applying the same rule to them, it is easily understood that they will vary, by reason of this better nourishment, in a direction corresponding to that of their parent.
This discussion gives a very simple explanation of the acknowledged fact that the seeds of the extremes are in the main the best for the propagation of the race. It does not include however, all the causes for this preferment. Some are of older date and due to previous influences.
A second point in our discussion is the appreciation of the fact that a single individual may be chosen to gather the seed from, and that these seeds, and the young plants they yield, are as a rule, numerous. Hence it follows that we are to compare their average and their extremes with the qualities of the parents. Both are of practical as well as of theoretical interest. The average of the progeny is to be considered as the chief result of the selection in the previous generation, while the extremes, at least those which depart in the same direction, are obviously the means of further improvement of the race.
Thus our discussion should be divided into [773] two heads. One of these comprises the relation of the average of the progeny to the exceptional qualities of the chosen parent, and the other the relation of exceptional offspring to the exceptional parents.
Let us consider the averages first. Are they to be expected to be equal to the unique quality of the parent, or perhaps to be the same as the average of the whole unselected race? Neither of these cases occur. Experience is clear and definite on this important point. Vilmorin, when making the first selections to improve the amount of sugar in beets, was struck with the fact that the average of the progeny lies between that of the original strain and the quality of the chosen parent. He expressed his observation by stating that the progeny are grouped around and diverge in all directions from some point, placed on the line which unites their parent with the type from which it sprang. All breeders agree on this point, and in scientific experiments it has often been confirmed. We shall take up some illustrative examples presently, but in order to make them clear, it is necessary to give a closer consideration to the results of Vilmorin.
From his experience it follows that the average of the progeny is higher than that of the race at large, but lower than the chosen parent. [774] In other words, there is a progression and a regression. A progression in relation to the whole race, and a regression in comparison with the parent. The significance of this becomes clear at once, if we recall the constancy of the variety which could be obtained from the selected extreme in the case of vegetative multiplication. The progression is what the breeder wants, the regression what he detests. Regression is the permanency of part of the mediocrity which the selection was invoked to overcome. Manifestly it is of the highest interest that the progression should be as large, and the regression as small as possible. In order to attain this goal the first question is to know the exact measure of progression and regression as they are exhibiting themselves in the given cases, and the second is to inquire into the influences, on which this proportion may be incumbent.
At present our notions concerning the first point are still very limited and those concerning the second extremely vague. Statistical inquiries have led to some definite ideas about the importance of regression, and these furnish a basis for experimental researches concerning the causes of the phenomenon. Very advantageous material for the study of progression and regression in the realm of fluctuating variability is afforded by the [775] ears of corn or maize. The kernels are arranged in longitudinal rows, and these rows are observed to occur in varying, but always even, numbers. This latter circumstance is due to the fact that each two neighboring rows contain the lateral branches of a single row of spikelets, the ages of which however, are included in the fleshy body of the ear. The variation of the number of the rows is easily seen to comply with Quetelet’s law, and often 30 or 40 ears suffice to give a trustworthy curve. Fritz Muller made some experiments upon the inheritance of the number of the rows, in Brazil. He chose a race which averaged 12 rows, selected ears with 14, 16 and 18 rows, etc., and sowed their kernels separately. In each of-these cultures he counted the rows of the seeds on the ears of all the plants when ripe, and calculated their average. This average, of course, does not necessarily correspond to a whole number, and fractions should not be neglected.
According to Vilmorin’s rule he always found some progression of the average and some regression. Both were the larger, the more the parent-ear differed from the general average, but the proportion between both remained the same, and seems independent of the amount of the deviation. Putting the deviation at 5, the progression calculated from his figures is [776] 2 and the regression 3. In other words the average of the progeny has gained over the average of the original variety slightly more than one-third, and slightly less than one-half of the parental deviation. I have repeated this experiment of Fritz Miller’s and obtained nearly the same regression of three-fifths, though working with another variety, and under widely different climatic conditions.
The figures of Fritz Muller were, as given below, in one experiment. In the last column I put the improvement calculated for a proportion of two-fifths above the initial average of 12.
| Rows on parent ears | Average of rows of progeny | 12 + 2/5ths of Difference |
| 14 | 12.6 | 12.8 |
| 16 | 14.1 | 13.6 |
| 18 | 15.2 | 14.4 |
| 20 | 15.8 | 15.2 |
| 22 | 16.1 | 16 |
Galton, in his work on natural inheritance, describes an experiment with the seeds of the sweet pea or Lathyrus odoratus. He determined the average size in a lot of purchased seeds, and selected groups of seeds of different, but for each group constant, sizes. These were sown, and the average of the seeds was determined anew in the subsequent harvest they yielded. These figures agreed with the rule of Vilmorin and were calculated in the manner [777] given for the test of the corn. The progression and regression were found to be proportionate to the amount of the deviation. The progression of the average was one-third, and the regression in consequence two-thirds of the total deviation. The amelioration is thus seen to be nearly, though not exactly, the same as in the previous case.
From the evidence of the other corresponding experiments, and from various statistical inquiries it seems that the value of the progression is nearly the same in most cases, irrespective of the species used and the quality considered. It may be said to be from one-third to one-half of the parental deviation, and in this form the statement is obviously of wide and easy applicability.
Our figures also demonstrate the great preeminence of vegetative varieties above the improved strains multiplied by seeds. They have a definite relation. Asexually multiplied strains may be said to be generally two times or even three times superior to the common offspring. This is a difference of great practical importance, and should never be lost sight of in theoretical considerations of the productive capacity of selection. Multiplication by seed however, has one great advantage over the asexual method; it may be repeated. The [778] selection is not limited to a single choice, but may be applied in two or more succeeding generations. Obviously such a repetition affords a better chance of increasing the progression of the average and of ameliorating the race to a greater degree than would be possible by a single choice. This principle of repeated selection is at present the prominent feature of race improvement. Next to variety-testing and hybridizing it is the great source of the steady progression of agricultural crops. From a practical standpoint the method is clear and as perfect as might be expected, but this is not the side of the problem with which we are concerned here. The theoretical analysis and explanation of the results obtained, however, is subject to much doubt, and to a great divergence of conceptions. So it is also with the application of the practical processes to those occurring in nature. Some assume that here repeated selection is only of subordinate importance, while others declare that the whole process of evolution is due to this agency. This very important point however, will be reserved for the next lecture, and only the facts available at present will be considered here.
