Species and Varieties, Their Origin by Mutation

Lecture V Characters of Retrograde Varieties

Every one admires the luxuriance of garden-flowers, and their diversity of color and form. All parts of the world have contributed to their number and every taste can find its preference among them. New forms produced by the skill of the breeder are introduced every year. This has been done mostly by crossing and intermingling the characters of introduced species of the same genus. In some of the cases the history of our flowers is so old that their hybrid origin is forgotten, as in the case of the pansies. Hybridizations are still going on in other groups on a large scale, and new forms are openly claimed to be of hybrid origin.
     Breeders and amateurs generally have more interest in the results than in the way in which they have been brought about. Excellent flowers and fruit recommend themselves and there seems to be no reason for inquiring [122] about their origin. In some cases the name of the originator may be so widely known that it adds weight to the value of the new form, and therefore may advantageously be coupled with it. The origin and history of the greater part of our garden-flowers, fruits and vegetables are obscure; we see them as they are, and do not know from whence they came. The original habitat for a whole genus or for a species at large, may be known, but questions as to the origin of the single forms, of which it is built up, ordinarily remain unanswered.
     For these reasons we are restricted in most cases to the comparison of the forms before us. This comparison has led to the general use of the term “variety” in opposition to “species.” The larger groups of forms, which are known to have been introduced as such are called species. All forms which by their characters belong to such a species are designated as varieties, irrespective of their systematic relation to the form, considered as the ancestor of the group.
     Hence, we distinguish between “hybrid varieties” and “pure varieties” according to their origin from different parents or from a single line of ancestors. Moreover, in both groups the forms may be propagated by seeds, or in the vegetative way by buds, by grafting or [123] by cutting, and this leads to the distinction of “seed-varieties” and “vegetative varieties.” In the first case the inheritance of the special characters through the seeds decides the status of the variety, in the latter case this point is left wholly out of consideration.
     Leaving aside all these different types, we are concerned here only with the “seed-varieties” of pure origin, or at least with those, that are supposed to be so. Hybridization and vegetative multiplication of the hybrids no doubt occur in nature, but they are very rare, when compared with the ordinary method of propagation by seed. “Seed-varieties” may further be divided into constant and inconstant ones. The difference is very essential, but the test is not always easy to apply. Constant varieties are as sharply defined and as narrowly limited as are the best wild species, while inconstant types are cultivated chiefly on account of their wide range of form and color. This diversity is repeated yearly, even from the purest seed. We will now discuss the constant seed-varieties, leaving the inconstant and eversporting types to a subsequent lecture.
     In this way we may make an exact inquiry into the departures from the species which are ordinarily considered to constitute the essential character of such a constant and pure seed-variety [124] and need only compare these differences with those that distinguish the elementary species of one and the same group from each other.
     Two points are very striking. By far the greatest part of the ordinary garden-varieties differ from their species by a single sharp character only. In derivative cases two, three or even more such characters may be combined in one variety, for instance, a dwarfed variety of the larkspur may at the same time bear white flowers, or even double white flowers, but the individuality of the single characters is not in the least obscured by such combinations.
     The second point is the almost general occurrence of the same variety in extended series of species. White and double flowers, variegated leaves, dwarfs and many other instances may be cited. It is precisely this universal repetition of the same character that strikes us as the essential feature of a variety.
     And again these two characteristics may now be considered separately. Let us begin with the sharpness of the varietal characters. In this respect varieties differ most obviously from elementary species. These are distinguished from their nearest allies in almost all organs. There is no prominent distinctive feature between the single forms of Draba [125] Verna, Helianthemum or of Taraxacum; all characters are almost equally concerned. The elementary species of Draba are characterized, as we have seen, by the forms and the hairiness of the leaves, the number and height of the flower-stalks, the breadth and incision of the petals, the forms of the fruits, and so on. Every one of the two hundred forms included in this collective species has its own type, which it is impossible to express by a single term. Their names are chosen arbitrarily. Quite the contrary is the case with most of the varieties, for which one word ordinarily suffices to express the whole difference.
     White varieties of species with red or blue flowers are the most common instances. If the species has a compound color and if only one of the constituents is lost, partially colored types arise as in Agrostemma Coronaria bicolor. Or the spots may disappear and the color become uniform as in Gentiana punctata concolor and the spotless Arum or Arum maculatum immaculatum. Absence of hairs produces forms as Biscutella laevigata glabra; lack of prickles gives the varieties known as inermis, as for instance, Ranunculus arvensis inermis. Cytisus prostratus has a variety ciliata, and Solanum Dulcamara, or the bitter-sweet, has a variety called tomentosum. The curious monophyllous [126] variety of the strawberry and many other forms will be discussed later.
     To enlarge this list it would only be necessary to extract from a flora, or from a catalogue of horticultural plants, the names of the varieties enumerated therein. In nearly every instance, where true varieties and not elementary species are concerned, a single term expresses the whole character.
     Such a list would also serve to illustrate the second point since the same names would recur frequently. Long lists of varieties are called alba, or inermis, or canescens or lutea, and many genera contain the same appellations. In some instances the systematists use a diversity of names to convey exactly the same idea, as if to conceal the monotony of the character, as for instance in the case of the lack of hairs, which is expressed by the varietal names of Papaver dubium glabrum, Arabis ciliata glabrata, Arabis hirsuta glaberrima, Veronica spicata nitens, Amygdalus persica laevis, Paeonia corallina Leiocarpa, &c.
     On the contrary we find elementary species in different genera based on the greatest possible diversity of features. The forms of Taraxacum or Helianthemum do not repeat those of Draba or Viola. In roses and brambles the distinguishing features are characteristic of the type, as [127] they are evidently derived from it and limited to it. And this is so true that nobody claims the grade of elementary species for white roses or white brambles, but everyone recognizes that forms diverging from the nearest species by a single character only, are to be regarded as varieties.
     This general conviction is the basis on which we may build up a more sharply defined distinction between elementary species and varieties. It is an old rule in systematic botany, that no form is to be constituted a species upon the basis of a single character. All authors agree on this point; specific differences are derived from the totality of the attributes, not from one organ or one quality. This rule is intimately connected with the idea that varieties are derived from species. The species is the typical, really existing form from which the variety has originated by a definite change. In enumerating the different forms the species is distinguished by the term of genuine or typical, often only indicated as a or the first; then follow the varieties sometimes in order of their degree of difference, sometimes simply in alphabetical order. In the case of elementary species there is no real type; no one of them predominates because all are considered to be equal in rank, and the systematic species to which they [128] are referred is not a really existing form, but is the abstraction of the common type of all, just as it is in the case of a genus or of a family.
     Summarizing the main points of this discussion, we find that elementary species are of equal rank and together build up the collective or systematic ideal species. Varieties on the other hand are derived from a real and commonly, still existing type.
     I hope that I have succeeded in showing that the difference between elementary species, or, as they are often called, smaller or subspecies, on the one hand and varieties on the other, is quite a marked one. However, in order to recognize this principle it is necessary to limit the term variety, to those propagating themselves by seed and are of pure and not of hybrid origin.
     But the principle as stated here, does not involve an absolute contrast between two groups of characters. It is more a difference in our knowledge and appreciation of them than a difference in the things themselves. The characters of elementary species are, as a rule, new to us, while those of varieties are old and familiar. It seems to me that this is the essential point.
     And what is it that makes us familiar with them? Obviously the continuous recurrence of the same changes, because by a constant repetition they must of course lose their novelty.
     [129] Presently we shall look into these characters more in detail and then we shall find that they are not so simple as might be supposed at first sight; but precisely because we are so familiar with them, we readily see that their different features really belong to a single character; while in elementary species everything is so new that it is impossible for us to discern the unities of the new attributes.
     If we bear in mind all these difficulties we cannot wonder at the confusion on this question that seems to prevail everywhere. Some authors following Linnaeus simply call all the subdivisions of species, varieties; others follow Jordan and avoid the difficulty by designating all smaller forms directly as species. The ablest systematists prefer to consider the ordinary species as collective groups, calling their constituents “The elements of the species,” as was done by A.P. De Candolle, Alph. De Candolle and Lindley.
     By this method they clearly point out the difference between the subdivisions of wild species as they ordinarily occur, and the varieties in our gardens, which would be very rare, were they not singled out and preserved.
     Our familiarity with a character and our grounds for calling it an old acquaintance may result from two causes, which in judging a new [130] variety are essentially different. The character in question may be present in the given species or it may be lacking, but present in the other group. In the first case a variety can only be formed by the loss of the character, in the second case it arises by the addition of a new one.
     The first mode may be called a negative process, while the second is then to be designated as positive. And as it is more easy to lose what one has than to obtain something new, negative varieties are much more common than are positive ones.
     Let us now take an instance of a character that is apt to vary in both ways, for this is obviously the best way of making clear what is meant by a negative and a positive change.
     In the family of the composites we find a group of genera with two forms of florets on each flower-head. The hermaphrodite ones are tubular with 5, or rarely 4, equal teeth, and occupy the center of the head. These are often called the flosculous florets or disk-florets. Those of the circumference are ligulate and ordinarily unisexual, without stamens. In many cases they are sterile, having only an imperfect ovary. They are large and brightly colored and are generally designated as ray-florets. As instances we may cite the camomile (Anthemis nobilis), the wild camomile (Matricaria Chamomilla), [131] the yarrow (Achillea Millefolium), the daisies, the Dahlia and many others. Species occur in this group of plants from time to time that lack the ray-florets, as in the tansy (Tanacetum vulgare) and some artemisias. And the genus of the marigolds or Bidens is noted for containing both of these types. The smaller and the three-toothed marigold (B. cernua and B. tripartita) are very common plants of wet soil and swamps, ordinarily lacking the ray-florets, and in some countries they are very abundant and wholly constant in this respect, never forming radiate flower-heads. On the other hand the white-flowered and the purple marigold (B. leucantha and B. atropurpurea) are cultivated species of our gardens, prized for their showy flower-heads with large white or deeply colored, nearly black-purple florets.
     Here we have opportunity to observe positive and negative varieties of the same character. The smaller, and the three-toothed marigold occur from time to time, provided with ray florets, showing a positive variation. And the white marigold has produced in our gardens a variety without rays. Such varieties are quite constant, never returning to the old species. Positive and negative varieties of this kind are by no means rare among the compositae.
     [132] In systematic works the positive ones are as a rule called “radiate,” and the negative ones “discoid.” Discoid forms of the ordinary camomile, of the daisy, of some asters (Aster Tripolium), and of some centauries have been described. Radiate forms have been observed in the tansy (Tanacetum vulgare), the common horse-weed or Canada fleabane (Erigeron canadensis) and the common groundsel (Senecio vulgaris). Taken broadly the negative varieties seem to be somewhat more numerous than the positive ones, but it is very difficult to come to a definite conclusion on this point.
     Quite the contrary is the case with regard to the color-varieties of red and blue flowers. Here the loss of color is so common that every one could give long lists of examples of it. Linnaeus himself supposed that no blue or red-colored wild species would be without a white variety. It is well known that he founded his often criticized prescript never to trust to color in recognizing or describing a species, on this belief.
     On the other hand there are some red varieties of white-flowered species. But they are very rare, and little is known about their characters or constancy. Blue varieties of white species are not found. The yarrow (Achillea Millefolium) has a red-flowered form, which occurs [133] from time to time in sunny and sandy localities. I have isolated it and cultivated it during a series of years and during many generations. It is quite true to its character, but the degree of its coloring fluctuates between pink and white and is extremely variable. Perhaps it can be considered as an inconstant variety. A redflowered form of the common Begonia semperflorens is cultivated under the name of “Vernon,” the white hawthorn (Crataegus Oxyacantha) is often seen with red flowers, and a pink-flowered variety of the “Silverchain” or “Bastard acacia” (Robinia Pseud-Acacia) is not rarely cultivated. The “Crown” variety of the yellow wall-flower and the black varieties, are also to be considered as positive color variations, the black being due in the latter cases to a very great amount of the red pigment.
     Among fruits there are also some positive red varieties of greenish or yellowish species, as for instance the red gooseberry (Ribes Grossularia) and the red oranges. The red hue is far more common in leaves, as seen among herbs, in cultivated varieties of Coleus and in the brown leaved form of the ordinary white clover, among trees and shrubs in the hazelnut (Corylus), the beech (Fagus), the birch (Betula), the barberry (Berberis) and many others. But though most of these forms are very ornamental and abundant [134] in parks and gardens, little is as yet known concerning the origin of their varietal attributes and their constancy, when propagated by seeds. Besides the ray-florets and the colors, there are of course a great many other characters in which varieties may differ from their species. In most of the cases it is easy to discern whether the new character is a positive or a negative one. And it is not at all necessary to scrutinize very narrowly the list of forms to become convinced that the negative form is the one which prevails nearly everywhere, and that positive aberrations are in a general sense so rare that they might even be taken for exceptions to the rule.
     Many organs and many qualities may be lost in the origination of a variety. In some instances the petals may disappear, as in Nigella, or the stamens, as in the Guelder-rose (Viburnum Opulus) and the Hortensia and in some bulbs even the whole flowers may be wanting, as in the beautiful “Plumosa” form of the cultivated grape-hyacinth or Muscari comosum. Fruits of the pineapples and bananas without seeds are on record as well as some varieties of apples and pears, of raisins and oranges. And some years ago Mr. Riviere of Algeria described a date growing in his garden that forms fruit without pits. The stoneless plum of Mr. [135] Burbank of Santa Rosa, California, is also a very curious variety, the kernel of which is fully developed but naked, no hard substance intervening between it and the pulp.
     More curious still are the unbranched varieties consisting of a single stem, as may be seen sometimes in the corn or maize and in the fir. Fir-trees of some three or four meters in height without a single branch, wholly naked and bearing leaves only on the shoots of the last year’s growth at the apex of the tree, may be seen. Of course they cannot bear seed, and so it is with the sterile maize, which never produces any seed-spikes or staminate flowers. Other seedless varieties can be propagated by buds; their origin is in most cases unknown, and we are not sure as to whether they should be classified with the constant or with the inconstant varieties.
     A very curious loss is that of starch in the grains of the sugar-corn and the sugar-peas. It is replaced by sugar or some allied substance (dextrine). Equally remarkable is the loss of the runners in the so-called “Gaillon” strawberries.
     Among trees the pendulous or weeping, and the broomlike or fastigiate forms are very marked varieties, which occur in species belonging to quite different orders. The ash, the beach, some willows, many other trees and some [136] finer species of garden-plants, as Sophora japonica, have given rise to weeping varieties, and the yew-tree or Taxus has a fastigiate form which is much valued because of its ascending branches and pyramidal habit. So it is with the pyramidal varieties of oaks, elms, the bastard-acacia and some others.
     It is generally acknowledged that these forms are to be considered as varieties on the ground of their occurrence in so wide a range of species, and because they always bear the same attributes. The pendulous forms owe their peculiarity to a lengthening of the branches and a loss of their habit of growing upwards; they are too weak to retain a vertical position and the response to gravity, which is ordinarily the cause of the upright growth, is lacking in them. As far as we know, the cause of this weeping habit is the same in all instances. The fastigiate trees and shrubs are a counterpart of the weeping forms. Here the tendency to grow in a horizontal direction is lacking, and with it the bilateral and symmetric structure of the branches has disappeared. In the ordinary yew-tree the upright stem bears its needles equally distributed around its circumference, but on the branches the needles are inserted in two rows, one to the left and one to the right. All the needles turn their upper surfaces upwards, [137] and their lower surfaces downwards, and all of them are by this means placed in a single horizontal plane, and branching takes place in the same plane. Evidently this general arrangement is another response to gravity, and it is the failure of this reaction which induces the branches to grow upwards and to behave like stems.
     Both weeping and fastigiate characters are therefore to be regarded as steps in a negative direction, and it is highly important that even such marked departures occur without transitions or intermediate forms. If these should occur, though ever so rarely, they would probably have been brought to notice, on account of the great prospect the numerous instances would offer. The fact that they are lacking, proves that the steps, though apparently great, are in reality to be considered as covering single units, that cannot be divided into smaller parts. Unfortunately we are still in the dark as to the question of the inheritance of these forms, since in most cases it is difficult to obtain pure seed.
     We now consider the cases of the loss of superficial organs, of which the nectarines are example. These are smooth peaches, lacking the soft hairy down, that is a marked peculiarity of the true peaches. They occur in different [138] races of the peach. As early as the beginning of the past century, Gallesio described no less than eight subvarieties of nectarines, each related to a definite race of peach. Most of them reproduce themselves truly from seed, as is well known in this country concerning the clingstones, freestones and some other types. Nectarines have often varied, giving rise to new sorts, as in the case of the white nectarine and many others differing greatly in appearance and flavor. On the other hand it is to be remarked, that the trees do not differ in other respects and cannot be distinguished while young, the varietal mark being limited to the loss of the down on the fruit. Peaches have been known to produce nectarines, and nectarines to yield true peaches. Here we have another instance of positive and negative steps with reference to the same character, but I cannot withhold an expression of some doubt as to the possibility of crossing and subsequently splitting up of the hybrids as a more probable explanation of at least some of the cases quoted by various writers.
     Smooth or glabrous varieties often occur, and some of them have already been cited as instances of the multiplication of varietal names. Positive aberrations are rather rare, and are mostly restricted to a greater density of the [139] pubescence in some hairy species, as in Galeopsis Ladanum canescens, Lotus corniculatus hirsutus and so on. But Veronica scutellata is smooth and has a pubescent variety, and Cytisus prostratus and C. spinescens are each recorded to have a ciliate form.
     Comparable with the occurrence and the lack of hairs, is the existence or deficiency of the glaucous effect in leaves, as is well known in the common Ricinus. Here the glaucous appearance is due to wax distributed in fine particles over the surface of the leaves, and in the green variety this wax is lacking. Other instances could be given as in the green varieties of Papaver alpinum and Rumex scutatus. No positive instances are recorded in this case.
     Spines and prickles may often disappear and give rise to unarmed and defenceless types. Of the thorn-apples both species, the whiteflowered Datura Stramonium and the purple D. Tatula have such varieties. Spinach has a variety called the “Dutch,” which lacks the prickles of the fruit; it is a very old form and absolutely constant, as are also the thornless thorn-apples. Last year a very curious instance of a partial loss of prickles was discovered by Mr. Cockerell of East Las Vegas in New Mexico. It is a variety of the American cocklebur, often called sea-burdock, or the [140] hedgehog-burweed, a stout and common weed of the western states. Its Latin name is Xanthium canadense or X. commune and the form referred to is named by Mr. Cockerell, X. Wootoni, in honor of Professor E.o. Wooton who described the first collected specimens.
     The burs of the common species are densely covered with long prickles, which are slightly hooked at the apex. In the new form, which is similar in all other respects to the common cocklebur, the burs are more slender and the prickles much less numerous, about 25 to the bur and mostly stouter at the base. It occurs abundantly in New Mexico, always growing with the common species, and seems to be quite constant from seed. Mr. Cockerell kindly sent me some burs of both forms, and from these I raised in my garden last year a nice lot of the common, as well as of the Wootoni plants.
     Spineless varieties are recorded for the bastard-acacia, the holly and the garden gooseberry (Ribes Grossularia, or R. Uva-crispa). A spineless sport of the prickly Broom (Ulex europaeus) has been seen from time to time, but it has not been propagated.
     Summarizing the foregoing facts, we have excellent evidence of varieties being produced either by the loss of some marked peculiarity or by the acquisition of others that are already [141] present in allied species. There are a great many cases however, in which the morphologic cause of the dissimilarity is not so easily discerned. But there is no reason to doubt that most of them will be found to conform to the rule on closer investigation. Therefore we can consider the following as the principal difference between elementary species and varieties; that the first arise by the acquisition of entirely new characters, and the latter by the loss of existing qualities or by the gain of such peculiarities as may already be seen in other allied species.
     If we suppose elementary species and varieties originated by sudden leaps or mutations, then the elementary species have mutated in the line of progression, some varieties have mutated in the line of retrogression, while others have diverged from their parental types in a line of depression, or in the way of repetition. This conception agrees quite well with the current idea that in the building up of the vegetable kingdom according to the theory of descent, it is species that form the links of the chain from the lower forms to the more highly organized later derivatives. Otherwise expressed, the system is built up of species, and varieties are only local and lateral, but never of real importance for the whole structure.
     [142] Heretofore we have generally assumed, that varieties differ from the parent-species in a single character only, or at least that only one need be considered. We now come to the study of those varieties, which differ in more than one character. Of these there are two types. In the first the points of dissimilarity are intimately connected with one another, in the second they are more or less independent.
     The mutually related peculiarities may be termed correlative, and we therefore speak, in such cases, of correlative variability. This phenomenon is of the highest importance and is of general occurrence. But before describing some examples, it is as well to note that in the lecture on fluctuating variability, cases of a totally different nature will be dealt with, which unfortunately are designated by the same term. Such merely fluctuating variations are therefore to be left out of the present discussion.
     The purple thorn-apple, which is considered by some writers as a variety of the white-flowered species or Datura Stramonium, and by others as a separate species, D. Tatula, will serve as an illustration. But as its distinguishing attributes, as far as we are concerned with them here, are of the nature described above as characteristic of varietal peculiarities no objection [143] can be made to our using them as a case of correlative variability.
     The essential character of the purple thornapple lies in the color of the flowers, which are of a very beautiful pale blue. But this color is not limited to the corolla. It is also to be seen in the stems and in the stalks and veins of the leaves, which are stained with a deep purple, the blue color being added to the original green. Even on the surface of the leaves it may spread into a purplish hue. On the stems it is to be met with everywhere, and even the young seedlings show it. This is of some importance, as the young plants when unfolding their cotyledons and primary leaves, may be distinguished by this means from the seedlings of the white flowered species.
     In crossing experiments it is therefore possible to distinguish the whites and the blues, even in young seedlings, and experience shows that the correlation is quite constant. The color can always be relied upon; if lacking in the seedlings, it will be lacking in the stems and flowers also; but if the axis of the young plant is ever so slightly tinged, the color will show itself in its beauty in the later stages of the life of the plant.
     This is what we term correlation. The colors of the different organs are always in agreement. It is true that they require the concurrence of [144] light for development, and that in the dark or in a faint light the seedlings are apt to remain green when they should become purple, but aside from such consideration all organs always come true to their color, whether pure green and white, or whether these are combined with the blue tinge. This constancy is so absolute that the colors of the different organs convey the suggestion, that they are only separate marks of a single character.
     It is on this suggestion that we must work, as it indicates the cause of the correlation. Once present, the faculty of producing the anthocyan, the color in question, will come into activity wherever and whenever opportunity presents itself. It is the cell-sap of the ordinary cell tissue or parenchyma, which is colored by the anthocyan, and for this reason all organs possessing this tissue, may exhibit the color in question.
     Thus the color is not a character belonging to any single organ or cell, nor is it bound to a morphologic unit; it is a free, physiologic quality. It is not localized, but belongs to the entire plant. If we wish to assume for its basis material representative particles, these particles must be supposed to be diffused throughout the whole body of the plant.
     This conception of a physiologic unit as the [145] cause of colors and other qualities is evidently opposed to the current idea of the cells and tissues as the morphologic units of the plants. But I do not doubt, that in the long run it will recommend itself as much to the scientist as to the breeder. For the breeder, when desiring to keep his varieties up to their standard, or when breeding to a definite idea, obviously keeps his standard and his ideal for the whole plant, even if he breeds only for flowers or for fruit.
     I have chosen the color of the purple thornapple as a first example, but the colors of other plants show so many diverging aspects, all pointing so clearly to the same conclusion, that it would be well to take a more extensive view of this interesting subject.
     First we must consider the correlation in the colors of flowers and fruits. If both are colored in the species, whether red or brown or purple or nearly black, and a variety lacking this hue is known, it will be lacking in both organs. If the color is pure, the flowers and berries will become white, but such cases are rare. Ordinarily a yellowish or greenish tinge underlies the ornamental color, and if this latter disappears, the yellowish ground will become manifest. So for instance in the Belladonna, a beautiful perennial herb with great shiny black, but very poisonous, fruits. Its flowers are brown, but in [146] some woods a variety with greenish flowers and bright yellow berries occurs, which is also frequently seen in botanic gardens. The anthocyan dye is lacking in both organs, and the same is the case with the stems and the leaves. The lady’s laurel or Daphne Mezereum has red corollas, purple leaves and red fruits; its white flowered variety may be distinguished by lack of the red hue in the stems and leaves, and by their beautiful yellow berries. Many other instances could be given, since the loss of color in berries is a very common occurrence, so common that for instance, in the heath-family or Ericaceae, with only a few exceptions, all berry-bearing species have white-fruited varieties.
     The same correlation is observed in the seeds. The white-flowered flax may be seen to yield yellow and not brown seeds as in the blue species. Many varieties of flowers may be recognized by the color of their seeds, as in the poppies, stocks and others. Other white-flowered varieties may be distinguished when germinating, their young axes being of a pure instead of a purplish green. It is a test ordinarily used by gardeners, to purify their flower beds long before the blooming time, when thinning or weeding them. Even in wild plants, as in Erodium, Calluna, Brunella and others, a botanist may recognize the rare white-flowered [147] variety by the pure green color of the leaves, at times when it is not in flower. Some sorts of peas bear colored flowers and a red mark on the stipules of their leaves. Among bulbous plants many varieties may be recognized even in the dry bulbs by the different tinges of the outer scales.
     Leaving the colors, we come now to another instance of correlation, which is still more astonishing. For it is as rare, as color-varieties are common. It is afforded by some plants the leaves of which, instead of being entire or only divided into large parts, are cleft to a greater extent by repeated fissures of the marginal lobes. Such foliar variations are often seen in gardens, where they are cultivated for their beauty or singularity, as the laciniated alders, fern-leaved, beeches and limes, oakleaved laburnums, etc. Many of them are described under the varietal name of laciniata. In some cases this fissure extends to the petals of the flowers, and changes them in a way quite analogous to the aberrancy of the leaves.
     This is known to occur with a variety of brambles, and is often seen in botanic gardens in one of the oldest and most interesting of all anomalies, the laciniated variety of the greater celandine or Chelidonium majus. Many other instances could be given. Most of them belong to the [148] group of negative variations, as we have defined them. But the same thing occurs also with positive varieties, though of course, such cases are very rare. The best known instance is that of the ever-flowering begonia, Begonia semperflorens, which has green leaves and white flowers, but which has produced garden varieties with a brown foliage and pink flowers. Here also the new quality manifests itself in different organs.
     Enough has now been said on correlative changes, to convince us that they are as a rule to be considered as the expression of some general internal or physiologic quality, which is not limited to a single organ, but affects all parts of the organism, provided they are capable of undergoing the change. Such characters are therefore to be considered as units, and should be referred to the group of single characters.
     Opposed to these are the true compound characters, which consist of different units. These may be segregated by the production of varieties, and thereby betray the separate factors of the complex group.
     The most beautiful instances of such complex characters are offered by the colors of some of the most prized garden-flowers. Rarely these are of a single hue, often two or three shades contribute to the effect, and in some cases special [149] spots or lines or tracings are to be seen on a white or on a colored background. That such spots and lines are separate units is obvious and is demonstrated by the fact that sometimes spotless varieties occur, which in all other respects have kept the colors of the species. The complexity of the color is equally evident, whenever it is built up of constituents of the anthocyan and of the yellow group. The anthocyan dye is limited to the sap-cavity of the cells, while the yellow and pure orange colors are fixed in special organs of the protoplasm. The observation under the microscope shows at once the different units, which though lying in the same cell and in almost immediate vicinity of each other are always wholly separated from one another by the wall of the vacuole or sapfilled cell-cavity.
     The combination of red and yellow gives a brown tinge, as in the cultivated wall-flower, or those bright hues of a dark orange-red, which are so much sought in tulips. By putting such flowers for a short time in boiling water, the cells die and release the red pigment, which becomes diffused in the surrounding fluids and the petals are left behind with their yellow tinge. In this way it is easy to separate the constituents, and demonstrate the compound nature of the original colors.
     [150] But the diversity of the color patterns is far from being exhausted with these simple instances. Apart from them, or joined to them, other complications are frequently seen, which it is impossible to analyze in such an artificial way. Here we have to return to our former principle, the comparison of different varieties. Assuming that single units may be lost, irrespective of the others, we may expect to find them segregated by variation, wherever a sufficiently wide range of color-varieties is in cultivation. In fact, in most cases a high degree of dissimilarity may be reached in the simplest way by such a separation of the components, and by their combination into most diverse smaller groups. A very nice instance of such an analysis of flower-colors is afforded by the ordinary snapdragon. The beautiful brown red color of this common garden-plant is composed on one side of yellow elements, on the other of red units. Of the yellow there are two, one staining the whole corolla with a light hue, as is to be seen in the pure yellow variety called luteum. This form has been produced by the loss of the whole group of the red constituents. If the yellow tinge is also lost, there arises a white variety, but this is not absolutely colorless, but shows the other yellow constituent. This last stains only some small parts [151] of the lips of the flower around the throat, brightening, as it seems, the entrance for the visiting insects. In many of the red or reddish varieties this one yellow patch remains, while the general yellow hue fails. In the variety called “Brilliant” the yellow ground makes the red color more shiny, and if it is absent the pure carmine tinge predominates.
     It is readily seen, that in the ordinary form the lips are of a darker red than the tube. This evident dissimilarity indicates some complexity. And in fact we have two varieties which exhibit the two causes of this attribute separately. One of them is called “Delila,” and has the red color limited to the lips, whilst the tube is pure white. The other is called “Fleshy,” and is of a pale pink throughout the whole corolla. Adding these two units to one another, we get the original dark red of the wild type, and it may be briefly stated here, that the way of effecting such an addition is given us in the crossing of the “Fleshy” and the “Delila” variety, the hybrid showing the two colors and returning thereby to the old prototype.
     Other cases of compound flower colors or of color patterns might be given as in the Mimulus and the poppy, and in most of these cases some varieties are to be seen in our gardens which show only the single constituents of the group.
     [152] Many dark flowers have an intermediate bright hued form besides the white variety, as in the case of roses, asters, Nicandra and so on.
     Intermediate forms with respect to stature may also be seen. The opium-poppy, the snapdragon, peas, the Nicandra, and many other garden-plants have not only dwarf varieties, but also some of intermediate height. These, though they are intermediate between the tall and dwarf types, cannot be considered as transitions, as between them and the extremes, intermediates are, as a rule wholly lacking. Instances of the same occurrence of three types may be seen in the seeds of maize (“Cuzco,” “Horse-dent” and “Gracillima”) of beans and some other plants. The Xanthium Wootoni, above referred to, with only part of the prickles of Xanthium commune is also a very curious instance of the demonstration of the compound nature of a character.
     Summarizing the conclusions that may be drawn from the evidence given in this lecture, we have seen that varieties differ from elementary species in that they do not possess anything really new. They originate for the greater part in a negative way, by the apparent loss of some quality, and rarely in a positive manner by acquiring a character, already seen in allied species. These characters are not of the nature of [153] morphologic entities, but are to be considered as physiologic units, present in all parts of the organisms, and manifesting themselves where ever occasion is afforded. They are units in the sense that they may appear and disappear singly. But very often they are combined to yield compound characters, which are capable of analysis. Opportunities for such an analysis are afforded by these groups of cultivated varieties, of which some members show a single distinguishing quality, or a number of them.