As a first example we may take the ray-florets of the composites. On a former occasion we have dealt with their fluctuation in number and [779] found that it is highly variable and complies in the main with Quetelet’s law. Madia elegans, a garden species, has on the average 21 rays on each head, fluctuating between 16 and 25 or more. I saved the seeds of a plant with only 17 rays on the terminal head, and got from them a culture which averaged 19 rays, which is the mean between 21 and 17. In this second generation I observed the extremes to be 22 and 12, and selected a plant with 13 rays as the parent for a continuation of the experiment. The plants, which I got from its seeds, averaged 18 and showed 22 and 13 as extremes. The total progression of the average was thus, in two generations, from 21 to 18, and the total regression from 13 to 18, and the proportion is thus seen to diminish by the repetition rather than to increase.
This experiment, however, is of course too imperfect upon which to found general conclusions. It only proves the important fact that the improved average of the second generation is not the starting-point for the further improvement. But the second generation allows a choice of an extreme, which diverges noticeably more from the mean than any individual of the first culture, and thereby gives a larger amount of absolute progression, even if the proportion between progression and regression remains [780] the same. The repetition is only an easy method of getting more widely deviating extremes; whether it has, besides this, another effect, remains doubtful. In order to be able to decide this question, it is necessary to repeat the selection during a series of generations. In this way the individual faults may be removed as far as possible. I chose an experiment of Fritz Muller, relating to the number of rows of grains on the ears exactly as in the case above referred to, and which I have repeated in my experimental garden at Amsterdam.
I started from a variety known to fructify fairly regularly in our climate, and exhibiting in the mean 12-14 rows, but varying between 8 and 20 as exceptional cases. I chose an ear with 16 rows and sowed its seeds in 1887. A number of plants were obtained, from each of which, one ear was chosen in order to count its rows. An average of 15 rows was found with variations complying with Quetelet’s law. One ear reached 22 rows, but had not been fertilized, some others had 20 rows, and the best of these was chosen for the continuation of the experiment. I repeated the sowing during 6 subsequent generations in the same way, choosing each time the most beautiful ear from among those with the greatest number of rows. Unfortunately with the increase of the number the [781] size of the grains decreases, the total amount of nourishment available for all of them remaining about the same. Thus the kernels and consequently the new plants became smaller and weaker, and the chance of fertilization was diminished in the ears with the highest number of rows. Consequently the choice was limited, and after having twice chosen a spike with 20 and once one with 24 rows, I finally preferred those with the intermediate number of 22.
This repeated choice has brought the average of my race up from 13 to 20, and thus to the extreme limit of the original variety. Seven years were required to attain this result, or on an average the progression was one row in a year. This augmentation was accompanied by an accompanying movement of the whole group in the same direction. The extreme on the side of the small numbers came up from 8 to 12 rows, and cobs with 8 or 10 rows did not appear in my race later than the third generation. On the other side the extreme reached 28, a figure never reached by the original variety as cultivated with us, and ears with 24 and 26 rows have been seen during the four last generations in increasing numbers.
This slow and gradual amelioration was partly due to the mode of pollination of the corn. [782] The pollen falls from the male spikes on the ears of the same plant, but also is easily blown on surrounding spikes. In order to get the required amount of seed it is necessary in our climate to encroach as little as possible upon free pollination, aiding the self-pollination, but taking no precautions against intercrossing. It is assumed that the choice of the best ears indicates the plants which have had the best pollen-parents as well as the best pistil parents, and that selection here, as in other cases, corrects the faults of free intercrossing. But it is granted that this correction is only a slow one, and accounts in a great degree for the slowness of the progression. Under better climatic conditions and with a more entire isolation of the individuals, it seems very probable that the same result could have been reached in fewer generations.
However this may be, the fact is that by repeated selection the strain can be ameliorated to a greater extent than by a single choice. This result completely agrees with the general experience of breeders and the example given is only an instance of a universal rule. It has the advantage of being capable of being recorded in a numerical way, and of allowing a detailed and definite description of all the succeeding generations. The entire harvest of all [783] of them has been counted and the figures combined into curves, which at once show the whole course of the pedigree-experiment. These curves have in the main taken the same shape, and have only gradually been moved in the chosen direction.
Three points are now to be considered in connection with this experiment. The first is the size of the cultures required for the resulting amelioration. In other words, would it have been possible to attain an average of 20 rows in a single experiment? This is a matter of calculation, and the calculation must be based upon the experience related above, that the progression in the case of maize is equal to two-fifths of the parental deviation. A cob with 20 rows means a deviation of 7 from the average of 13, the incipient value of my race. To reach such an average at once, an ear would be required with 7 x 5/2 = 17-1/2 rows above the average, or an ear with 30-32 rows. These never occur, but the rule given in a previous lecture gives a method of calculating the probability of their occurrence, or in other words, the number of ears required to give a chance of finding such an ear. It would take too long to give this calculation here, but I find that approximately 12,000 ears would be required to give one with 28 rows, which was the highest number attained in [784] my experiment, while 100,000 ears would afford a chance of one with 32 rows*. Had I been able to secure and inspect this number of ears, perhaps I would have needed only a year to get an average of 20 rows. This however, not being the case, I have worked for seven years, but on the other hand have cultivated all in all only about one thousand individuals for the entire experiment.
Obviously this reduction of the size of the experiment is of importance. One hundred thousand ears of corn could of course, be secured directly from trade or from some industrial culture, but corn is cultivated only to a small extent in Holland, and in most cases the requisite number of individuals would be larger than that afforded by any single plantation.