[154]

LECTURE VI

STABILITY AND REAL ATAVISM

It is generally believed that varieties are principally distinguished from species by their inconstancy. This conception is derived from some special cases and transferred to others, and in its common form this belief must have originated from the confusion which exists as to the meaning of the term variety. It is true that vegetative varieties as a rule run back, when propagated by seeds; they are an obvious instance of inconstancy. In the second place we have considered the group of inconstant or sporting varieties, which of course we must exclude when studying the stability of other types. However, even these sporting varieties are unstable only to a certain degree, and in a broader sense will prove to be as true to their character as the most constant types.
     Having separated these two groups, which include also the wide range of hybrid forms, we may next consider only those varieties of pure origin, and ordinarily propagated by seeds, [155] which have been discussed in former chapters. Their general character lies in their fidelity to type, and in the fact that this is single, and not double, as in the sporting varieties.
     But the current belief is, that they are only true to their peculiarities to a certain degree, and that from time to time, and not rarely, they revert to the type from which they have arisen. Such reversion is supposed to prove that they are mere varieties, and at the same time to indicate empirically the species from which they have sprung.
     In the next lecture we shall examine critically the evidence on which this assumption rests. Before doing so however, it will be necessary to collate the cases in which there is no reversion at all, or in which the reversion is absent at least in experimental and pure sowings.
     In the present state of our knowledge it is very difficult to decide, whether or not true reversion occurs in constant varieties. If it does occur, it surely does so very rarely and only under unusual circumstances, or in particular individuals. However when such individuals are multiplied by buds and especially when they are the only representatives of their type, the reversion, though theoretically rare, will be shown by nearly every specimen of the variety. Examples of this will be given below.
     [156] They are generally called atavists or reversionists, but even these terms are sometimes used in a different sense.
     Lastly it is to be said that the empirical and experimental evidence as to the question of constancy is not as extensive as it should be. The experimental conditions are seldom described, and it is only recently that an interest in the matter has been awakened. Much remains to be done. Among other things the innumerable varieties of trees, shrubs and perennial herbs should be tested as to their constancy when grown from purely fertilized seeds. Many of them may be included among the number that sport constantly.
     Leaving aside the doubtful or insufficiently studied cases, we may now turn our attention to the facts that prove the absolute stability of a large number of varieties, at least as far as such completeness can be attained by experiment or observation.
     The best proof is afforded by the varieties which grow wild in localities where they are quite isolated from the species, and where for this reason, no possibility of crossing disturbs the significance of the proof. As one instance the rayless form of the wild camomile, or the Matricaria Chamomilla discoidea may be mentioned. Many systematists have been so strongly [157] impressed with its absolute constancy and its behavior as an ordinary species, that they have elevated it, as it is called, to the rank of a species. As such it is described under the name of Matricaria discoidea DC. It is remarkable for its rapid and widespread distribution, as of late years it has become naturalized in different parts of America and of Europe, where it is to be seen especially in France and in Norway. Experimentally I raised in succeeding years between 1000 and 2000 seedlings, but observed no trace of reversion, either in the strongest or in the numerous very small and weak individuals which appeared in the cultures.
     The tansy-ragwort or Senecio Jacobaea may be chosen as a second instance. It is a perennial herb with short rootstocks and stout stems bearing numerous short-peduncled heads in large compact corymb; it multiplies itself abundantly by seeds and is very common on the sand dunes of Holland. It has two forms, differing only in the occurrence or the lack of the ray florets. But these two varieties occupy different localities and are even limited to different provinces. As far as I have been able to ascertain on numerous excursions during a series of years, they never sport, and are only intermingled on the outskirts of their habitats. The rayless form is generally considered as the [158] variety but it is quite as stable as the radiate species.
     The radiate varieties of marigold, quoted in a former lecture, seem to be equally constant, when growing far away from their prototypes. I sowed the seeds of a single plant of the radiate form of Bidens cernua, and found all of the seedlings came true, and in the next year I had from their seed between 2,000 and 3,000 flowering individuals, all equally radiate. Many species of composites have been tried, and they are all constant. On the other hand rare sports of this kind have been observed by Murr and other authors.
     Many kinds of vegetables and of fruits give instances of stability. White strawberries, green grapes, white currants, crisped lettuce, crisped parsley and some other crisped forms may be cited. The spinage without prickles is a widely known instance. White-flowered flax never reverts to the blue prototype, if kept pure. Sugar-peas and sugar-corn afford further instances. Strawberries without runners have come true from seed ever since their first appearance, over a hundred years ago.
     Many garden-varieties, the stability of which under ordinary circumstances is doubtful, because of their being sown too close to other varieties of the same species, have been tested in [159] respect to their stability by different writers and at different times. In doing this it is plain that it is very essential to be sure of the purity of the seed. Specimens must be grown in positions isolated from their allies, and if possible be pollinated artificially with the exclusion of the visits of insects. This may be done in different ways. If it is a rare species, not cultivated in the neighborhood, it is often sufficient to make sure of this fact. Pollen may be conveyed by bees from distances of some ten or twenty meters, or in rare cases from some hundred meters and more, but a greater distance is ordinarily sufficient for isolation. If the flowers fertilize themselves, as is more often the case than is generally supposed, or if it is easy to pollinate them artificially, with their own pollen or in small groups of similar individuals, the best way is to isolate them by means of close coverings. When flowering, the plants are as a rule too large to be put under bell-glasses, and moreover such coverings would keep the air moist, and cause the flower-buds to be thrown off. The best coverings are of netting, or of canvas of sufficiently wide mesh, although after a long experience I greatly prefer cages of fine iron-wire, which are put around and over the whole plant or group of plants, and fastened securely and tightly to the ground.
     [160] Paper bags also may be made use of. They are slipped over the flowering branches, and bound together around the twigs, thus enclosing the flowers. It is necessary to use prepared papers, in order that they may resist rain and wind. The best sort, and the one that I use almost exclusively in my fertilization-experiments, is made of parchment-paper. This is a wood-pulp preparation, freed artificially from the so-called wood-substance or lignin. Having covered the flowers with care, and having gathered the seeds free from intermixtures and if possible separately for each single individual, it only remains to sow them in quantities that will yield the greatest possible number of individuals. Reversions are supposed to be rare and small groups of seedlings of course would not suffice to bring them to light. Only sowings of many hundreds or thousands of individuals are decisive. Such sowings can be made in one year, or can be extended over a series of years and of generations. Hildebrand and Hoffman have preferred the last method, and so did Hofmeister and many others. Hildebrand sowed the white hyacinth, and the white varieties of the larkspur, the stock and the sweet pea. Hoffman cultivated the white flax and many other varieties and Hofmeister extended his sowings [161] over thirty years with the white variety of the yellow foxglove (Digitalis parviflora). White-flowered varieties of perennial garden plants were used in my own experiments. I bought the plants, flowered them under isolation in the way described above, gathered the seeds from each individual separately and sowed them in isolated groups, keeping many hundreds and in some cases above a thousand plants up to the time of flowering. Among them I found only one inconstant variety, the white form of the yellow columbine, Aquilegia chrysantha. It evidently belonged to the group of sporting varieties already referred to. All others came absolutely true to type without any exception. The species experimented with, were Campanula persicifolia, Hyssopus officinalis, Lobelia syphilitica, Lychnis chalcedonica, Polemonium dissectum, Salvia sylvestris and some others. Tested in the same way I found the white varieties of the following annual plants also quite true: Chrysanthemum coronarium, Godetia amoena, Linum usitatissimum, Phlox drummondi, and Silene Armeria. To these may be added the white hemlock stork’s-bill (Erodium cicutarium album) which grows very abundantly in some parts of my fatherland, and is easily recognizable by its pure green leaves and stems, even when not flowering. I cultivated it, in large numbers [162] during five succeeding generations, but was never able to find even the slightest indication of a reversion to the red prototype. The scarlet pimpernel or Anagallis arvensis has a blue variety which is absolutely constant. Even in Britton and Brown’s “Flora,” which rarely enumerates varieties, it is mentioned as being probably a distinct species. Eight hundred blooming seedlings were obtained from isolated parents, all of the same blue color. The New Zealand spinage (Tetragonia expansa) has a greenish and a brownish variety, the red color extending over the whole foliage, including the stems and the branches. I have tried both of them during several years, and they never sported into each other. I raised more than 5,000 seedlings, from the different seeds of one lot of the green variety in succeeding years, but neither those germinating in the first year, nor the others coming into activity after two, three or four years of repose gave any sign of the red color of the original species.
     It is an old custom to designate intermediate forms as hybrids, especially when both the types are widely known and the intermediates rare. Many persons believe that in doing so, they are giving an explanation of the rarer forms. But since the laws of hybridism are coming to be known we shall have to break with [163] all such usages. So for instance there are numerous flowers which are of a dark red or a dark blue color, and which, besides a white variety, have a pink or a pale blue form. Such pale varieties are of exactly the same value as others, and on testing they are found to be equally stable. So for instance the pink variety of the Sweet William (Silene Armeria rosea), the Clarkia pulchella carnea and the pale variety of the corn-cockle, called usually Agrostemma Githago nicaeensis or even simply A. nicaeensis. The latter variety I found pure during ten succeeding generations. Another notable stable intermediate form is the poppy bearing the Danish flag (Papaver somniferum Danebrog). It is an old variety, and absolutely pure when cultivated separately. A long list of other instances might easily be given.
     Many garden-varieties, that are still universally prized and cultivated are very old. It is curious to note how often such forms have been introduced as novelties. The common foxglove is one of the best examples. It has a monstrous variety, which is very showy because it bears on the summit of its raceme and branches, large erect cup-shaped flowers, which have quite a different aspect from the normal thimbleshaped side-blossoms. These flowers are ordinarily described as belonging to the anomaly [164] known as “peloria,” or regular form of a normally symmetric type; they are large and irregular on the stems and the vigorous branches but slender and quinate on the weaker twigs. Their beauty and highly interesting anomalous character has been the cause of their being described many times, and nearly always as a novelty; they have been recently re-introduced into horticulture as such, though they were already cultivated before the middle of the last century. About that time very good descriptions with plates were published in the journal “Flora” by Vrolik, but afterwards they seem to have been forgotten. The peloric variety of the foxglove always comes true from seed, though in the strict sense of the word which we have chosen for our discussion, it does not seem to be a constant and pure variety.
     It is very interesting to compare old botanical books, or even old drawings and engravings containing figures of anomalous plants. The celebrated Pinacothec of Munich contains an old picture by Holbein (1495-1543) representing St. Sebastian in a flower-garden. Of the plants many are clearly recognizable, and among others there is one of the “one-leaved” variety of the strawberry, which may still be met with in botanical gardens. In the year 1671 a Dutch botanist, Abraham Munting published [165] a large volume on garden-plants, containing a great number of very good engravings. Most of them of course show normal plants, but intermixed with these are varieties, that are still in cultivation and therefore must be at least two centuries old. Others, though not figured, are easily recognized by their names and descriptions. The cockscomb is the most widely known, but many white or double flowered varieties were already cultivated at that time. The striped Jalappa, the crested Sedum, the fasciated crown-imperial, white strawberries, red gooseberries and many others were known to Munting.
     Some varieties are as old as culture itself, and it is generally known that the Romans cultivated the white form of the opium-poppy and used the foliage of the red variety of the sugarbeet as a vegetable.
     In our time flowers and fruits are changing nearly as rapidly as the fancies and tastes of men. Every year new forms are introduced and usurp the place of older ones. Many are soon forgotten. But if we look at old country gardens, a goodly number of fine and valued old sorts are still to be found. It would be worth while to make special collections of living plants of old varieties, which surely would be a good and interesting work and bring about a conviction [166] of the stability of pure strains. Coming now to the other side of the question, we may consider those cases of reversion which have been recorded from time to time, and which always have been considered as direct proofs of the varietal character of the reverting form. Reversion means the falling back or returning to another type, and the word itself expresses the idea that this latter type is the form from which the variety has arisen.
     Some instances of atavism of this kind are well known, as they are often repeated by individuals that are multiplied by buds or by grafting. Before looking attentively into the different features of the many cases of rare reversions it will be advisable to quote a few examples.
     The flowering-currant of the Pacific Coast or North American scarlet ribes (Ribes sanguineum), a very popular ornamental shrub, will serve as a good example. It is prized because of its brilliant red racemes of flowers which blossom early in the spring, before the appearance of the leaves. From this species a white form has arisen, which is an old and widely cultivated one, but not so highly prized because of its pale flowers. These are not of a pure white, but have retained a faint reddish hue. The young twigs and the stalks of the [167] leaves afford an instance of correlated variability since in the species the red color shows itself clearly mixed with the green, while in the variety this tinge is wholly wanting.
     Occasionally this white-flowered currant reverts back to the original red type and the reversion takes place in the bud. One or two buds on a shrub bearing perhaps a thousand bunches of white flowers produce twigs and leaves in which the red pigment is noticeable and the flowers of which become brightly colored. If such a twig is left on the shrub, it may grow further, ramify and evolve into a larger group of branches. All of them keep true to the old type. Once reverted, the branches remain forever atavistic. It is a very curious sight, these small groups of red branches among the many white ones. And for this reason attention is often called to it, and more than once I myself have had the opportunity of noting its peculiarities. It seems quite certain that by planting such shrubs in a garden, we may rely upon seeing sooner or later some new buds reverting to the prototype.
     Very little attention seems hitherto to have been given to this curious phenomenon, though in many respects it deserves a closer investigation. The variety is said to have originated from seed in Scotland, many years ago, and [168] seems to be propagated only by cuttings or by grafting. If this is true, all specimens must be considered as constituting together only one individual, notwithstanding their wide distribution in the gardens and parks of so many countries. This induces me to suppose, that the tendency to reversion is not a character of the variety as such, but rather a peculiarity of this one individual. In other words it seems probable that when the whitish variety arises a second time from the red species, it is not at all necessary that it should exhibit this same tendency to revert. Or to put it still in another way, I think that we may suppose that a variety, which might be produced repeatedly from the same original stock, would only in rare individuals have a tendency to revert, and in most cases would be as absolutely constant as the species itself.
     Such a conception would give us a distinct insight into the cause of the rarity of these reversions. Many varieties of shrubs and trees have originated but once or twice. Most of them must therefore, if our supposition is correct, be expected to be stable and only a few may be expected to be liable to reversions.
     Among the conifers many very good cases of reversions by buds are to be found in gardens and glasshouses. They behave exactly like the whitish currant. But as the varietal characters [169] are chiefly found in the foliage and in the branches, these aberrations are to be seen on the plants during the whole year. Moreover they are in some cases much more numerous than in the first instance. The Cryptomeria of Japan has a variety with twigs resembling ropes. This is not caused by a twisting, but only by a curvature of the needles in such a way that they seem to grow in spiral lines around the twigs. This variety often reverts to the type with widely spread, straight needles. And on many a specimen four, five, or more reverted branches may be seen on different parts of the same shrub. Still more widely cultivated is the shrub called Cephalotaxus pedunculata fastigiata, and more commonly known under its old name of Podocarpus koraiana. It is the broomlike variety of a species, nearly allied to the common American and European species of yew, (Taxus minor and T. baccata). It is a low shrub, with broadly linear leaves of a clear green. In the species the leaves are arranged in two rows, one to the left and one to the right of the horizontally growing and widely spreading branches. In the variety the branches are erect and the leaves inserted on all sides. When sporting, it returns to the bilateral prototype and flat wings of fan-shaped twigs are produced laterally on its dense broom-like tufts.
     [170] Wherever this variety is cultivated the same reversion may be seen; it is produced abundantly, and even under seemingly normal circumstances. But as in the case of the Ribes all the specimens are derived by buds from a single original plant. The variety was introduced from Japan about the year 1860, but is probably much older. Nothing is known as to its real origin. It never bears flowers or fruits. It is curious to note that the analogous variety of the European yew, Taxus baccata fastigiata, though much more commonly cultivated than the Cephalotaxus, never reverts, at least as far as I have been able to ascertain. This clearly corroborates the explanation given above.
     After considering these rare instances of more widely known reversions, we may now examine the question of atavism from a broader point of view. But in doing so it should once more be remembered, that all cases of hybridism and also all varieties sporting annually or frequently, are to be wholly excluded. Only the very rare occurrence of instances of atavism in varieties that are for the rest known to be absolutely constant, is to be considered.
     Atavism or reversion is the falling back to a prototype. But what is a prototype? We may take the word in a physiologic or in a systematic sense. Physiologically the signification is a [171] very narrowly restricted one; and includes only those ancestors from which a form is known to have been derived. But such evidence is of course historic. If a variety has been observed to spring from a definite species, and if the circumstances have been sufficiently ascertained not to leave the slightest doubt as to its pure origin, and if moreover all the evidence has been duly recorded, we may say that the origin of the variety is historically known. In most cases we must be content with the testimony, given somewhat later, and recorded after the new variety had the opportunity of showing its greater merits.
     If it now happens that such a variety of recorded origin should occasionally revert to its parent-species, we have all we can wish for, in the way of a thoroughly proved case of atavism. But such instances are very rare, as the birth of most varieties has only been very imperfectly controlled.
     Next to this comes the systematic relation of a variety to its species. The historic origin of the variety may be obscure, or may simply be forgotten. But the distinguishing marks are of the order described in our last lecture, either in the positive or in the negative direction, and on this ground the rarer form is considered to be a variety of the more wide-spread one. If [172] now the presumed variety sports and runs over to the presumed type, the probability of the supposed relation is evidently enhanced. But it is manifest that the explanation rests upon the results of comparative studies, and not upon direct observations of the phenomena themselves.
     The nearer the relations between the two types in question, the less exposed to doubt and criticism are the conclusions. But the domain of atavism is not restricted to the cases described. Quite on the contrary the facts that strike us most forcibly as being reversions are those that are apt to give us an insight into the systematic affinity of a higher degree. We are disposed to make use of them in our attempts to perfect the natural system and to remould it in such a way as to become a pedigree of the related groups. Such cases of atavism no doubt occur, but the anomalies referred to them must be interpreted merely on the ground of our assumptions as to the relative places in the system to be assigned to the different forms.
     Though such instances cannot be considered as belonging strictly to the subject we are dealing with, I think it may be as well to give an example, especially as it affords an occasion for referring to the highly important researches of Heinricher on the variability and atavistic [173] tendencies of the pale blue flag or Iris pallida. The flowers of the blue flags have a perianth of six segments united below into a tube. The three outer parts are dilated and spreading, or reflexed, while the three inner usually stand erect, but in most species are broad and colored like the outer ones. Corresponding to the outer, perianth-segments are the three stamens and the three, petal-like divisions of the style, each bearing a transverse stigma immediately above the anther. They are pollinated by bumble-bees, and in some instances by flies of the genus Rhingia, which search for the honey, brush the pollen out of the anthers and afterwards deposit it on the stigma. According to systematic views of the monocotyledons the original prototype of the genus Iris must have had a whorl of six equal, or nearly equal perianth-segments and six stamens, such as are now seen in the more primitive types of the family of the lilies, as for instance in the lilies themselves, the tulips, hyacinths and others. As to the perianth this view is supported by the existence of one species, the Iris falcifolia, the perianth of which consists of six equal parts. But species with six stamens are wholly lacking. Heinricher however, in cultivating some anomalous forms of Iris pallida, succeeded in filling out this gap and in producing [174] flowers with a uniform perianth and six stamens, recalling thereby the supposed ancestral type. The way in which he got these was as follows: he started from some slight deviations observed in the flowers of the pale species, sowed the seeds in large numbers and selected from the seedlings only those which clearly showed anomalies in the expected atavistic direction. By repeating this during several generations he at last reached his goal and was able to give reality to the prototype, which formerly was only a hypothetical one. The Iris kaempferi, a large-flowered Japanese species much cultivated in gardens, is very variable in the number of the different parts of its flowers, and may in some instances be seen even with six stamens. If studied in the same way as Heinricher’s iris, it no doubt will yield highly interesting and confirmatory results.
     Many other instances of such systematic atavism could be given, and every botanist can easily add some from memory. Many anomalies, occurring spontaneously, are evidently due to the same principle, but it would take too long to describe them.
     Reversion may occur either by buds or by seeds. It is highly probable that it occurs more readily by sexual than by asexual propagation. But if we restrict the discussion to the limits [175] hitherto observed, seed-reversions must be said to be extremely rare. Or rather cases which are sufficiently certain to be relied upon, are very rare, and perhaps wholly lacking. Most of the instances, recorded by various writers, are open to question. Doubts exist as to the purity of the seeds and the possibility of some unobserved cross disturbing the results.
     In the next lecture we shall deal in general with the ordinary causes and results of such crosses. We shall then see that they are so common and occur so regularly under ordinary circumstances that we can never rely on the absolute purity of any seeds, if the impossibility of an occasional cross has not been wholly excluded, either by the circumstances themselves, or by experimental precautions taken during the flowering period.
     For these reasons cases of atavism given without recording the circumstances, or the precautions that guarantee the purity of the fertilization, should always be disregarded. And moreover another proof should always be demanded. The parent which yielded the seeds might be itself a hybrid and liable to reversions by the ordinary laws of the splitting up of hybrids. Such cases should likewise be discarded, since they bring in confusing elements. If we review the long list of recorded cases by these [176] strict methods of criticism very few instances will be found that satisfy legitimate demands. On this ground it is by far safer in the present state of our knowledge, to accept bud-variations only as direct proofs of true atavism. And even these may not always be relied on, as some hybrids are liable to split up in a vegetative way, and in doing so to give rise to bud-variations that are in many respects apparently similar to cases of atavism. But fortunately such instances are as yet very rare.
     After this discussion it would be bold indeed to give instances of seed-atavism, and I believe that it will be better to refrain wholly from doing so.
     Many instances of so-called atavism are of purely morphologic nature. The most interesting cases are those furnished by the forms which some plants bear only while young, and which evidently connect them with allied species, in which the same features may be seen in the adult state. Some species of the genus Acacia bear bipinnate leaves, while others have no leaves at all, but bear broadened and flattened petioles instead. The second type is presumed to be descended from the first by the loss of the leaflets and the modification of the stalks into flat and simple phyllodes. But many of them are liable to recall this primitive form [177] when very young, in the first two or three, or sometimes in eight or ten primary leaves. These leaves are small because of the weakness of the young plant and therefore often more or less reduced in structure. But they are usually strictly bipinnate and thereby give testimony as to their descent from species which bear such leaves throughout their life.
     Other similar cases could be given, but this will suffice. They once more show how necessary it is to separate the different cases, thrown together until now, under this general name of atavism. It would be far better to give them all special names, and as long as these are not available we must be cautious not to be misguided by the name, and especially not to confuse different phenomena with one another, because at the present time they bear the same names.
     Taking into consideration the relatively numerous restrictions resulting from this discussion, we will now make a hasty survey of some of the more notable and generally acknowledged cases of atavism by bud-propagation. But it should be repeated once more that most of the highly cultivated plants, grown as vegetables, or for their fruit or flowers, have so many crosses in their ancestry, that it seems better to exclude them from all considerations, in which purity of [178] descent is a requisite. By so doing, we exclude most of the facts which were until now generally relied upon. For the roses, the hyacinths, the tulips, the chrysanthemums always have furnished the largest contributions to the demonstrations of bud-variation. But they have been crossed so often, that doubt as to the purity of the descent of any single form may recur, and may destroy the usefulness of their many recorded cases of bud-variation for the demonstration of real atavism. The same assertion holds good in many other cases, as with Azalea and Camellia. And the striped varieties of these genera belong to the group of ever-sporting forms, and therefore will be considered later on. So it is with carnations and pinks, which occasionally vary by layering, and of which some kinds are so uncertain in character that they are called by floriculturists “catch-flowers.” On the other hand there is a larger group of cases of reversion by buds, which is probably not of hybrid nature, nor due to innate inconstancy of the variety, but must be considered as pure atavism. I refer to the bud-variations of so many of our cultivated varieties of shrubs and trees. Many of them are cultivated because of their foliage. They are propagated by grafting, and in most cases it is probable that all the numerous specimens [179] of the same variety have been derived in this way from one primitive, aberrant individual. We may disregard variegated leaves, spotted or marked with white or yellow, because they are too inconstant types.
     We may next turn our attention to the varieties of trees with cut leaves, as the oakleaved Laburnum, the parsley-leaved vine and the fern-leaved birch. Here the margin of the leaves is deeply cut and divided by many incisions, which sometimes change only the outer parts of the blade, but in other cases may go farther and reach, or nearly reach, the midvein, and change the simple leaf into a seemingly compound structure. The anomaly may even lead to the almost complete loss of all the chorophyll-tissue and the greater part of the lateral veins, as in the case of the cut-leaved beech or Fagus sylvatica pectinata.
     Such varieties are often apt to revert by buds to the common forms. The cut-leaved beech sometimes reverts partially only, and the branches often display the different forms of cut-leaved, fern-like, oak-leaved and other variously shaped leaves on the same twigs. But this is merely due to the wide variability of the degree of fissure and is to be considered only as a fluctuation between somewhat widely distant extremes, which may even apparently include [180] the form of the common beech-leaves. It is not a bud-variation at all, and it is to be met with quite commonly while the true reversions by buds are very rare and are of the nature of sports appearing suddenly and remaining constant on the same twig. Analogous phenomena of wide variability with true reversion may be seen in the variety of the European hornbeam called Carpinus Betulus heterophylla. The leaves of this tree generally show the greatest diversity in form. Some other cases have been brought together by Darwin. In the first place a subvariety of the weeping-willow with leaves rolled up into a spiral coil. A tree of this kind kept true for twenty-five years and then threw out a single upright shoot bearing flat leaves. The barberry (Berberis) offers another case; it has a well known variety with seedless fruit, which can be propagated by cuttings or layers, but its runners are said always to revert to the common form, and to produce ordinary berries with seeds. Most of the cases referred to by Darwin, however, seem to be doubtful and cannot be considered as true proofs of atavism until more is known about the circumstances under which they were produced.
     Red or brown-leaved varieties of trees and shrubs also occasionally produce green-leaved branches, and in this way revert to the type [181] from which they must evidently have arisen. Instances are on record of the hazel, Corylus Avellana, of the allied Corylus tubulosa, of the red beech, the brown birch and of some other purple varieties. Even the red bananas, which bear fruits without seeds and therefore have no other way of being propagated than by buds, have produced a green variety with yellow fruits. The Hortensia of our gardens is another instance of a sterile form which has been observed to throw out a branch with cymes bearing in their center the usual small staminate and pistillate flowers instead of the large radiate and neutral corollas of the variety, thereby returning to the original wild type. Crisped weeping-willows, crisped parsley and others have reverted in a similar manner.
     All such cases are badly in need of a closer investigation. And as they occur only occasionally, or as it is commonly stated, by accident, the student of nature should be prepared to examine carefully any case which might present itself to him. Many phases of this difficult problem could no doubt be solved in this way. First of all the question arises as to whether the case is one of real atavism, or is only seemingly so, being due to hybrid or otherwise impure descent of the varying individual, and secondly whether it may be only an instance of the regularly [182] occurring so-called atavism of the sporting varieties with which we shall deal in a later lecture. If it proves to be real atavism and rare, the case should be accurately described and figured, or photographed if possible; and the exact position of the reverting bud should be ascertained. Very likely the so-called dormant or resting buds are more liable to reversions than the primary ones in the arils of the leaves of young twigs. Then the characters of the atavistic branches should be minutely compared with those of the presumed ancestor; they may be quite identical with them or slightly divergent, as has been asserted in some instances. The atavism may be complete in one case, but more or less incomplete in others. By far the most interesting point is the question, as to what is to be expected from the seeds of such an atavistic branch. Will they keep true to the reverted character, or return to the characters of the plant which bears the retrograde branch? Will all of them do so, or only part of them, and how large a part? It is very astonishing that this question should still be unsolved where so many individual trees bear atavistic branches that remain on them through long series of years. But then many such branches do not flower at all, or if they flower and bear seed, no care is taken to prevent [183] cross-fertilization with the other flowers of the same plant, and the results have no scientific value. For anyone who cares to work with the precautions prescribed by science, a wide field is here open for investigation, because old reverted branches may be met with much less rarely than new ones.
     Finally the possibility is always to be considered that the tendency to bud-reversions may be a special feature of some individuals, and may not be met with in others of the same variety. I have spoken of this before. For the practical student it indicates that a specimen, once observed to produce atavistic buds, may be expected to do the same thing again. And then there is a very good chance that by combining this view with the idea that dormant buds are more apt to revert than young ones, we may get at a method for further investigation, if we recur to the practice of pruning. By cutting away the young twigs in the vicinity of dormant buds, we may incite these to action. Evidently we are not to expect that in so doing they will all become atavistic. For this result is not at all assured; on the contrary, all that we might hope to attain would be the possibility of some of them being induced to sport in the desired direction.
     Many questions in scientific research can only [184] be answered by long and arduous work in well equipped laboratories; they are not to be attempted by every one. But there are other problems which the most complete of institutions are not able to study if opportunity is not offered them, and such opportunities are apt to occur more often in fields, gardens, parks, woods and plains, than in the relatively small experimental gardens of even the largest institution. Therefore, whosoever has the good fortune to find such sports, should never allow the occasion to pass without making an investigation that may bring results of very great importance to science.