Repeated selection is thereby seen to be the means of reducing the size of the required cultures to possible measures, not only in the experimental-garden, but also for industrial purposes. A selection from among 60,000-100,000 individuals may be within reach of Burbank, but of few others. As a rule they prefer a longer time with a smaller lot of plants. This
[785] is exactly what is gained by repeated selections. To my mind this reduction of the size of the cultures is probably the sole effect of the repetition. But experience is lacking on this point, and exact comparisons should be made whenever possible, between the descendants of a unique but extreme choice, and a repeated but smaller selection. The effect of the repetition on the nourishment of the chosen representatives should be studied, for it is clear that a plant with 22 rows, the parents and grandparents of which had the same number, indicates a better condition of internal qualities than one with the same number of rows, produced accidentally from the common race. In this way it may perhaps be possible to explain, why in my experiment an ear with 22 rows gave an average offspring with 20, while the calculation, founded on the regression alone would require a parental ear with 32 rows. However, as already stated, this discussion is only intended to convey some general idea as to the reduction of the cultures by means of repeated selections, as the material at hand is wholly inadequate for any closer calculation. This important point of the reduction may be illustrated in still another manner. The sowing of very large numbers is only required because it is impossible to tell from the [786] inspection of the seeds which of them will yield the desired individual. But what is impossible in the inspection of the seeds may be feasible, at least in important measure, in the inspection of the plants which bear the seeds. Whenever such an inspection demonstrates differences, in manifest connection with the quality under consideration, any one will readily grant that it would be useless to sow the seeds of the worst plants, and that even the whole average might be thrown over, if it were only possible to point out a number of the best. But it is clear that by this inspection of the parent plants the principle of repeated selection is introduced for two succeeding generations, and that its application to a larger series of generations is only a question of secondary importance. Summing up our discussion of this first point we may assert that repeated selection is only selection on a small and practical scale, while a single choice would require numbers of individuals higher than are ordinarily available. A second discussion in connection with our pedigree-culture of corn is the question whether the amelioration obtained was of a durable nature, or only temporary. In other words, whether the progeny of the race would remain constant, if cultivated after cessation of the selection. In order to ascertain this, [787] I continued the culture during several generations, choosing ears with less than the average number of rows. The excellence of the race at once disappeared, and the ordinary average of the variety from which I had started seven years before, returned within two or three seasons. This shows that the attained improvement is neither fixed nor assured and is dependent on continued selection. This result only confirms the universal experience of breeders, which teaches the general dependency of improved races on continued selection. Here a striking contrast with elementary species or true varieties is obvious. The strains which nature affords are true to their type; their average condition remains the same during all the succeeding generations, and even if it should be slightly altered by changes in the external conditions, it returns to the type, as soon as these changes come to an end. It is a real average, being the sum of the contribution of all the members of the strain. Improved races have only an apparent average, which is in fact biased by the exclusion of whole groups of individuals. If left to themselves, their appearance changes, and the real average soon returns. This is the common experience of breeders. A third point is to be discussed in connection [788] with the detailed pedigree-cultures. It is the question as to what might be expected from a continuation of improvement selection. Would it be possible to obtain any imaginable deviation from the original type, and to reach independency from further selection? This point has not until now attracted any practical interest, and from a practical point of view and within the limits of ordinary cultures, it seems impossible to obtain a positive answer. But in the theoretical discussion of the problems of descent it has become of the highest importance, and therefore requires a separate treatment, which will be reserved for the next lecture. Here we come upon another equally difficult problem. It relates to the proportion of embryonic or individual fluctuation, to partial variation as involved in the process of selection. Probably all qualities which may be subjected to selection vary according to both principles, the embryonic decision giving only a more definite average, around which the parts of the individual are still allowed to oscillate. It is so with the corn, and whenever two or more ears are ripening or even only flowering on the same plant, differences of a partial nature may be seen in the number of their rows. These fluctuations are only small however, ordinarily not exceeding two and rarely four [789] rows. Choosing always the principal ear, the figures may be taken to indicate the degree of personal deviation from the average of the race. But whenever we make a mistake, and perchance sow from an ear, the deviation of which was largely due to partial variation, the regression should be expected to become considerably larger. Hence it must be conceded that exact calculations of the phenomena of inheritance are subject to much uncertainty, resulting from our very imperfect knowledge concerning the real proportion of the contributing factors, and the difficulty of ascertaining their influence in any given case. Here also we encounter more doubts than real facts, and much remains to be done before exact calculations may become of real scientific value. Returning to the question of the effects of selection in the long run, two essentially different cases are to be considered. Extremes may be selected from among the variants of ordinary fluctuating variability, or from ever-sporting varieties. These last we have shown to be double races. Their peculiar and wide range of variability is due to the substitution of two characters, which exclude one another, or if combined, are diminished in various degrees. Striped flowers and stocks, “five-leaved” clover, pistilloid opium-poppies and numerous other [790] monstrosities have been dealt with as instances of such ever-sporting varieties. Now the question may be put, what would be the effect of selection if in long series of years one of the two characters of such a double race were preferred continuously, to the complete exclusion of the other. Would the race become changed thereby? Could it be affected to such a degree as to gradually lose the inactive quality, and cease to be a double race? Here manifestly we have a means by which to determine what selection is able to accomplish. Physiologic experiments may be said to be too short to give any definite evidence. But cases may be cited where nature has selected during long centuries and with absolute constancy in her choice. Moreover unconscious selections by man have often worked in an analogous manner, and many cultivated plants may be put to the test concerning the evidence they might give on this point. Stating beforehand the result of this inquiry, we may assert that long-continued selection has absolutely no appreciable effect. Of course I do not deny the splendid results of selection during the first few years, nor the necessity of continued selection to keep the improved races to the height of their ameliorated qualities. I only wish to state that the work [791] of selection here finds its limit and that centuries and perhaps geologic periods of continued effort in the same direction are not capable of adding anything more to the initial effect. Some illustrative examples may suffice to prove the validity of this assertion. Every botanist who has studied the agricultural practice of plant-breeding, or the causes of the geographic distribution of plants, will easily recall to his mind numerous similar cases. Perhaps the most striking instance is afforded by cultivated biennial plants. The most important of them are forage-beets and sugar-beets. They are, of course, cultivated only as biennials, but some annual specimens may be seen each year and in nearly every field. They arise from the same seed as the normal individuals, and their number is obviously dependent on external conditions, and especially on the time of sowing. Ordinary cultures often show as much as 1% of these useless plants, but the exigencies of time and available labor often compel the cultivator to have a large part of his fields sown before spring. In central Europe, where the climate is unfavorable at this season, the beets respond by the production of far larger proportions of annual specimens, their number coming often up to 20% or more, thus constituting noticeable losses in