[185]

LECTURE VII

FALSE ATAVISM OR VICINISM

About the middle of the last century Louis de Vilmorin showed that it was possible to subject plants to the methods of amelioration of races then in use for domestic animals, and since that time atavism has played a large part in all breeding-processes. It was considered to be the greatest enemy of the breeder, and was generally spoken of as a definite force, working against and protracting the endeavors of the horticulturist.
     No clear conception as to its true nature had been formulated, and even the propriety of designating the observed phenomena by the term atavism seemed doubtful. Duchesne used this word some decades ago to designate those cases in which species or varieties revert spontaneously, or from unknown internal causes, to some long-lost characters of their ancestors. Duchesne’s definition was evidently a sharp and useful one, since it developed for the first time the idea of latent or dormant qualities, [186] formerly active, and awaiting probably through centuries an occasion to awaken, and to display the lost characters.
     Cases of apparent reversion were often seen in nurseries, especially in flower culture, which under ordinary circumstances are rarely wholly pure, but always sport more or less into the colors and forms of allied varieties. Such sporting individuals have to be extirpated regularly, otherwise the whole variety would soon lose its type and its uniformity and run over to some other form in cultivation in the vicinity. For this reason atavism in nurseries causes much care and labor, and consequently is to be dealt with as a very important factor.
     From time to time the idea has suggested itself to some of the best authorities on the amelioration of plants, that this atavism was not due to an innate tendency, but, in many cases at least, was produced by crosses between neighboring varieties. It is especially owing to Verlot that this side of the question was brought forward. But breeders as a rule have not attached much importance to this supposition, chiefly because of the great practical difficulties attending any attempt to guard the species of the larger cultures against intermixture with other varieties. Bees and humble-bees fly from bud to bud, and carry the pollen from one [187 ] sort to another, and separation by great distances would be required to avoid this source of impurity. Unfortunately the arrangements and necessities of large cultures make it impossible to isolate the allied varieties from each other.
     From a theoretical point of view the origin of these impurities is a highly important question. If the breeders’ atavism is due to crosses, and only to this cause, it has no bearing at all on the question of the constancy of varieties. And the general belief, that varieties are distinguished from true species by their repeated reversion and that even such reversibility is the real distinction of a variety, would not hold.
     For this reason I have taken much trouble in ascertaining the circumstances which attend this form of atavism. I have visited a number of the leading nurseries of Europe, tested their products in various ways, and made some experiments on the unavoidable conditions of hybridizing and on their effect on the ensuing generations. These investigations have led me to the conclusion, that atavism, as it is generally described, always or nearly always is due to hybridization, and therefore it is to be considered as untrue or false atavism.
     True atavism, or reversion caused by an innate latent tendency, seems to be very rare, [188] and limited to such cases as we have spoken of under our last heading. And since the definition, given to this term by its author, Duchesne, is generally accepted in scientific works, it seems better not to use it in another sense, but rather to replace it in such cases by another term. For this purpose I propose the word vicinism, derived from the Latin vicinus or neighbor, as indicating the sporting of a variety under the influence of others in its vicinity. Used in this way, this term has the same bearing as the word atavism of the breeders, but it has the advantage of indicating the true cause thereof.
     It is well known that the term variability is commonly employed in the broadest possible sense. No single phenomenon can be designated by this name, unless some primary restriction be given. Atavism and vicinism are both cases of variability, but in wholly different sense. For this reason it may be as well, to insert here a short survey of the general meanings to be conveyed by the term variation. It implies in the first place the occurrence of a wide range of forms and types, irrespective of their origin, and in the second place the process of the change in such forms. In the first signification it is nearly identical with polymorphy, or richness of types, especially so when these [189] types are themselves quite stable, or when it is not at all intended to raise the question of their stability. In scientific works it is commonly used to designate the occurrence of subspecies or varieties, and the same is the case in the ordinary use of the term when dealing with cultivated plants. A species may consist of larger or smaller groups of such units, and they may be absolutely constant, never sporting if hybridization is precluded, and nevertheless it may be called highly variable. The opium-poppy affords a good instance. It “varies” in height, in color of foliage and flowers; the last are often double or laciniated; it may have white or bluish seeds, the capsules may open themselves or remain closed and so on. But every single variety is absolutely constant, and never runs into another, when the flowers are artificially pollinated and the visits of insects excluded. So it is with many other species. They are at the same time wholly stable and very variable.
     The terms variation and variety are used frequently when speaking of hybrids. By crossing forms, which are already variable in the sense just mentioned, it is easy to multiply the number of the types, and even in crossing pure forms the different characters may be combined in different ways, the resulting combinations [190] yielding new, and very often, valuable varieties. But it is manifest that this form of variation is of quite another nature from the variations of pure races. Many hybrid varieties are quite constant, and remain true to their type if no further crosses are made; many others are artificially propagated only in a vegetative way, and for this reason are always found true. Hybrid varieties as a rule were formerly confused with pure varieties, and in many instances our knowledge as to their origin is quite insufficient for sharp distinctions. To every student of nature it is obvious, that crossing and pure variability are wholly distinct groups of phenomena, which should never be treated under the same head, or under the same name.
     Leaving aside polymorphy, we may now discuss those cases of variability, in which the changes themselves, and not only their final results play a part. Of such changes two types exist. First, the ever-recurring variability, never absent in any large group of individuals, and determining the differences which are always to be seen between parents and their children, or between the children themselves. This type is commonly called “individual variability” and since this term also has still other meanings, it has of late become customary to use instead the term “fluctuating variability.” [191] And to avoid the repetition of the latter word it is called “fluctuation.” In contrast to these fluctuations are the so-called sports or single varieties, not rarely denominated spontaneous variations, and for which I propose to use the term “mutations.” They are of very rare occurrence and are to be considered as sudden and definite steps.
     Lastly, we have to consider those varieties, which vary in a much wider range than the ordinary ones, and seem to fluctuate between two opposite extremes, as for instance variegated leaves, cultivated varieties with variegated or striped flowers, double flowers and some other anomalies. They are eversporting and ever-returning from one type to the other. If however, we take the group of these extremes and their intermediates as a whole, this group remains constant during the succeeding generations. Here we find once more an instance of the seemingly contradictory combination of high variability and absolute constancy. It means that the range of variability has quite definite limits, which in the common course of things, are never transgressed.
     We may infer therefore that the word variability has such a wide range of meanings that it ought never be used without explanation. [192] Nothing indeed, is more variable than the signification of the term variable itself.
     For this reason, we will furthermore designate all variations under the influence of neighbors with the new and special term “vicinism.” It always indicates the result of crossing.
     Leaving this somewhat lengthy terminological discussion, we now come to the description of the phenomenon itself. In visiting the plantations of the seedsmen in summer and examining the large fields of garden-flowers from which seed is to be gathered, it is very rare to find a plot quite pure. On the contrary, occasional impurities are the rule. Every plot shows anomalous individuals, red or white flowers among a field of blue, normal among laciniated, single among double and so on. The most curious instance is afforded by dwarf varieties, where in the midst of hundreds and thousands of small individuals of the same height, some specimens show twice their size. So for instance, among the dwarfs of the larkspur, Delphinium Ajacis.
     Everywhere gardeners are occupied in destroying these “atavists,” as they call them. When in full bloom the plants are pulled up and thrown aside. Sometimes the degree of impurity is so high, that great piles of discarded plants of the same species lie about the [193] paths, as I have seen at Erfurt in the ease of numerous varieties of the Indian cress or Tropaeolum.
     Each variety is purified at the time when it shows its characters most clearly. With vegetables, this is done long before flowering, but with flowers only when in full bloom, and with fruits, usually after fertilization has been accomplished. It needs no demonstration to show that this difference in method must result in very diverging degrees of purity.
     We will confine ourselves to a consideration of the flowers, and ask what degree of purity may be expected as the result of the elimination of the anomalous plants during the period of blooming.
     Now it is evident that the colors and forms of the flowers can only be clearly distinguished, when they are fully displayed. Furthermore it is impossible to destroy every single aberrant specimen as soon as it is seen. On the contrary, the gardener must wait until all or nearly all the individuals of the same variety have displayed their characters, as only in this way can all diverging specimens be eliminated by a single inspection. Unfortunately the insects do not wait for this selection. They fertilize the flowers from the beginning, and the damage will have been done [194] long before the day of inspection comes around. Crosses are unavoidable and hybrid seeds will unavoidably come into the harvest. Their number may be limited by an early eradication of the vicinists, or by the elimination of the first ripe seeds before the beginning of the regular harvest, or by other devices. But some degree of impurity will remain under ordinary circumstances.
     It seems quite superfluous to give more details. In any case in which the selection is not done before the blooming period, some impurities must result. Even if it is done before that time, errors may occur, and among hundreds and thousands of individuals a single anomalous one may escape observation.
     The conclusion is, that flower seeds as they are offered in commerce, are seldom found absolutely pure. Every gardener knows that he will have to weed out aberrant plants in order to be sure of the purity of his beds. I tested a large number of samples of seeds for purity, bought directly from the best seed growers. Most of them were found to contain admixtures and wholly pure samples were very rare.
     I will now give some illustrative examples. From seeds of a yellow snapdragon, I got one red-flowered specimen among half a hundred [195] yellow ones, and from the variety “Delila” of the same species two red ones, a single white and two belonging to another variety called “Firefly.” Calliopsis tinctoria has three varieties, the ordinary type, a brown-flowered one and one with tubular rays. Seeds of each of these three sorts ordinarily contain a few belonging to the others. Iberis umbellata rosea often gives some white and violet examples. The “Swan” variety of the opium-poppy, a dwarfish double-flowered form of a pure white, contained some single-flowered and some red-flowered plants, when sown from commercial seed are said to be pure. But these were only occasional admixtures, since after artificial fertilization of the typical specimens the strain at once became absolutely pure, and remained so for a series of generations, as long as the experiment was continued. Seeds of trees often contain large quantities of impurities, and the laciniated varieties of birch, elder and walnut have often been observed to come true only in a small number of seedlings.
     In the case of new or young varieties, seed merchants often warn their customers as to the probable degree of purity of the seeds offered, in order to avoid complaints. For example the snow-white variety of the double daisy, Bellis perennis plena, was offered at the start as containing [196] as much as 20% of red-flowered specimens.
     Many fine varieties are recorded to come true from seed, as in the case of the holly with yellow fruits, tested by Darwin. Others have been found untrue to a relatively high degree, as is notorious in the case of the purple beech. Seeds of the laciniated beech gave only 10% of laciniated plants in experiments made by Strasburger; seeds of the monophyllous acacia, Robinia Pseud-Acacia monophylla, were found to be true in only 30% of the seedlings. Weeping ashes often revert to the upright type, red May-thorns (Crataegus) sometimes revert nearly entirely to the white species and the yellow cornel berry is recorded to have reverted in the same way to the red berries of the Cornus Mas.
     Varieties have to be freed by selection from all such impurities, since isolation is a means which is quite impracticable under ordinary circumstances. Isolation is a scientific requirement that should never be neglected in experiments, indeed it may be said to be the first and most important requisite for all exact research in questions of variability and inheritance. But in cultivating large fields of allied varieties for commercial purposes, it is impossible to grow them at such distances from each other [197] as to prevent cross-pollination by the visits of bees.
     This purification must be done in nearly every generation. The oldest varieties are to be subjected to it as well as the latest. There is no regular amelioration, no slow progression in the direction of becoming free from these admixtures. Continuous selection is indispensable to maintain the races in the degree of purity which is required in commerce, but it does not lead to any improvement. Nor does it go so far as to become unnecessary in the future. This shows that there must be a continuous source of impurities, which in itself is not neutralized by selection, but of which selection can only eliminate the deteriorating elements.
     The same selection is usually applied to new varieties, when they occasionally arise. In this case it is called “fixing,” as gardeners generally believe that through selection the varieties are brought to the required degree of purity. This belief seems to rest mainly on observations made in practice, where, as we have seen, isolation is of very rare application. Most varieties would no doubt be absolutely pure from the first moment of their existence, if it were only possible to have them purely fertilized. But in practice this is seldom to be obtained. Ordinarily the breeder is content with such slow [198] improvement as may be obtained with a minimum of cost, and this mostly implies a culture in the same part of the nursery with older varieties of the same species. Three, four or five years are required to purify the novelty, and as this same length of time is also required to produce sufficient quantities of seed for commercial purposes, there is no strong desire to shorten the period of selection and fixation. I had occasion to see this process going on with sundry novelties at Erfurt in Germany. Among them a chamois-colored variety of the common stock, a bluish Clarkia elegans and a curiously colored opium-poppy may be mentioned. In some cases the crossfertilization is so overwhelming, that in the next generation the novelty seems entirely to have disappeared.
     The examples given may suffice to convey a general idea of the phenomenon, ordinarily called atavism by gardeners, and considered mostly to be the effect of some innate tendency to revert to the ancestral form. It is on this conception that the almost universal belief rests, that varieties are distinguished, as such, from species by their inconstancy. Now I do not deny the phenomenon itself. The impurity of seeds and cultures is so general and so manifest, and may so easily be tested by every one [199] that it cannot reasonably be subjected to any doubt. It must be conceded to be a fact, that varieties as a rule revert to their species under the ordinary circumstances of commercial culture. And I cannot see any reason why this fact should not be considered as stating a principal difference between varieties and species, since true species never sport into one another.
     My objection only refers to the explanation of the observed facts. According to my view nearly all these ordinary reversions are due to crosses, and it is for this reason that I proposed to call them by a separate name, that of “vicinists.” Varieties then, by means of such spontaneous intercrossing sport into one another, while species either do not cross, or when crossing produce hybrids that are otherwise constituted and do not give the impression of atavistic reversion.
     I must not be content with proposing this new conception, but must give the facts on which this assumption rests. These facts are the results of simple experiments, which nevertheless are by no means easy to carry out, as they require the utmost care to secure the absolute purity of the seeds that are employed. This can only be guaranteed by previous cultures of isolated plants or groups of plants, or by artificial pollination.
     [200] Once sure of this preliminary condition, the experiment simply consists in growing a variety at a given distance from its species and allowing the insects to transfer the pollen. After harvesting the seed thus subjected to the presumed cause of the impurities, it must be sown in quantities, large enough to bring to light any slight anomaly, and to be examined during the period of blooming.