the product [792] of the whole field. Rimpau, who has made a thorough study of this evil and has shown its dependency on various external conditions, has also tried to find methods of selection with the aim of overcoming it, or at least of reducing it to uninjurious proportions. But in these efforts he has reached no practical result. The annuals are simply inexterminable. Coming to the alternative side of the problem it is clear that annuals have always been excluded in the selection. Their seeds cannot be mixed with the good harvest, not even accidentally, since they have ripened in a previous year. In order to bear seeds in the second year beets must be taken from the field, and kept free from frost through the winter. The following spring they are planted out, and it is obvious that even the most careless farmer is not liable to mix them with annual specimens. Hence we may conclude that a strict and unexcelled process of selection has been applied to the destruction of this tendency, not only for sugar-beets, since Vilmorin’s time, when selection had become a well understood process, but also for forage-beets since the beginning of beet culture. Although unconscious, the selection of biennials must have been uninterrupted and strict throughout many centuries. It has had no effect at all. Annuals are seen [793] to return every year. They are ineradicable. Every individual is in the possession of this latent quality and liable to convert it into activity as soon as the circumstances provoke its appearance, as proved by the increase of annuals in the early sowings. Hence the conclusion that selection in the long run is not adequate to deliver plants from injurious qualities. Other proofs could be given by other biennials, and among them the stray annual plants of common carrots are perhaps the most notorious. In my own cultures of evening-primroses I have preferred the annuals and excluded the biennials, but without being able to produce a pure annual race. As soon as circumstances are favorable, the biennials return in large numbers. Cereals give analogous proofs. Summer and winter varieties have been cultivated separately for centuries, but in trials it is often easy to convert the one into the other. No real and definite isolation has resulted from the effect of the long continued unconscious selection. Striped flowers, striped fruits, and especially striped radishes afford further examples. It would be quite superfluous to dwell upon them. Selection always tends to exclude the monochromatic specimens, but does not prevent their return in every generation. Numerous [794] rare monstrosities are in the same category, especially when they are of so rare occurrence as not to give any noticeable contribution to the seed-production, or even if they render their bearers incapable of reproduction. In such cases the selection of normal plants is very severe or even absolute, but the anomalies are by no means exterminated. Any favorable circumstances, or experimental selection in their behalf shows them to be still capable of full development. Numerous cases of such subordinate hereditary characters constitute the greater part of the science of vegetable teratology. If it should be objected that all these cases cover too short a time to be decisive, or at least fail in giving evidence relative to former times, alpine plants afford a proof which one can hardly expect to be surpassed. During the whole present geologic epoch they have been subjected to the never failing selection of their climate and other external conditions. They exhibit a full and striking adaptation to these conditions, but also possess the latent capacity for assuming lowland characters as soon as they are transported into such environment. Obviously this capacity never becomes active on the mountains, and is always counteracted by selection. This agency is evidently without any effect, for as we have seen when dealing [795] with the experiments of Nageli, Bonnier and others, each single individual may change its habits and its aspect in response to transplantation. The climate has an exceedingly great influence on each individual, but the continuance of this influence is without permanent result. So much concerning ever-sporting varieties and double adaptations. We now come to the effects of a continuous selection of simple characters. Here the sugar-beets stand preeminent. Since Vilmorin’s time they have been selected according to the amount of sugar in their roots, and the result has been the most striking that has ever been attained, if considered from the standpoint of practice. But if critically examined, with no other aim than a scientific appreciation of the improvement in comparison with other processes of selection, the support of the evidence for the theory of accumulative influence proves to be very small. The amount of sugar is expressed by percentage-figures. These however, are dependent on various causes, besides the real quantity of sugar produced. One of these causes is the quantity of watery fluid in the tissues, and this in its turn is dependent on the culture in dryer or moister soil, and on the amount of moisture in the air, and the same variety of sugar-beets [796] yields higher percentage-figures in a dry region than in a wet one. This is seen when comparing, for instance, the results of the analyses from the sandy provinces of Holland with those from the clay-meadows, and it is very well known that Californian beets average as high as 26% or more, while the best European beets remain at about 20%. As far as I have been able to ascertain, these figures however, are not indicative of any difference of race, but simply direct responses to the conditions of climate and of soil. Apart from these considerations the improvement reached in half a century or in about twenty to thirty generations is not suggestive of anything absolute. Everything is fluctuating now, even as it was at the outset, and equally dependent on continual care. Vilmorin has given some figures for the beets of the first generations from which he started his race. He quotes 14% as a recommendable amount, and 7 and 21 as the extreme instances of his analyses. However incorrect these figures may be, they coincide to a striking degree with the present condition of the best European races. Of course minor values are excluded each year by the selection, and in consequence the average value has increased. For the year 1874 we find a standard of 10-14% considered as normal, [797] bad years giving 10%, good years from 12% to 14% in the average. Extreme instances exceeded 17%. From that time the practice of the polarization of the juice for the estimate of the sugar has rapidly spread throughout Europe, and a definite increase of the average value soon resulted. This however, often does not exceed 14%, and beets selected in the field for the purpose of polarization come up to an average of 15 to 16%, varying downward to less than 10% and upward to 20 and 21%. In the main the figures are the same as those of Vilmorin, the range of variability has not been reduced, and higher extremes are not reached. An average increase of 1% is of great practical importance, and nothing can excel the industry and care displayed in the improvement of the beet-races. Notwithstanding this a lasting influence has not been exercised; the methods of selection have been improved, and the number of polarized beets has been brought up to some hundreds of thousands in single factories, but the improvement is still as dependent upon continuous selection as it was half a century ago. The process is practically very successful, but the support afforded by it to the selection theory vanishes on critical examination.
[798]
LECTURE XXVIII
ARTIFICIAL AND NATURAL SELECTION
The comparison of artificial and natural selection has furnished material support for the theory of descent, and in turn been the object of constant criticism since the time of Darwin. The criticisms, in greater part, have arisen chiefly from an imperfect knowledge of both processes. By the aid of distinctions recently made possible, the contrast between elementary species and improved races has become much more vivid, and promises to yield better results on which to base comparisons of artificial and natural selection.
Elementary species, as we have seen in earlier lectures, occur in wild and in cultivated plants. In older genera and systematic species they are often present in small numbers only, but many of the more recent wild types and also many of the cultivated forms are very rich in this respect. In agriculture the choice of the most adequate elementary forms for any special purpose is acknowledged [799] as the first step in the way of selection, and is designated by the name of variety-testing, applying the term variety to all the subdivisions of systematic species indiscriminately. In natural processes it bears the title of survival of species. The fact that recent types show large numbers, and in some instances even hundreds of minor constant forms, while the older genera are considerably reduced in this respect, is commonly explained by the assumption of extinction of species on a correspondingly large scale. This extinction is considered to affect the unfit in a higher measure than the fit. Consequently the former vanish, often without leaving any trace of their existence, and only those that prove to be sufficiently adapted to the surrounding external conditions, resist and survive.