     The wild seashore aster, Aster Tripolium, will serve as an example. It has pale violet or bluish rays, but has given rise to a white variety, which on testing, I have found pure from seed. Four specimens of this white variety were cultivated at a distance of nearly 100 meters from a large lot of plants of the bluish species. I left fertilization to the bees, harvested the seeds of the four whites separately and had from them the following year more than a thousand flowering plants. All of them were of the purest white, with only one exception, which was a plant with the bluish rays of the species, wholly reverting to its general type. As the variety does not give such reversions when cultivated in isolation, this sport was obviously due to some cross in the former year. In the same way I tried the white Jacob’s ladder, Polemonium coeruleum album in the neighborhood of the blue-flowered species, the distance [202] in this case being only 40 meters. Of two hundred seeds one became a blue atavist, or rather vicinist, while all others remained true to the white type. The same was observed in the white creeping thyme, or Thymus Serpyllum album, and the white self-heal, Brunella vulgaris alba, gave even so much as 28% seedlings with purple corollas out of some 400 specimens, after being cultivated in close proximity to its parent-species. I have tried many other species, but always with the same result. Such atavists only arise by cultivation in the proximity of allied varieties, never in isolation. They are not real atavists, but only vicinists.
     In order to show this yet more clearly, I made another experiment with the white selfheal. I had a lot of the pinnate-leaved variety with purple flowers and somewhat stouter stems, and cultivated single plants of the whiteflowering sort at distances that varied from 2-16 meters. The seeds of each plant were collected and sown separately, those of the nearest gave up to 5 or 6 hybrids from the seeds of one parent, while those of the farthest gave only one purple-flowered plant for each parent. Evidently the chance of the pollen being carried by bees is much greater on short than on longer distances.
     True hybrids between species may arise in quite the same way, and since it is obviously impossible to attribute them to an innate tendency to reversion, they afford an absolutely irrefutable proof of the assertion that pollen is often brought by insects from one lot of plants to another. In this way I obtained a hybrid between the common Jacob’s ladder and the allied species Polemonium dissectum. With a distance of 100 meters between them I had two hybrid seeds among a hundred of pure ones. At a similar distance pollen was carried over from the wild radish, Raphanus Raphanistrum, to the allied Raphanus caudatus, and I observed the following year some very nice hybrids among my seedlings. A hybrid-bean between Phaseolus nanus and P. multiflorus, and some hybrids between the yellow daisy, Chrysanthemum segetum and the allied Chrysanthemum coronarium or ox-eye daisy which also arose spontaneously in my garden between parents cultivated at recorded distances, might further be noted. Further details of these experiments need not be given. Suffice to say, that occasional crosses between species do occur, and not even rarely, that they are easily recognized as such and cannot be confused with cases of atavism, and that therefore they give proof to the assumption that in the same way crosses ordinarily occur also between varieties [203] of the same species, if cultivated at small distances apart, say 40-50 meters or even more. Vicinism therefore, may play a part in all such cultures, enough to account for all the impurities observed in the nurseries or in commercial seed-samples.
     Of course this whole discussion is limited to such species as are not only as a rule visited by insects, but are dependent on these visits for their fertilization. Most of our garden-flowers are included in this category. If not then we may expect to find the cultures and seeds pure, irrespective of the distances between allied varieties, as for instance with peas, which are known to be self-fertilizing. Another instance is given by the barley. One of the most curious anomalous varieties of this cereal, is the “Nepaul-barley,” with its small adventitious flowers on the palets or inner scales. It is a very old, widely cultivated sort, which always comes true from seed, and which has been tested in repeated experiments in my garden. The spikelets of this curious plant are oneflowered and provided with two linear glumes or outer scales. Of the inner scales or palets, the outer one is three-lobed at the summit, hence the varietal name of Hordeum vulgare trifurcatum. The central lobe is oblong and hollow, covering a small supernumerary floret inserted [204] at its base. The two lateral lobes are narrower, sometimes linear, and are often prolonged into an awn, which is generally turned away from the center of the spike. The central lobe sometimes bears two florets at its base, although but one is usually present and it may be incomplete.
     I might give one more instance from my own experience. A variety of the evening-primrose with small linear petals was once found by one of my sons growing wild near Amsterdam. It was represented by only one individual, flowering among a great many of the ordinary type with broad petals. But the evening-primroses open their anthers in the morning, fertilize themselves during the day, and only display their beautiful flowers in the evening, after the pollination has been accomplished. They then allure evening moths, such as Agrotis and Plusia, by their bright color, their sweet honeysmell and their nectar. Since the fertilization is accomplished many hours before opening, crosses are effected only in rare instances, and the seeds commonly remain true to the parent type. The seeds of this one plant, when sown separately in my garden, produced exclusively flowers with the small linear petals of their parent. Although I had a hundred individuals bearing many thousands of flowers, there was not an instance of reversion. And such would [205] immediately have been observed, had it occurred, because the hybrids between the cruciate and the normal flowers are not intermediate, but bear the broad petals of the O. biennis.
     We may now take up another phase of the question, that of the running out of new varieties, shortly after their introduction into a new country, or later.
     The most widely known instance of this is that of the American corn in Baden, recorded by Metzger and quoted by Darwin as a remarkable instance of the direct and prompt action of climate on a plant. It has since been considered as a reversion to the old type. Such reversions invariably occur, according to Wallace, in cases of new varieties, which have been produced quickly. But as we now know, such reversions are due to spontaneous crosses with the old form, and to the rule, that the hybrids of such origin are not intermediate, but assume the features of the older of the two parents. In the light of this experience, Metzger’s observation becomes a typical instance of vicinism. It relates to the “Tuscarora” corn of St. Louis, a variety with broad and flat white seeds.
     About the year 1840, this corn was introduced into Baden in Germany, and cultivated by Metzger. In the first year it came true to type, and [206] attained a height of 12 feet, but the season did not allow its seeds to ripen normally. Only a few kernels were developed before the winter. From this seed plants of a wholly different type came the next year, of smaller stature, and with more brownish and rounded kernels. They also flowered earlier and ripened a large number of seeds. The depression on the outer side of the seed had almost disappeared, and the original white had become darker. Some of the seeds had even become yellow and in their rounded form they approached the common European maize. Obviously they were hybrids, assuming the character of their pollen-parent, which evidently was the ordinary corn, cultivated all around. The observation of the next year showed this clearly, for in the third generation nearly all resemblance to the original and very distinct American species was lost. If we assume that only those seeds ripened which reverted to the early-ripening European type, and that those that remained true to the very late American variety could not reach maturity, the case seems to be wholly comprehensible, without supposing any other factors to have been at work than those of vicinism, which though unknown at the period of Metzger’s and Darwin’s writings, seems now to be fully understood. No innate tendency to run out and no changing influence of the climate are required for an adequate explanation of the facts.
     In the observation quoted, what astonishes us most, is the great rapidity of the change, and the short time necessary for the offspring of the accidental crosses to completely supplant the introduced type. In the lecture on the selection of elementary species, closely analogous cases were described. One of them was the wild oat or Avena fatua which rapidly supplants the cultivated oats in bad years in parts of the fields. Other instances were the experiments of Risler with the “Galland”” wheat and the observation of Rimpau on “Rivett’s bearded” wheat.
     Before leaving the question of vicinism and its bearing on the general belief of the instability of varieties, which when tested with due care, prove to be quite stable, it may be as well to consider the phenomena from another point of view. Our present knowledge of the effects of crosses between varieties enables us to formulate some general rules, which may be used to calculate, and in some way to predict, the nature of the impurities which necessarily attend the cultivation of allied species in close vicinity. And this mode of cultivation being in almost universal use in the larger nurseries, [208] we may, by this discussion, arrive at a more scientific estimation of the phenomena of vicinism, hitherto described.
     The simplest case that may be given, is when an ordinary retrograde variety is cultivated with the species to which it belongs. For instance, if dwarfs are cultivated next to the taller type, or a white variety next to the red or blue-flowering species, or thornless forms in neighboring beds with the armed species. Bees and Bumble-bees, butterflies and moths are seen flying from flower to flower, collecting the honey and carrying pollen. I frequently saw them cross the limits of the neighboring beds. Loaded with the pollen of the variety they visit the flowers of the different species and impregnate the stigma with it. And returning to the variety they bring about similar crosses in the flowers of the latter. Hybrid seeds will develop in both cases and become mixed with the crop. We now have to ask the question, what sort of plants will arise from these hybrid seeds. As a general rule we may state, first, that the hybrids of either form of cross are practically the same, secondly that they are not intermediate, but that the character of one parent prevails to the almost absolute exclusion of the other and in the third place that the older character dominates the younger.
     [209] The hybrid offspring will therefore, in the main, have the character of the species and be indistinguishable from it, or show only such differences as escape ordinary observation. When occurring in the seeds of the variety they betray themselves as soon as the differential characters are displayed. Between the thousands of flowering plants of a white variety the hybrids will instantly catch the eye by their red or blue corollas. Quite the contrary effect results from the admixture of hybrids with the seeds of the species itself. Here no difference will show itself, even in the fullest bloom. The effect of the spontaneous crosses will pass unobserved. The strain, if pure in the first year, will seem to be still in the same condition. Or in other terms, the unavoidable spontaneous crosses will disturb the purity of the variety in the second year, while they do not seem to interfere at all with the uniformity of the species. The direct effect of the visits of the insects is evident in the first case, but passes unobserved in the latter.
     From this it would seem, that spontaneous crosses are hurtful to varieties, but are innocuous to true species. Certainly this would be so, were there no selection. But it is easily seen, that through this operation the effect becomes quite the opposite. For when the fields [210] are inspected at the time of the fullest display of the varietal characters, the obvious hybrids will be eliminated, but the hidden ones will of necessity be spared, as they are concealed among the species by the similarity of their type. Hence, the harvest of the variety may be rendered pure or nearly so, while the harvest of the species will retain the seeds of the hybrids. Moreover it will contain seeds originated by the spontaneous but numerous crosses of the true plants with the sparsely intermingled hybrids.
     This brings us to the question, as to what will be the visible consequences of the occurrence of such invisible hybrids in the following generation. In opposition to the direct effects just described, we may call them indirect. To judge of their influence, we must know how hybrid seeds of the first generation behave.
     In one of our lectures we will deal with the laws that show the numerical relations known as the laws of Mendel. But for our present purpose, these numerical relations are only of subordinate importance. What interests us here is the fact that hybrids of varieties do not remain constant in the second generation but usually split as it is said, remaining hybrid only in part of their offspring, the other portion returning to the parental types. This however, will show itself only in those individuals [211] which reassume the character of the varietal parent, all the others apparently remaining true to the type of the species. Now it is easy to foresee what must happen in the second generation if the first generation after the cross is supposed to be kept free from new vicinistic influences, or from crosses with neighboring varieties.
     We may limit ourselves in the first place to the seeds of the unobserved hybrids. For the greater part they will repeat the character of their parents and still remain concealed. But a small number will display the varietal marks, as for example showing white flowers in a field of blue ones. Hence, the indirect consequence of the spontaneous crosses will be the same in the species, as was the direct effect in the variety, only that it appears a year later. It will then be eliminated in the process of selection.
     Obviously, this elimination conduces only to a partial purification. The conspicuous plants will be destroyed, but a greater number of hybrids will remain, still concealed by their resemblance to the general type and will be spared to repeat the same process next year. So while the variety may be freed every year from the impurities brought into it in the preceeding summer, the admixtures of the species [212] will continue during a number of years, and it may not be possible to get rid of them at all.
     It is an often recurring assertion that white varieties of colored species are the most stable of all horticultural races. They are often said to be at least as constant as the species itself, and even to surpass it in this quality. With our present state of knowledge, the explanation of this general experience is easily given. For selection removes the effect of spontaneous crosses from the variety in each year, and renders it practically pure, while it is wholly inadequate to produce the same effects on the species, because of the concealed hybrids.
     The explanation given in this simple instance may be applied to the case of different varieties of the same species, when growing together and crossed naturally by insects.
     It would take too long to go into all the details that present themselves here to the student of nature and of gardens. I will only state, that since varieties differ principally from their species by the lack of some sharp character, one variety may be characterized by the lack of color of the flowers, another by the lack of pubescence, a third by being dwarfed, and so on. Every character must be studied separately in its effects on the offspring [213] of the crosses. And it is therefore easily seen, that the hybrids of two varieties may resemble neither of them, but revert to the species itself. This is necessarily and commonly the case, since it is always the older or positive characters that prevail in the hybrids and the younger or negative that lie hidden. So for instance, a blue dwarf larkspur, crossed with a tall white variety, must give a tall blue hybrid, reassuming in both characters the essentials of the species.
     Keeping this rule in view, it will be easy to calculate what may be expected from spontaneous crosses for a wide range of occurrences, and thus to find an explanation of innumerable cases of apparent variability and reversion in the principle of vicinism. Students have only to recollect that specific characters prevail over varietal ones, and that every character competes only with its own antagonist. Or to give a sharper distinction: whiteness of flowers cannot be expected to be interchanged with pubescence of leaves.
     In concluding I will point out another danger which in the principle of vicinism may be avoided. If you see a plant in a garden with all the characteristics of its species, how can you be sure that it is truly a representative of the species, and not a hybrid? The prevailing [214] characters are in either case the same. Perhaps on close inspection you may find in some cases a slight difference, some character being not as fully developed in the hybrid as in the species. But when such is not the case, or where the opportunity for such a closer examination is wanting, a hybrid may easily be taken for a specimen of the pure race. Now take the seeds of your plant and sow them. If you had not supposed it to be hybrid you will be astonished at finding among its progeny some of a wholly different type. You will be led to conclude that you are observing a sudden change in structure such as is usually called a sport.
     Or in other words you may think that you are assisting at the origination of a new variety. If you are familiar with the principle of vicinism, you will refrain from such an inference and consider the supposition of a hybrid origin. But in former times, when this principle was still unknown and not even guessed at, it is evident that many mistakes must have been made, and that many an instance, which until now has been considered reliable proof of a so-called single variation, is in fact only a case of vicinism. In reading the sparse literature on sports, numerous cases will be found, which cannot stand this test. In many instances crossing must be looked to as an explanation, [215] and in other cases the evidence relied upon does not suffice to exclude this assumption. Many an old argument has of late lost its force by this test.
     Returning to our starting point we may now state that regular reversions to a specific type characterize a form as a variety of that species. These reversions, however, are not due to an innate tendency, but to unobserved spontaneous crosses.