This selection exhibits far-reaching analogies between the artificial and the natural processes, and is in both cases of the very highest importance. In nature the dying out of unfit mutations is the result of the great struggle for life. In a previous lecture we have compared its agency with that of a sieve. All elements which are too small or too weak fall through, and only those are preserved which resist the sifting process. Reduced in number they thrive and multiply and are thus enabled to [800] strike out new mutative changes. These are again submitted to the sifting tests, and the frequent repetition of this process is considered to give a good explanation of the manifold, highly complicated, and admirable structures which strike the beginner as the only real adaptations in nature.
Exactly in the same way artificial selection isolates and preserves some elementary species, while it destroys others. Of course the time is not sufficient to secure new mutations, or at least these are only rare at present, and their occurrence is doubtful in historic periods. Apart from this unavoidable difference the analogy between natural and artificial selection appears to me to be very striking.
This form of selection may be termed selection between species. Opposed to it stands the selection within the elementary species or variety. It has of late, alone come to be known as selection, though in reality it does not deserve this distinction. I have already detailed the historical evidence which gives preference to selection between species. The process can best be designated by the name of intraspecific selection, if it is understood that the term intraspecific is meant to apply to the conception of small or elementary species.
I do not wish to propose new terms, but [801] I think that the principal differences might better become understood by the introduction of the word election into the discussion of questions of heredity. Election meant formerly the preferential choice of single individuals, while the derivation of the word selection points to a segregation of assemblies into their larger parts. Or to state it in a shorter way, individual selection is exactly what is usually termed election. Choosing one man from among thousands is to elect him, but a select party is a group of chosen persons. There would be no great difficulty in the introduction of the word election, as breeders are already in the habit of calling their choice individuals “elite,” at least in the case of beets and of cereals.
This intraspecific selection affords a second point for the comparison between natural and artificial processes. This case is readily granted to be more difficult than the first, but there can be no doubt that the similarity is due to strictly comparable causes. In practice this process is scarcely second in importance to the selection between species, and in numerous cases it rests upon it, and crowns it, bringing the isolated forms up to their highest possible degree of usefulness. In nature it does quite the same, adapting strains of individuals to the local conditions of their environment. Improved [802] races do not generally last very long in practice; sooner or later they are surpassed by new selections. Exactly so we may imagine the agency of natural intraspecific selection. It produces the local races, the marks of which disappear as soon as the special external conditions cease to act. It is responsible only for the smallest lateral branches of the pedigree, but has nothing in common with the evolution on the main stems. It is of very subordinate importance.
These assertions of course, are directly opposed to the current run of scientific belief, but they are supported by facts. A considerable part of the evidence has already been dealt with and for our closing discussion only an exact comparison remains to be made between the two detailed types of intraspecific selection. In coming to this I will first dwell upon some intermediate types and conclude with a critical discussion of the features of artificial selection, which to my mind prove the invalidity of the conclusions drawn from it in behalf of an explanation of the processes of nature.
Natural selection occurs not only in the wild state, but is also active in cultivated fields. Here it regulates the struggle of the selected varieties and improved races with the older types, and even with the wild species. In a previous [803] lecture I have detailed the rapid increase of the wild oats in certain years, and described the experiments of Risler and Rimpau in the running out of select varieties. The agency is always the same. The preferred forms, which give a larger harvest, are generally more sensitive to injurious influences, more dependent on rich manure and on adequate treatment. The native varieties have therefore the advantage, when climatic or cultural conditions are unfavorable for the fields at large. They suffer in a minor degree, and are thereby enabled to propagate themselves afterwards more rapidly and to defeat the finer types. This struggle for life is a constant one, and can easily be followed, whenever the composition of a strain is noted in successive years. It is well appreciated by breeders and farmers, because it is always liable to counteract their endeavors and to claim their utmost efforts to keep their races pure. There can be no doubt that exactly the same struggle exempt from man’s intrusion is fought out in the wild state.
Local races of wild plants have not been the object for field observations recently. Some facts however, are known concerning them. On the East Friesian Islands in the North Sea the flowers are strikingly larger and brighter colored than those of the same species on the [804] neighboring continent. This local difference is ascribed by Behrens to a more severe selection by the pollinating insects in consequence of their lesser frequency on these very windy isles. Seeds of the pines from the Himalayas yield cold-resisting young plants if gathered from trees in a high altitude, while the seeds of the same species from lower regions yield more sensitive seedlings. Similar instances are afforded by Rhododendron and other mountain species. According to Cieslar corresponding differences are shown by seeds of firs and larches from alpine and lowland provinces.
Such changes are directly dependent on external influences. This is especially manifest in experiments extending the cultures in higher or in more northern regions. The shorter summer is a natural agent of selection; it excludes all individuals which cannot ripen their seeds during so short a period. Only the short lived ones survive. Schubeler made very striking experiments with corn and other different cereals, and has succeeded in making their culture possible in regions of Norway where it formerly failed. In the district of Christiania, corn had within some few years reduced its lifetime from 123 to 90 days, yielding smaller stems and fewer kernels, but still sufficient to make its culture profitable under the existing conditions. [805] This change was not permanent, but was observed to diminish rapidly and to disappear entirely, whenever the Norwegian strain was cultivated in the southern part of Germany. It was a typical improved race, dependent on continual selection by the short summers which had produced it. Similar results have been reached by Von Wettstein in the comparison of kinds of flax from different countries. The analogy between such cultivated local races and the local races of nature is quite striking. The practice of seed exchange rests for a large part on the experience that the characters, acquired under the definite climatic and cultural conditions of some select regions, hold good for one or two, and sometimes even more generations, before they decrease to practical uselessness. The Probstei, the Hanna and other districts owe their wealth to this temporary superiority of their wheat and other cereals.
Leaving these intermediate forms of selection, we now come to our principal point. It has already been discussed at some length in the previous lecture, but needs further consideration. It is the question whether intraspecific selection may be regarded as a cause of lasting and ever-increasing improvement. This is assumed by biologists who consider fluctuating variability as the main source of progression [806] in the organic world. But the experience of the breeders does not support this view, since the results of practice prove that selection according to a constant standard soon reaches a limit which it is not capable of transgressing. In order to attain further improvements the method of selection itself must be improved. A better and sharper method assures the choice of more valuable representatives of the race, even if these must be sought for in far larger numbers of individuals, as is indicated by the law of Quetelet.
Continuous or even prolonged improvement of a cultivated race is not the result of frequently repeated selection, but of the improvement of the standard of appreciation. Nature, as far as we know, changes her standard from time to time only in consequence of the migrations of the species, or of local changes of climate. Afterwards the new standard remains unchanged for centuries.
Selection, according to a constant standard, reaches its results in few generations. The experience of Van Mons and other breeders of apples shows that the limit of size and lusciousness may be soon attained. Vilmorin’s experiments with wild carrots and those of Carriere with radishes lead to the same conclusion as regards roots. Improvements of flowers in [807] size and color are usually easy and rapid in the beginning, but an impassable limit is soon reached. Numerous other instances could be given.
Contrasted with these simple cases is the method of selecting sugar beets. More than once I have alluded to this splendid example of the influence of man upon domestic races, and tried to point out how little support it affords to the current scientific opinion concerning the power of natural selection. For this reason it is interesting to see how a gradual development of the methods of selection has been, from the very outset, one of the chief aims of the breeders. None of them doubts that an improvement of the method alone is adequate to obtain results. This result, in the main, is the securing of a few percent more of sugar, a change hardly comparable with that progress in evolution, which our theories are destined to explain.
Vilmorin’s original method was a very simple one. Polarization was still undiscovered in his time. He determined the specific weight of his beets, either by weighing them as a whole, or by using a piece cut from the base of the roots and deprived of its bark, in order to test only the sugar tissues. The pieces were floated in solutions of salt, which were diluted until the pieces [808] began to sink. Their specific weight at that moment was determined and considered to be a measure of the corresponding value of the beet. This principle was afterwards improved in two ways. The first was a selection after the salt solution method, but performed on a large scale. After some few determinations, a solution was made of such strength as to allow the greater number of the beets to float, and only the best to sink down. In large vessels thousands of beets could be tested in this way, to select a few of the very heaviest. The other improvement was the determination of the specific weight of the sap, pressed out from the tissue. It was more tedious and more expensive, but more direct, as the influence of the air cavities of the tissue was excluded. It prepared the way for polarization.
This was introduced about the year 1874 in Germany, and soon became generally accepted. It allowed the amount of sugar to be measured directly, and with but slight trouble. Thousands of beets could be tested yearly by this method, and the best selected for the production of seed. In some factories a standard percentage is determined by previous inquiries, and the mass of the beets is tested only by it. In others the methods of taking samples and clearing the sap have been improved so far as to allow the [809] exact determination of three hundred thousand polarization values of beets within a few weeks. Such figures give the richest material for statistical studies, and at once indicate the best roots, while they enable the breeder to change his standard in accordance with the results at any time. Furthermore they allow the mass of the beets to be divided into groups of different quality, and to produce, besides the seeds for the continuation of the race, a first class and second-class product and so on. In the factory of Messrs. Kuhn & Co., at Naarden, Holland, the grinding machine has been markedly improved, so as to tear all cell walls asunder, open all cells, and secure the whole of the sap within less than a minute, and without heating.
It would take too long to go into further details, or to describe the simultaneous changes that have been applied to the culture of the elite strains. The detailed features suffice to show that the chief care of the breeder in this case is a continuous amelioration of the method of selecting. It is manifest that the progression of the race is in the main due to great technical improvements, and not solely to the repetition of the selection.
Similar facts may be seen on all the great lines of industrial selection. An increasing appreciation [810] of all the qualities of the selected plants is the common feature. Morphological characters, and the capacity of yielding the desired products, are the first points that strike the breeder. The relation to climate and the dependence on manure soon follow; but the physiological and chemical sides of the problem are usually slow of recognition in the methods of selection. When visiting Mr. de Vilmorin at Paris some years ago, I inspected his laboratory for the selection of potatoes. In the method in use, the tubers were rubbed to pulp and the starch was extracted and measured. A starch percentage figure was determined for each plant, and the selection of the tubers for planting was founded upon this result. In the same way wheat has been selected by Dippe at Quedlinburg, first by a determination of its nitrogenous contents in general, and secondly by the amount of the substances which determine its value for baking purposes.
The celebrated rye of Schlanstedt was produced by the late Mr. Rimpau in a similar manner and was put on the market between 1880 and 1890 and was received with great favor throughout central Europe, especially in Germany and in France. It is a tall variety, with vigorous stems and very long heads, the kernels of which are nearly double the size of those of the [811] ordinary rye, and are seen protruding, when ripe, from between the scales of the spikelets. It is unfit for poor soils, but is one of the very best varieties for soils of medium fertility in a temperate climate. It is equal in the production of grain to the best French sorts, but far surpassing them in its amount of straw. It was perfected at the farm of Schlanstedt very slowly, according to the current conceptions of the period. The experiment was started in the year 1866, at which time Rimpau collected the most beautiful heads from among his fields, and sowed their kernels in his experiment garden. From this first culture the whole race was derived. Every year the best ears of the strain were chosen for repeated culture, under experimental care, while the remainder was multiplied in a field to furnish the seeds for large and continually increasing areas of his farms.
Two or three years were required to produce the quantity of seed of each kind required for all the fields of Schlanstedt. The experiment garden, which through the kindness of Mr. Rimpau I had the good fortune of visiting more than once between 1875 and 1878, was situated in the middle of his farm, at some distance from the dwellings. Of course it was treated with more care, and especially kept [812] in better conditions of fertility than was possible for the fields at large. A continued study of the qualities and exigencies of the elite plants accompanied this selection, and gave the means of gradually increasing the standard. Resistance against disease was observed and other qualities were ameliorated in the same manner. Mr. Rimpau repeatedly told me that he was most anxious not to overlook any single character, because he feared that if any of them might become selected in the wrong way, perchance unconsciously, the whole strain might suffer to such a degree as to make all the other ameliorations quite useless. With this purpose the number of plants per acre was kept nearly the same as those in the fields, and the size of the culture was large enough every year to include the best kernels of quite a number of heads. These were never separated, and exact individual pedigrees were not included in the plan. This mixture seemed to have the advantage of keeping up an average value of the larger number of the characters, which either from their nature or from their apparent unimportance had necessarily to be neglected.