[217]

LECTURE VIII

LATENT CHARACTERS

No organism exhibits all of its qualities at any one time. Many of them are generally dormant and await a period of activity. For some of them this period comes regularly, while in others the awakening depends upon external influences, and consequently occurs very irregularly. Those of the first group correspond to the differences in age; the second constitute the responses of the plant to stimuli including wound-injuries.
     Some illustrative examples may be quoted in order to give a precise idea of this general conception of dormant or latent characters. Seed leaves are only developed in the seed and the seedling; afterwards, during the entire lifetime of the plant, the faculty of producing them is not made use of. Every new generation of seeds however, bears the same kind of seed leaves, and hence it is manifest that it is the same quality, which shows itself from time to time.
     The primary leaves, following the seed-leaves, are different in many species, from the later ones, and the difference is extremely pronounced in some cases of reduction. Often, when leaves are lacking in the adult plant, being replaced by flattened stalks as in the case of the acacias, or by thorns, or green stems and twigs as in the prickly broom or Ulex europaeus, the first leaves of the young plant may be more highly differentiated, being pinnate in the first case and bearing three leaflets in the second instance. This curious behavior which is very common, brings the plants, when young, nearer to their allies than in the adult state, and manifestly implies that the more perfect state of the leaves is latent throughout the life of the plant, with the exception of the early juvenile period.
Eucalyptus Globulus, the Australian gum tree, has opposite and broadly sessile leaves during the first years of its life. Later these disappear and are replaced by long sickle-shaped foliage organs, which seem to be scattered irregularly along the branches. The juvenile characters manifestly lie dormant during the adult period, and that this is so, may be shown artificially by cutting off the whole crown of the tree, when the stem responds by producing numerous new branches, which assume the [218] shape proper to the young trees, bearing sessile and opposite leaves.
     It seems quite unnecessary to give further instances. They are familiar to every student. It is almost safe to say that every character has its periods of activity and of inactivity, and numbers of flowers and fruits can be mentioned as illustrations. One fact may be added to show that nearly every part of the plant must have the power of producing all or nearly all the characters of the individual to which it belongs. This proof is given by the formation of adventitious buds. These, when once formed, may grow out into twigs, with leaves and flowers and roots. They may even be separated from the plants and used as cuttings to reproduce the whole. Hence we may conclude that all tissues, which possess the power of producing adventitious buds, must conceal in a latent state, all the numerous characters required for the full development of the whole individual.
     Adventitious buds may proceed from specialized cells, as on the margin of the leaves of Bryophyllum calycinum; or from the cells of special tissues, as in the epidermis of the begonias; or they may be provoked by wounds in nearly every part of the plant, provided it be able to heal the wound by swelling tissues or [219] callus. The best instance is afforded by elms and by the horse-chestnut. If the whole tree is hewn down the trunk tries to repair the injury by producing small granulations of tissue between the wood and the bark, which gradually coalesce while becoming larger. From this new ring of living matter innumerable buds arise, that expand into leafy branches, showing clearly that the old trunk possesses, in a latent state, all the qualities of the whole crown. Indeed, such injured stumps may be used for the production of copses and hedges.
     All the hitherto recorded cases of latency have this in common, that they may become active during the life-time of any given individual once, or oftener. This may be called the ordinary type of latency.
     Besides this there is another form of latent characters, in which this awakening power is extremely limited, or wholly absent. It is the systematic latency, which may be said to belong to species and varieties in the same way as the ordinary latency belongs to individuals. As this individual latency may show itself from time to time during the life of a given plant, the first may only become active from time to time during the whole existence of the variety or the species. It has no regular period of activity, nor may it be incited by artificial stimulation.
     [220] It emerges from concealment only very rarely and only on its own initiative. Such instances of atavism have been described in previous lectures, and their existence has been proved beyond doubt.
     Systematic latency explains the innumerable instances in which species are seen to lack definite characteristics which ordinarily do not fail, either in plants at large, or in the group or family to which the plant belongs. If we take for instance the broom-rape or Orobanche, or some other pale parasite, we explain their occurrence in families of plants with green leaves, by the loss of the leaves and of the green color. But evidently this loss is not a true one, but only the latency of those characters. And even this latency is not a complete one, as little scales remind us of the leaves, and traces of chlorophyll still exist in the tissues. Numerous other cases will present themselves to every practical botanist.
     Taking for granted that characters, having once been acquired, may become latent, and that this process is of universal occurrence throughout the whole vegetable and animal kingdom, we may now come to a more precise and clear conception of the existing differences between species and varieties.
     For this purpose we must take a somewhat [221] broader view of the whole evolution of the vegetable kingdom. It is manifest that highly developed plants have a larger number of characters than the lower groups. These must have been acquired in some way, during preceding times. Such evolution must evidently be called a process of improvement, or a progressive evolution. Contrasted to this is the loss, or the latency of characters, and this may be designated retrogressive or retrograde evolution. But there is still a third possibility. For a latent character may reassume its activity, return to the active state, and become once more an important part of the whole organization. This process may be designated as degressive evolution; it obviously completes the series of the general types of evolution.
     Advancement in general in living nature depends on progressive evolution. In different parts of the vegetable kingdom, and even in different families this progression takes place on different lines. By this means it results in an ever increasing divergency between the several groups. Every step is an advance, and many a step must have been taken to produce flowering plants from the simplest unicellular algae.
     But related to, and very intimately connected with this advancement is the retrogressive [222] evolution. It is equally universal, perhaps never failing. No great changes have been attained, without acquiring new qualities on one side, and reducing others to latency. Everywhere such retrogressions may be seen. The polypetalous genera Pyrola, Ledum, and Monotropa among the sympetalous heaths, are a remarkable instance of this. The whole evolution of the monocotyledons from the lowest orders of dicotyledons implies the seeming loss of cambial growth and many other qualities. In the order of aroids, from the calamus-root or sweet flag, with its small but complete flowers, up to the reduced duckweeds (Lemna), almost an unbroken line of intermediate steps may be traced showing everywhere the concurrence of progressive and retrogressive evolution.
     Degressive evolution is not so common by far, and is not so easy to recognize, but no doubt it occurs very frequently. It is generally called atavism, or better, systematic atavism, and the clearest cases are those in which a quality which is latent in the greater part of a family or group, becomes manifest in one of its members. Bracts in the inflorescence of crucifers are ordinarily wanting, but may be seen in some genera, Erucastrum pollichii being perhaps the [223] most widely known instance, although other cases might easily be cited.
     For our special purpose we may take up only the more simple cases that may be available for experimental work. The great lines of evolution of whole families and even of genera and of many larger species obviously lie outside the limits of experimental observation. They are the outcome of the history of the ancestors of the present types, and a repetition of their history is far beyond human powers. We must limit ourselves to the most recent steps, to the consideration of the smallest differences. But it is obvious that these may be included under the same heads as the larger and older ones. For the larger movements are manifestly to be considered only as groups of smaller steps, going in the same direction.
     Hence we conclude, that even the smallest steps in the evolution of plants which we are able to observe, may be divided into progressive, retrogressive and degressive ones. The acquisition of a single new quality is the most simple step in the progressive line, the becoming latent and the reactivating of this same quality are the prototypes of the two other classes.
     Having taken this theoretical point of view, it remains to inquire, how it concurs with the [224] various facts, given in former lectures and how it may be of use in our further discussions.
     It is obvious that the differences between elementary species and varieties on the one hand, and between the positive and negative varieties as distinguished above, are quite comparable with our theoretical views. For we have seen that varieties can always be considered as having originated by an apparent loss of some quality of the species, or by the resumption of a quality which in allied species is present and visible. In our exposition of the facts we have of course limited ourselves to the observable features of the phenomena without searching for a further explanation. For a more competent inquiry however, and for an understanding of wider ranges of facts, it is necessary to penetrate deeper into the true nature of the implied causes.
     Therefore we must try to show that elementary species are distinguished from each other by the acquisition of new qualities, and that varieties are derived from their species either by the reduction of one or more characteristics to the latent state, or by the energizing of dormant characters.
     Here we meet with a great difficulty. Hitherto varieties and subspecies have never been clearly defined, or when they have been, it was [225] not by physiological, but only by morphological research. And the claims of these two great lines of inquiry are obviously very diverging. Morphological or comparative studies need a material standard, by which it may be readily decided whether certain groups of animals and plants are to be described or de-nominated as species, as subspecies or as varieties. To get at the inner nature of the differences is in most cases impossible, but a decision must be made. The physiological line of inquiry has more time at its disposal; it calls for no haste. Its experiments ordinarily cover years, and a conclusion is only to be reached after long and often weary trials. There is no making a decision on any matter until all doubtful points have been cleared up. Of course, large groups of facts remain uncertain, awaiting a closer inquiry, and the teacher is constrained to rely on the few known instances of thoroughly investigated cases. These alone are safe guides, and it seems far better to trust to them and to make use of them for the construction of sharp conceptions, which may help us to point out the lines of inquiry which are still open.
     Leaving aside all such divisions and definitions, as were stamped with the name of provisional species and varieties by the great systematist, [226] Alphonse De Candolle, we may now try to give the proofs of our assertion, by using only those instances that have been thoroughly tested in every way.
     We may at once proceed to the retrogressive or negative varieties. The arguments for the assumption that elementary species owe their origin to the acquisition of new qualities may well be left for later lectures when we shall deal with the experimental proofs in this matter.
     There are three larger groups of facts, on which the assumption of latent characters in ordinary varieties rests. These are true atavism, incomplete loss of characters, and systematic affinity. Before dealing with each of these separately, it may be as well to recall once more that in former lectures we have treated the apparent losses only as modifications in a negative way, without contemplating the underlying causes.
     Let us recall the cases of bud-atavism given by the whitish variety of the scarlet Ribes, by peaches and nectarines, and by conifers, including Cephalotaxus and Cryptomeria. These and many other analogous facts go to prove the relation of the variety to the species. Two assumptions are allowable. In one the variety differs from the species by the total loss of the [227] distinctive character. In the other this character is simply reduced to an inactive or dormant state. The fact of its recurrence from time to time, accompanied by secondary characters previously exhibited, is a manifest proof of the existence of some relation between the lost and the resumed peculiarity. Evidently this relation cannot be accounted for on the assumption of an absolute disappearance; something must remain from which the old features may be restored.
     This lengthy discussion may be closed by the citation of the cases, in which plants not only show developmental features of a former state, but also reproduce the special features they formerly had, but seemingly have lost. Two good illustrative examples may be given. One is afforded by the wheat-ear carnation, the other by the green dahlias, and both have occurred of late in my own cultures.
     A very curious anomaly may from time to time be observed in large beds of carnations. It bears no flowers, but instead of them small green ears, which recall the ears of wheat. Thence the name of “Wheat-ear” carnation. On closer inspection it is easily seen how they originate. The normal flowers of the carnations are preceded by a small group of bracts, [228] which are arranged in opposite pairs and therefore constitute four rows.
     In this variety the flower is suppressed and this loss is attended by a corresponding increase of the number of the pairs of bracts. This malformation results in square spikes or somewhat elongated heads consisting only of the greenish bracts. As there are no flowers, the variety is quite sterile, and as it is not regarded by horticulturists as an improvement on the ordinary bright carnations, it is seldom multiplied by layering. Notwithstanding this, it appears from time to time and has been seen in different countries and at different periods, and, what is of great importance for us, in different strains of carnations. Though sterile, and obviously dying out as often as it springs into existence, it is nearly two centuries old. It was described in the beginning of the 18th century by Volckamer, and afterwards by Jaeger, De Candolle, Weber, Masters, Magnus and many other botanists. I have had it twice, at different times and from different growers.
     So far as I have been able to ascertain reversions of this curious carnation to normal flowers have not yet been recorded. Such a modification occurred last summer in my garden on a plant which had not been divided or layered, but on which the slender branches had [229] been left on the stem. Some of them remained true to the varietal type and bore only green spikes. Others reverted wholly or partially to the production of normal flowers. Some branches bore these only, others had spikes and flowers on neighboring twigs, and in still other instances little spikes had been modified in such manner that a more or less well developed flower was preceded by some part of an ear.
     The proof that this retrograde modification was due to the existence of a character in the latent state was given by the color of the flowers. If the reverted bud had only lost the power of producing spikes, they would evidently simply have returned to the characteristics of the ordinary species, and their color would have been a pale pink. Instead of this, all flowers displayed corollas of a deep brown. They obviously reverted to their special progenitor, the chance variety from which they had sprung, and not to the common prototype of the species. Of course it was not possible to ascertain from which variety the plant had really originated, but the reproduction of any one clearly defined varietal mark is in itself proof enough of their origin, and of the latency of the dark brown flower-color in this special case.
     A still better proof is afforded by a new type of green dahlia. The ordinary green dahlia [230] has large tufts of green bracts instead of flowering heads, the scales of the receptacle having assumed the texture and venation of leaves, and being in some measure as fleshy. But the green heads retain the form of the ordinary flower-heads, and as they have no real florets that may fade away, they remain unchanged on the plants, and increase in number through the whole summer. The new types of green dahlia however, with which I have now to deal, are distinguished by the elongation of the axis of the head, which is thereby changed into a long leafy stalk, attaining a length of several inches. These stalks continue growing for a very long time, and for the most part die without producing anything else than green fleshy scales.
     This long-headed green dahlia originated at Haarlem some years ago, in the nursery of Messrs. Zocher & Co. It was seen to arise twice, from different varieties. Both of these were double-flowered, one a deep carmine with white tips on the rays, the other of a pale orange tint, known by the name of “Surprise.” As they did not bear any florets or seeds, they were quite sterile. The strain arising from the carmine variety was kindly given to me by Messrs. Zocher & Co., and was propagated in my garden, while the other was kept in the nursery. In the earlier cultures both remained true to [231] their types, never producing true florets. No mark of the original difference was to be seen between them. But last summer (1903) both reverted to their prototypes, bearing relatively large numbers of ordinary double flowerheads among the great mass of green stalks. Some intermediate forms also occurred consisting of green-scaled stalks ending in small heads with colored florets.
     Thus far we have an ordinary case of reversion. But the important side of the phenomenon was, that each plant exactly “recollected” from which parent it had sprung. All of those in my garden reverted to the carmine florets with white tips, and all of those in the nursery to the pale orange color and the other characteristics of the “Surprise” variety.
     It seems absolutely evident, that no simple loss can account for this difference. Something of the character of the parent-varieties must have remained in the plant. And whatever conception we may formulate of these vestigial characters it is clear that the simplest and most obvious idea is their preservation in a dormant or latent state. Assuming that the distinguishing marks have only become inactive by virescence, it is manifest that on returning each will show its own peculiarities, as recorded above. Our second point was the incomplete loss of [232] the distinguishing quality in some varieties. It is of general occurrence, though often overlooked. Many white varieties of colored flowers give striking instances, among them many of the most stable and most prized garden-flowers. If you look at them separately or in little bouquets they seem to be of irreproachable purity. But if you examine large beds a pale hue will become visible. In many cases this tinge is so slight as to be only noticeable in a certain illumination, or by looking in an oblique direction across the bed; in others it is at once evident as soon as it has been pointed out. It always reminds the observer of the color of the species to which the variety belongs, being bluish in violets and harebells, reddish in godetias and phloxes, in Silene Armeria and many others. It proves that the original color quality of the species has not wholly, but only partly disappeared. It is dormant, but not entirely obliterated; latent, but not totally concealed; inactive, but only partially so. Our terminology is an awkward one; it practically assumes, as it so often does in other cases, a conventional understanding, not exactly corresponding to the simple meaning of the words. But it would be cumbrous to speak always of partial inactivity, incomplete latency or half awakening qualities. Even such words as sub-latent, [233] which would about express the real state of things, would have little chance of coming into general use.
     Such sub-latent colors are often seen on special parts in white varieties of flowers. In many cases it is the outer side of the petals which recalls the specific color, as in some white roses. In violets it is often on the spur that the remains of the original pigment are to be seen. In many instances it is on the tips of the petals or of the segments of the corolla, and a large number of white or yellow flowers betray their affinity to colored species by becoming red or bluish at the edges or on the outer side.
     The reality of such very slight hues, and their relation to the original pigment of the species may in some cases be proved by direct experiment. If it is granted that latency is not an absolute quality, then it will be readily accepted, that even latency must be subjected to the laws of gradual variation or fluctuating variability. We will deal with these laws in a later lecture but every one knows that greater deviations than the ordinary may be attained by sowing very large numbers and by selecting from among them the extreme individuals and sowing anew from their seed. In this way the slightest tinge of any latent color may be [234] strengthened, not indeed to the restoration of the tinge of the species, but at least so far as to leave no doubt as to the identity of the visible color of the species and the latent or sublatent one of the variety.
     I made such an experiment with the peach leaved harebell or Campanula persicifolia. The white variety of this species, which is often met with in our gardens, shows a very pale bluish hue when cultivated in large quantities, which however is subject to individual variations. I selected some plants with a decided tinge, flowered them separately, sowed their seeds, and repeated this during two generations. The result was an increase of the color on the tips of the segments of the corolla in a few individuals, most of them remaining as purely white as the original strain. But in those few plants the color was very manifest, individually variable in degree, but always of the same blue as in the species itself.
     Many other instances could be given. Smooth varieties are seldom absolutely so, and if scattering hairs are found on the leaves or only on some more or less concealed parts, they correspond in their character to those of the species. So it is with prickles, and even the thornless thorn-apple has fruits with surfaces far from smooth. The thornless horse-chestnut [235] has in some instances such evident protuberances on the valves of its fruits, that it may seem doubtful whether it is a pure and stable variety.
     Systematic latency may betray itself in different ways, either by normal systematic marks, or by atavism. With the latter I shall deal at length on another occasion, and therefore I will give here only one very clear and beautiful example. It is afforded by the common red clover. Obviously the clovers, with their three leaflets in each leaf, stand in the midst of the great family of papilionaceous plants, the leaves of which are generally pinnate. Systematic affinity suggests that the “three leaved” forms must have been derived from pinnate ancestors, evidently by the reduction of the number of the leaflets. In some species of clover the middle of the three is more or less stalked, as is ordinarily the case in pinnate leaves; in others it is as sessile as are its neighbors. In a subsequent chapter I will describe a very fine variety, which sometimes occurs in the wild state and may easily be isolated and cultivated. It is an ordinary red clover with five leaflets instead of three, and with this number varying between three and seven, instead of being nearly wholly stable as in the common form. It produces from time to time pinnate leaves, [236] very few indeed, and only rarely, but then often two or three or even more on the same individual. Intermediate stages are not wanting, but are of no consequence here. The pinnate leaves obviously constitute a reversion to some prototype, to some ancestor with ordinary papilionaceous leaves. They give proof of the presence of the common character of the family, concealed here in a latent state. Any other explanation of this curious anomaly would evidently be artificial. On the other hand nothing is really known about the ancestors of clover, and the whole conception rests only on the prevailing views of the systematic relationships in this family. But, as I have already said, further proof must be left for a subsequent occasion.
     Many instances, noted in our former lectures, could be quoted here. The systematic distribution of rayed and rayless species and varieties among the daisy-group of the composites affords a long series of examples. Accidental variations in both directions occur. The Canada fleabane or Erigeron canadensis, the tansy or Tanacetum vulgare and some others may at times be seen with ray-florets, and according to Murr, they may sometimes be wanting in Aster Tripolium, Bellis perennis, some species of Anthemis, Arnica montana and in a number [237] of other well-known rayed species. Another instance may be quoted; it has been pointed out by Grant Allen, and refers to the dead-nettle or Lamium album. Systematically placed in a genus with red-flowering species, we may regard its white color as due to the latency of the general red pigment.
     But if the flower of this plant is carefully examined, it will be found in most cases not to be purely white, but to have some dusky lines and markings on its lower lip. Similar devices are observed on the lip of the allied Lamium maculatum, and in a less degree on the somewhat distant Lamium purpureum. With Lamium maculatum or spotted dead-nettle, the affinity is so close that even Bentham united the two in a single species, considering the ordinary dead-nettle only as a variety of the dappled purple type. For the support of this conception of a specific or varietal retrograde change many other facts are afforded by the distribution of the characteristic color and of the several patterns of the lips of other labiates, and our general understanding of the relationships of the species and genera in this family may in a broad sense be based on the comparison of these seemingly subordinate characteristics. The same holds good in many other cases, and systematists have often become uncertain [238] as to the true value of some form, by its relationship to the allied types in the way of retrogressive modification. Color-differences are so showy, that they easily overshadow other characters. The white and the blue thorn-apple, the white and the red campion (Lychnis vespertina and diurna) and many other illustrative cases could be given, in which two forms are specifically separated by some authors, but combined by others on the ground of the retrograde nature of some differentiating mark.
     Hitherto we have dealt with negative characters and tried to prove that the conception of latency of the opposite positive characteristics is a more natural explanation of the phenomenon than the idea of a complete loss. We have now to consider the positive varieties, and to show that it is quite improbable that here the species have struck out for themselves a wholly new character. In some instances such may have been the case, but then I should prefer to treat these rather as elementary species. But in the main we will have to assume the latency of the character in the species and its reassumption by the variety when originating, as the most probable explanation.
     Great stress is laid upon this conception by the fact, that positive varieties are so excessively rare when compared with the common occurrence [239] of negative ones. Indeed, if we put aside the radiate and the color-varieties of flowers and foliage, hardly any cases can be cited. We have dealt with this question in a former lecture, and may now limit ourselves to the positive color-varieties.
     The latency of the faculty of producing the red pigment in leaves must obviously be accepted for nearly the whole vegetable kingdom. Oaks and elms, the beautiful climbing species of Ampelopsis, many conifers, as for instance Cryptomeria japonica, some brambles, the Guelder-rose (Viburnum Opulus) and many other trees and shrubs assume a more or less bright red color in the fall. During summer this tendency must have been dormant, and that this is so, is shown by the young leaves of oaks and others, which, when unfolding in the spring show a similar but paler hue. Moreover, there is a way of awakening the concealed powers at any time. We have only to inflict small wounds on the leaves, or to cut through the nerves or to injure them by a slight bruising, and the leaves frequently respond with an intense reddening of the living tissues around and especially above the wounds. Azolla caroliniana, a minute mosslike floating plant allied to the ferns, responds to light and cold with a reddish tinge, and to shade or warmth with a pure green. The foliage [240] of many other plants behaves likewise, as also do apples and peaches on the insolated sides of the fruits. It is quite impossible to state these groups of facts in a more simple way than by the statement that the tendency to become red is almost generally present, though latent in leaves and stems, and that it comes into activity whenever a stimulus provokes it.
     Now it must be granted that the energizing of such a propensity under ordinary circumstances is quite another thing from the origination of a positive variety by the evolution of the same character. In the variety the activity has become independent of outer influences or dependent upon them in a far lesser degree. The power of producing the red pigments is shown to be latent by the facts given above, and we see that in the variety it is no longer latent but is in perfect and lasting activity throughout the whole life of the plant.
     Red varieties of white flowers are much more rare. Here the latency of the red pigment may be deduced partly from general arguments like those just given, partly from the special systematic relations in the given cases. Hildebrand has clearly worked out this mode of proof. He showed by the critical examination of a large number of instances that the occurrence of the red-flowered varieties is contingent upon the [241] existence of red species in the same genus, or in some rare cases, in nearly allied genera. Colors that are not systematically present in the group to which a white species belongs are only produced in its varieties in extremely rare cases.
     We may quote some special rules, indicated by Hildebrand. Blue species are n the main very rare, and so are blue varieties of white species also. Carnations, Asiatic or cultivated buttercups (Ranunculus asiaticus), Mirabilis, poppies, Gladiolus, Dahlia, and some other highly cultivated or very old garden-plants have not been able to produce true blue flowers. But the garden-anemone (Anemone coronaria) has allies with very fine blue flowers. The common stock has bluish varieties and is allied to Aubretia and Hesperis, and gooseberries have a red form, recalling the ordinary currant. In nearly all other instances of blue or red varieties every botanist will be able to point out some allied red or blue species, as an indication of the probable source of the varietal character.
     Dark spots on the lower parts of the petals of some plants afford another instance, as in poppies and in the allied Glaucium, where they sometimes occur as varietal and in other cases as specific marks.
     The yellow fails in many highly developed [242] flowers, which are not liable to produce yellow variations, as in Salvia, Aster, Centaurea, Vinca, Polygala and many others. Even the rare pale yellowish species of some of these genera have no tendency in this direction. The hyacinths are the most remarkable, if not the sole known instance of a species having red and blue and white and yellow varieties, but here the yellow is not the bright golden color of the buttercups.
     The existence of varietal colors in allied species obviously points to a common cause, and this cause can be no other than the latency of the pigment in the species that do not show it.
     The conception of latency of characters as the common source of the origination of varieties, either in the positive or in the negative way, leads to some rules on variability, which are known under the names given to them by Darwin. They are the rules of repeated, homologous, parallel and analogous variability. Each of them is quite general, and may be recognized in instances from the most widely distant families. Each of them is quite evident and easily understood on the principle of latency.
     By the term of repeated variability is meant the well-known phenomenon, that the same variety has sprung at different times and in different [243] countries from the same species. The repetition obviously indicates a common internal cause. The white varieties of blue- and red-flowered plants occur in the wild state so often, and in most of the instances in so few individuals that a common pedigree is absolutely improbable. In horticulture this tendency is widely and vexatiously known, since the repetition of an old variety does not bring any advantage to the breeder. The old name of “conquests,” given by the breeders of hyacinths, tulips and other flower-bulbs to any novelty, in disregard of the common occurrence of repetitions, is an indication of the same experience in the repeated appearance of certain varieties.
     The rule of parallel variations demands that the same character occasionally makes its appearance in the several varieties or races, descended from the same species, and even in widely distinct species. This is a rule, which is very important for the general conception of the meaning of the term variety as contrasted with elementary species. For the recurrence of the same deviation always impresses us as a varietal mark. Laciniated leaves are perhaps the most beautiful instance, since they occur in so many trees and shrubs, as the walnut tree, the beech, the birch, the hazelnut, and even in [244] brambles and some garden-varieties of the turnip (Brassica).
     In such cases of parallel variations the single instances obviously follow the same rules and are therefore to be designated as analogous. Pitchers or ascidia, formed by the union of the margins of a leaf, are perhaps the best proof. They were classified by Morren under two heads, according to their formation from one or more leaves. Monophyllous pitchers obey the same law, viz.: that the upper side of the leaf has become the inner side of the pitcher. Only one exception to this rule is known to me. It is afforded by the pitchers of the banyan or holy fig-tree, Ficus religiosus, but it does not seem to belong to the same class as other pitchers, since as far as it has been possible to ascertain the facts, these pitchers are not formed by a few leaves as in all other cases, but by all the leaves of the tree.
     In some cases pitchers are only built up of part of the leaf-blade. Such partial malformations obey a rule, that is common to them and to other foliar enations, viz.: that the side of the leaf from which they emerge, is always their outer side. The inner surface of these enations corresponds to the opposite side of the leaf, both in color and in anatomical structure. The last of the four rules above mentioned is [245] that of the homologous variability. It asserts that the same deviation may occur in different, but homologous parts of the same plant. We have already dealt with some instances, as the occurrence of the same pigment in the flowers and foliage, in the fruits and seeds of the same plant, as also illustrated by the loss of the red or blue tinge by flowers and berries. Other instances are afforded by the curious fact that the division of the leaves into numerous and small segments is repeated by the petals, as in the common celandine and some sorts of brambles.
     It would take too long to make a closer examination of the numerous cases which afford proof of these statements. Suffice it to say that everywhere the results of close inspection point to the general rule, that the failure of definite qualities both in species and in varieties must, in a great number of cases, be considered as only apparent. Hidden from view, occasionally reappearing, or only imperfectly concealed, the same character must be assumed to be present though latent.
     In the case of negative or retrogressive varieties it is the transition from the active into a dormant state to which is due the origin of the variety. Positive varieties on the contrary owe their origin to the presence of some character [246] in the species in the latent state, and to the occasional re-energizing thereof.
     Specific or varietal latency is not the same thing as the ordinary latency of characters that only await their period of activity, or the external influence which will awake them. They are permanently latent, and could well be designated by the word perlatent. They spring into activity only by some sudden leap, and then at once become independent of ordinary external stimulation.