After ten years of continuous labor, the rye of Rimpau caught the attention of his neighbors, being manifestly better than that of ordinary [813] sowings. Originally he had made his cultures for the improvement of his own fields only. Gradually however, he began to sell his product as seed to others, though he found the difference still very slight. After ten years more, about 1886, he was able to sell all his rye as seed, thereby making of course large profits. It is now acknowledged as one of the best sorts, though in his last letter Mr. Rimpau announced to me that the profits began to decline as other selected varieties of rye became known. The limit of productiveness was reached, and to surmount this, selection had to be begun again from some new and better starting point.
This new starting point invokes quite another principle of selection, a principle which threatens to make the contrast between artificial and natural selection still greater. In fact it is nothing new, being in use formerly in the selection of domestic animals, and having been applied by Vilmorin to his sugar beets more than half a century ago. Why it should ever have been overlooked and neglected in the selection of sugar beets now is not clear.
The principle in itself is very simple. It agrees that the visible characters of an animal or a plant are only an imperfect measure for its hereditary qualities, instead of being the real criterion to be relied upon, as is the current belief. [814] It further reasons that a direct appreciation of the capacity of inheritance can only be derived from the observation of the inheritance itself. Hence it concludes that the average value of the offspring is the only real standard by which to judge the representatives of a race and to found selection upon.
These statements are so directly opposed to views prevalent among plant breeders, that it seems necessary to deal with them from the theoretical and experimental, as well as from the practical side.
The theoretical arguments rest on the division of the fluctuating variability into the two large classes of individual or embryonic, and of partial deviations. We have dealt with this division at some length in the previous lecture. It will be apparent at once, if we choose a definite example. Let us ask what is the real significance of the percentage figure of a single plant in sugar beets. This value depends in the first place, on the strain or family from which the beet has been derived, but this primary point may be neglected here, because it is the same for all the beets of any lot, and determines the average, around which all are fluctuating.
The deviation of the percentage figure of a single beet depends on two main groups of external [815] causes. First come those that have influenced the young germs of the plant during its most sensitive period, when still an embryo within the ripening seed. They give a new limitation to the average condition, which once and forever becomes fixed for this special individual. In the second place the young seedling is affected during the development of its crown of leaves, and of its roots, by numerous factors, which cannot change this average, but may induce deviations from it, increasing or decreasing the amount of sugar, which will eventually be laid down in the root. The best young beet may be injured in many ways during periods of its lifetime, and produce less sugar than could reasonably be expected from it. It may be surpassed by beets of inferior constitution, but growing under more favorable circumstances.
Considered from this point of view the result of the polarization test is not a single value, but consists of at least two different factors. It may be equal to the algebraic sum of these, or to their difference, according to whether the external conditions on the field were locally and individually favorable or unfavorable. A large amount of sugar may be due to high individual value, with slight subsequent deviation from it, [816] or to a less prominent character combined with an extreme subordinate deviation.
Hence it is manifest that even the results of such a highly improved technical method do not deserve the confidence usually put in them. They are open to doubt, and the highest figures do not really indicate the best representatives of the race. In order to convey this conception to you in a still stronger manner, let us consider the partial variability as it usually shows itself. The various leaves of a plant may noticeably vary in size, the flowers in color, the fruits in flavor. They fluctuate around an average, which is assumed to represent the approximate value of the whole plant. But if we were allowed to measure only one leaf, or to estimate only one flower or fruit, and be compelled to conclude from it the worth of the whole plant, what mistakes we could make! We might indeed hit upon an average case, but we might as easily get an extreme, either in the way of increase or of decrease. In both cases our judgment would be badly founded. Now who can assure us that the single root of a given beet is an average representative of the partial variability? The fact that there is only one main root does not prove anything. An annual plant has only one stem, but a perennial species has many. The average height of the last is a [817] reliable character, but the casual height of the former is very uncertain.
So it is with the beets. A beet may be divided by its buds and give quite a number of roots, belonging to the same individual. These secondary roots have been tested for the amount of sugar, and found to exhibit a manifest degree of variability. If the first root corresponded to their average, it might be considered as reliable, but if not anyone will grant that an average is more reliable than a single determination. Deviations have as a fact been observed, proving the validity of our assertion. These considerations at once explain the disappointment so often experienced by breeders. Some facts may be quoted from the Belgian professor of agriculture at Gembloux, the late Mr. Laurent. He selected two beets, from a strain, with the exceptional amount of 23% sugar, but kept their offspring separate and analyzed some 60 of each. In both groups the average was only 11-12%, the extremes not surpassing 14-15%. Evidently the choice was a bad one, notwithstanding the high polarization value of the parent. Analogous cases are often observed, and my countrymen, Messrs. Kuhn & Co., go so far as to doubt all excessive variants, and to prefer beets with high, but less extraordinary percentages. Such are to be had in larger numbers [818] and their average has a good chance of exemption from a considerable portion of the doubts adhering to single excessive cases.
It is curious to note here what Louis de Vilmorin taught concerning this point in the year 1850. I quote his own words: “I have observed that in experiments on heredity it is necessary to individualize as much as possible. So I have taken to the habit of saving and sowing separately the seeds of every individual beet, and I have always found that among the chosen parent plants some had an offspring with a better average yield than others. At the end I have come to consider this character only, as a standard for amelioration.”
The words are clear and their author is the originator of the whole method of plant breeding selection. Yet the principle has been abandoned, and nearly forgotten under the impression that polarization alone was the supreme guide to be relied upon. However, if I understand the signs rightly, the time is soon coming when Vilmorin’s experience will become once more the foundation for progress in breeding.
Leaving the theoretical and historical aspects of the problem, we will now recall the experimental evidence, given in a former lecture, dealing with the inheritance of monstrosities. I have shown that in many instances monstrosities [819] constitute double races, consisting of monstrous and of normal individuals. At first sight one might be induced to surmise that the monstrous ones are the true representatives of the race, and that their seeds should be exclusively sown, in order to keep the strain up to its normal standard. One might even suppose that the normal individuals, or the so-called atavists, had really reverted to the original type of the species and that their progeny would remain true to this.
My experiments, however, have shown that quite the contrary is the case. No doubt, the seeds of the monstrous specimens are trustworthy, but the seeds of the atavists are not less so. Fasciated hawkweeds and twisted teasels gave the same average constitution of the offspring from highly monstrous, and from apparently wholly normal individuals. In other words the fullest development of the visible characteristic was not in the slightest degree an indication of better hereditary tendencies. In unfavorable years a whole generation of a fasciated race may exhibit exclusively normal plants, without transmitting a trace of this deficiency to the following generation. As soon as the suitable conditions return, the monstrosity reassumes its full development. The accordance of these facts with the experience [820] of breeders of domestic animals, and of Louis de Vilmorin, and with the result of the theoretical considerations concerning the factors of fluctuation has led me to suggest the method of selecting, which I have made use of in my experiments with tricotyls and syncotyls.
Seedling variations afford a means of counting many hundreds of individuals in a single germinating pan. If seed from one parent plant is sown only in each pan, a percentage figure for the amount of deviating seedlings may be obtained. These figures we have called the hereditary percentages. I have been able to select the parent plants after their death on the sole ground of these values. And the result has been that from varieties which, on an average, exhibited 50-55% deviating seedlings, after one or two years of selection this proportion in the offspring was brought up to about 90% in most of the cases. Phacelia and mercury with tricotylous seedlings, and the Russian sunflower with connate seed leaves, may be cited as instances.
Besides these tests, others were performed, based only on the visible characters of the seedlings. The result was that this characteristic was almost useless as a criterion. The atavists gave, in the main, nearly the same hereditary percentages as the tricotyls and syncotyls, and [821] their extremes were in each case far better constituted than the average of the chosen type. Hence, for selection purposes, the atavists must be considered to be in no way inferior to the typical specimens.
If it had been possible to apply this principle to twisted and fasciated plants, and perhaps even to other monstrosities, I think that it will readily be granted that the chance of bringing even these races up to a percentage of 90% would have been large enough. But the large size of the cultures required for the counting of numerous groups of offspring in the adult state has deterred me from making such trials. Recently however, I have discovered a species, Viscaria oculata which allows of counting twisted specimens in the pans, and I may soon be able to obtain proofs of this assertion. The validity of the hereditary percentage as a standard of selection has, within the last few years, been recognized and defended by two eminent breeders, W.A. Hays in this country and Von Lochow in Germany. Both of them have started from the experience of breeders of domestic animals. Von Lochow applied the principle to rye. He first showed how fallacious the visible characters often are. For instance the size of the kernels is often dependent on their number in the head, and if this number is [822] reduced by the injurious varietal mark of lacunae (Luckigkeit), the whole harvest will rapidly deteriorate by the selection of the largest kernels from varieties which are not quite free from this hereditary deficiency.
In order to estimate the value of his rye plants, he gathers the seed of each one separately and sows them in rows. Each row corresponds to a parent plant and receives 200 or 150 seeds, according to the available quantity. In this way from 700 to 800 parent plants are tested yearly. Each row is harvested separately. The number of plants gives the average measure of resistance to frost, this being the only important cause of loss. Then the yield in grain and straw is determined and calculated, and other qualities are taken into consideration. Finally one or more groups stand prominent above all others and are chosen for the continuation of the race. All other groups are wholly excluded from the “elite,” but among them the best groups and the very best individuals from lesser groups are considered adequate for further cultivation, in order to produce the commercial product of the race.
As a matter of fact the rye of Von Lochow is now one of the best varieties, and even surpasses the celebrated variety of Schlanstedt. It was only after obtaining proof of the validity [823] of his method that Von Lochow decided to give it to the public.
W.M. Hays has made experiments with wheat at the Minnesota Agricultural Experiment Station. He chose a hundred grains as a proper number for the appreciation of each parent plant, and hence has adopted the name of “centgener power” for the hereditary percentage.
The average of the hundred offspring is the standard to judge the parent by. Experience shows at once that this average is not at all proportional to the visible qualities of the parent. Hence the conclusion that the yield of the parent plant is a very uncertain indication of its value as a parent for the succeeding generation. Only the parents with the largest power in the centgener of offspring are chosen, while all others are wholly discarded. Afterwards the seeds of the chosen groups are propagated in the field until the required quantities of seed are obtained.
This centgener power, or breeding ability, is tested and compared for the various parent plants as to yield, grade, and percentage of nitrogenous content in the grain, and as to the ability of the plant to stand erect, resist rust, and other important qualities. It is evident that by this test of a hundred specimens a far better [824] and much more reliable determination can be made than on the ground of the minutest examination of one single plant. From this point of view the method of Hays commands attention. But the chief advantage lies in the fact that it is a direct proof of that which it is desired to prove, while the visible marks give only very indirect information.
Thus the results of the men of practice are in full accordance with those of theory and scientific experiment, and there can be little doubt that they open the way for a rapid and important improvement. Once attained, progress however, will be dependent on the selection principle, and the hereditary percentage, or centgener power or breeding ability, must be determined in each generation anew. Without this the race would soon regress to its former condition.
To return to our starting point, the comparison of artificial and natural selection. Here we are at once struck by the fact that it is hardly imaginable, how nature can make use of this principle. In some measure the members of the best centgener will manifestly be at an advantage, because they contain more fit specimens than the other groups. But the struggle for existence goes on between individuals, and not between groups of brethren against groups of [825] cousins. In every group the best adapted individuals will survive, and soon the breeding differences between the parents must vanish altogether. Manifestly they can, as a rule, have no lasting result on the issue of the struggle far existence.
If now we remember that in Darwin’s time this principle, breeding ability, enjoyed a far more general appreciation than at present, and that Darwin must have given it full consideration, it becomes at once clear that this old, but recently revived principle, is not adequate to support the current comparison between artificial and natural selection.
In conclusion, summing up all our arguments, we may state that there is a broad analogy between breeding selection in the widest sense of the word, including variety testing, race improvement and the trial of the breeding ability on one side, and natural selection on the other. This analogy however, points to the importance of the selection between elementary species, and the very subordinate role of intraspecific selection in nature. It strongly supports our view of the origin of species by mutation instead of continuous selection. Or, to put it in the terms chosen lately by Mr. Arthur Harris in a friendly criticism of my views: “Natural selection may explain the survival [826] of the fittest, but it cannot explain the arrival of the fittest.”
Notes
* On about 200 ears the variability ranges from 8-22 rows, and this leads approximately to one row more by each doubling of the numbers of instances. One ear with 22 rows in 200 would thus lead to the expectation of one ear with 32 rows in 100,000 ears.