[247]

LECTURE IX

CROSSES OF SPECIES AND VARIETIES

In the foregoing lectures I have tried to show that there is a real difference between elementary species and varieties. The first are of equal rank, and together constitute the collective or systematic species. The latter are usually derived from real and still existing types. Elementary species are in a sense independent of each other, while varieties are of a derivative nature.
     Furthermore I have tried to show that the ways in which elementary or minor species must have originated from their common ancestor must be quite different from the mode of origin of the varieties. We have assumed that the first come into existence by the production of something new, by the acquirement of a character hitherto unnoticed in the line of their ancestors. On the contrary, varieties, in most cases, evidently owe their origin to the loss of an already existing character, or in other less frequent cases, to the re-assumption of a quality [248] formerly lost. Some may originate in a negative, others in a positive manner, but in both cases nothing really new is acquired.
     This distinction holds good for all cases in which the relationship between the forms in question is well known. It seems entirely justifiable therefore to apply it also to cases in which the systematic affinity is doubtful, as well as to instances in which it is impossible to arrive at any taxonomic conclusions. The extreme application of the principle would no doubt disturb the limits between many species and varieties as now recognized. It is not to be forgotten however that all taxonomic distinctions, which have not been confirmed by physiologic tests are only provisional, a view acknowledged by the best systematists. Of course the description of newly discovered forms can not await the results of physiologic inquiries; but it is absolutely impossible to reach definite conclusions on purely morphologic evidence. This is well illustrated by the numerous discords of opinion of different authors on the systematic worth of many forms.
     Assuming the above mentioned principle as established, and disregarding doubtful cases as indicated, the term progressive evolution is used to designate the method in which elementary species must have originated. It is the [249] manner in which all advance in the animal and vegetable kingdoms must have taken place, continuously adding new characters to the already existing number. Contrasted with this method of growing differentiation, are the retrogressive modifications, which simply retrace a step, and the degressive changes in which a backward step is retraced and old characters revived. No doubt both of these methods have been operative on a large scale, but they are evidently not in the line of general advancement.
     In all of these directions we see that the differentiating marks show more or less clearly that they are built up of units. Allied forms are separated from each other without intermediates. Transitions are wholly wanting, although fallaciously apparent in some instances owing to the wide range of fluctuating variability of the forms concerned, or to the occurrence of hybrids and subvarieties.
     These physiologic units, which in the end must be the basis for the distinction of the systematic units, may best be designated by the term of “unit-characters.” Their internal nature is as yet unknown to us, and we will not now look into the theories, which have been propounded as to the probable material basis underlying them. For our present purpose the empirical evidence of the general occurrence of [250] sharp limits between nearly related characters must suffice. As Bateson has put it, species are discontinuous, and we must assume that their characters are discontinuous also.
     Moreover there is as yet no reason for trying to make a complete analysis of all the characters of a plant. No doubt, if attained, such an analysis would give us a deep insight into the real internal construction of the intricate properties of organisms in general. But taxonomic studies in this direction are only in their infancy and do not give us the material required for such an analysis. Quite on the contrary, they compel us to confine our study to the most recently acquired, or youngest characters, which constitute the differentiating marks between nearly allied forms.
     Obviously this is especially the case in the realm of the hybrids, since only nearly related forms are able to give hybrid offspring. In dealing with this subject we must leave aside all questions concerning more remote relationships.
     It is not my purpose to treat of the doctrine of hybridization at any length. Experience is so rapidly increasing both in a practical and in a purely scientific direction that it would take an entire volume to give only a brief survey of the facts and of all the proposed theories.
     [251] For our present purposes we are to deal with hybrids only in so far as they afford the means of a still better distinction between elementary species and varieties. I will try to show that these two contrasting groups behave in quite a different manner, when subjected to crossing experiments, and that the hope is justified that some day crosses may become the means of deciding in any given instance, what is to be called a species, and what a variety, on physiologic grounds. It is readily granted that the labor required for such experiments, is perhaps too great for the results to be attained, but then it may be possible to deduce rules from a small series of experiments, which may lead us to a decision in wider ranges of cases.
     To reach such a point of view it is necessary to compare the evidence given by hybrids, with the conclusions already attained by the comparison of the differentiating characteristics of allied forms.
     On this ground we first have to inquire what may be expected respecting the internal nature and the outcome of the process of crossing in the various cases cited in our former discussion.
     We must always distinguish the qualities, which are the same in both parents, from those that constitute the differentiating marks in every single cross. In respect to the first [252] group the cross is not at all distinguished from a normal fertilization, and ordinarily these characters are simply left out of consideration. But it should never be forgotten that they constitute the enormous majority, amounting to hundreds and thousands, whereas the differentiating marks in each case are only one or two or a few at most. The whole discussion is to be limited to these last-named exceptions. We must consider first what would be the nature of a cross when species are symmetrically combined, and what must be the case when varieties are subjected to the same treatment. In so doing, I intend to limit the discussion to the most typical cases. We may take the crosses between elementary species of the same or of very narrowly allied systematic species on the one side, and on the other, limit treatment to the crossing of varieties with the species, from which they are supposed to have sprung by a retrograde modification. Crosses of different varieties of the same species with one another obviously constitute a derivative case, and should only be discussed secondarily. And crosses of varieties with positive or depressive characters have as yet so rarely been made that we may well disregard them.
     Elementary species differ from their nearest allies by progressive changes, that is by the acquirement [253] of some new character. The derivative species has one unit more than the parent. All other qualities are the same as in the parent. Whenever such a derivative is combined with its parent the result for these qualities will be exactly as in a normal fertilization. In such ordinary cases it is obvious that each character of the pollen-parent is combined with the same character of the pistil-parent. There may be slight individual differences, but each unit character will become opposed to, and united with, the same unit-character in the other parent. In the offspring the units will thus be paired, each pair consisting of two equivalent units. As to their character the units of each single pair are the same, only they may exhibit slight differences as to the degree of development of this character.
     Now we may apply this conception to the sexual combination of two different elementary species, assuming one to be the derivative of the other. The differentiating mark is only present in one of the parents and wanting in the other. While all other units are paired in the hybrid, this one is not. It meets with no mate, and must therefore remain unpaired. The hybrid of two such elementary species is in some way incomplete and unnatural. In the ordinary course of things all individuals derive [254] their qualities from both parents; for each single mark they possess at least two units. Practically but not absolutely equal, these two opponents always work together and give to the offspring a likeness to both parents. No unpaired qualities occur in normal offspring; these constitute the essential features of the hybrids of species and are at the same time the cause of their wide deviations from the ordinary rules.
     Turning now to the varieties, we likewise need discuss their differentiating marks only. In the negative types, these consist of the apparent loss of some quality which was active in the species. But it was pointed out in our last lecture that such a change is an apparent loss. On a closer inquiry we are led to the assumption of a latent or dormant state. The presumably lost characters have not absolutely, or at least not permanently disappeared. They show their presence by some slight indication of the quality they represent, or by occasional reversions. They are not wanting, but only latent.
     Basing our discussion concerning the process of crossing on this conception, and still limiting the discussion to one differentiating mark, we come to the inference, that this mark is present and active in the species, and present but dormant in the variety. Thus it is present in both, and as all other characters not differentiating [255] find their mates in the cross, so these two will also meet one another. They will unite just as well as though they were both active or both dormant. For essentially they are the same, only differing in their degree of activity. From this we can infer, that in the crossing of varieties, no unpaired remainder is left, all units combining in pairs exactly as in ordinary fertilization.
     Setting aside the contrast between activity and latency in this single pair, the procedure in the inter-crossing of varieties is the same as in ordinary normal fertilization.
     Summarizing this discussion we may conclude that in normal fertilization and in the inter-crossing of varieties all characters are paired, while in crosses between elementary species the differentiating marks are not mated.
     In order to distinguish these two great types of fertilization we will use the term bisexual for the one and unisexual for the other. The term balanced crosses then conveys the idea of complete bisexuality, all unit-characters combining in pairs. Unbalanced crosses are those in which one or more units do not find their mates and therefore remain unpaired. This distinction was proposed by Macfarlane when studying the minute structure of plant-hybrids in comparison with that of their parents (1892).
     [256] In the first place it shows that a species hybrid may inherit the distinguishing marks of both parents. In this way it may become intermediate between them, having some characters in common with the pollen-parent and others with the pistil-parent. As far as these characters do not interfere with each other, they may be fully developed side by side, and in the main this is the way in which hybrid characters are evolved. But in most cases our existing knowledge of the units is far too slender to give a complete analysis, even of these distinguishing marks alone. We recognize the parental marks more or less clearly, but are not prepared for exact delimitations. Leaving these theoretical considerations, we will pass to the description of some illustrative examples.
     In the first place I will describe a hybrid between two species of Oenothera, which I made some years ago. The parents were the common evening-primrose or Oenothera biennis and of its small-flowered congener, Oenothera muricata. These two forms were distinguished by Linnaeus as different species, but have been considered by subsequent writers as elementary species or so-called systematic varieties of one species designated with the name of the presumably older type, the O. biennis. Varietal differences in a physiologic sense they [257] do not possess, and for this reason afford a pure instance of unbalanced union, though differing in more than one point.
     I have made reciprocal crosses, taking at one time the small-flowered and at the other the common species as pistillate parent. These crosses do not lead to the same hybrid as is ordinarily observed in analogous cases; quite on the contrary, the two types are different in most features, both resembling the pollen-parent far more than the pistil-parent. The same curious result was reached in sundry other reciprocal crosses between species of this genus. But I will limit myself here to one of the two hybrids.
     In the summer of 1895 I castrated some flowers of O. muricata, and pollinated them with O. biennis, surrounding the flowers with paper bags so as to exclude the visits of insects. I sowed the seeds in 1896 and the hybrids were biennial and flowered abundantly the next year and were artificially fertilized with their own pollen, but gave only a very small harvest. Many capsules failed, and the remaining contained only some few ripe seeds.
     From these I had in the following year the second hybrid generation, and in the same way I cultivated also the third and fourth. These were as imperfectly fertile as the first, and in [258] some years did not give any seed at all, so that the operation had to be repeated in order to continue the experiment. Last summer (1903) I had a nice lot of some 25 biennial specimens blooming abundantly. All in all I have grown some 500 hybrids, and of these about 150 specimens flowered.
     These plants were all of the same type, resembling in most points the pollen-parent, and in some others the pistil-parent of the original cross. The most obvious characteristic marks are afforded by the flowers, which in O. muricata are not half so large as in biennis, though borne by a calyx-tube of the same length. In this respect the hybrid is like the biennis bearing the larger flowers. These may at times seem to deviate a little in the direction of the other parent, being somewhat smaller and of a slightly paler color. But it is very difficult to distinguish between them, and if biennis and hybrid flowers were separated from the plants and thrown together, it is very doubtful whether one would succeed in separating them.
     The next point is offered by the foliage. The leaves of O. biennis are broad, those of O. muricata narrow. The hybrid has the broad leaves of O. biennis during most of its life and at the time of flowering. Yet small deviations in the [259] direction of the other parent are not wanting, and in winter the leaves of the hybrid rosettes are often much narrower than those of O. biennis, and easily distinguishable from both parents. A third distinction consists in the density of the spike. The distance between the insertion of the flowers of O. biennis is great when compared with that of O. muricata. Hence the flowers of the latter species are more crowded and those of O. biennis more dispersed, the spikes of the first being densely crowned with flowers and flower-buds while those of O. biennis are more elongated and slender. As a further consequence the O. biennis opens on the same evening only one, two or three flowers on the same spike, whereas O. muricata bears often eight or ten or more flowers at a time. In this respect the hybrid is similar to the pistil-parent, and the crowding of the broad flowers at the top of the spikes causes the hybrids to be much more showy than either of the parent types.
     Other distinguishing marks are not recorded by the systematists, or are not so sharply separated as to allow of the corresponding qualities of the hybrids being compared with them.
     This hybrid remains true to the description given. In some years I cultivated two generations [260] so as to be able to compare them with one another, but did not find any difference. The most interesting point however, is the likeness between the first generation, which obviously must combine in its internal structure the units of both parents, and the second and later generations which are only of a derivative nature. Next to this stands the fact that in each generation all individuals are alike. No reversion to the parental forms either in the whole type or in the single characteristics has ever been observed, though the leaves of some hundreds, and the spikes and flowers of some 150 individual plants have been carefully examined. No segregation or splitting up takes place.
     Here we have a clear, undoubted and relatively simple, case of a true and pure species hybrid. No occurrence of possible varietal characteristics obscures the result, and in this respect this hybrid stands out much more clearly than all those between garden-plants, where varietal marks nearly always play a most important part.
     From the breeder’s point of view our hybrid Oenothera would be a distinct gain, were it not for the difficulty of its propagation. But to enlarge the range of the varieties this simple and stable form would need to be treated anew, by [261] crossing it with the parent-types. Such experiments however, have miscarried owing to the too stable nature of the unit-characters.
     This stability and this absence of the splitting shown by varietal marks in the offspring of hybrids is one of the best proofs of unisexual unions. It is often obscured by the accompanying varietal marks, or overlooked for this reason. Only in rare cases it is to be met with in a pure state and some examples are given of this below.
     Before doing so, I must call your attention to another feature of the unbalanced unions. This is the diminution of the fertility, a phenomenon universally known as occurring in hybridizations. It has two phases. First, the diminished chance of the crosses themselves of giving full crops of seed, as compared with the pure fertilization of either parent. And, secondly, the fertility of the hybrids themselves. Seemingly, all grades of diminished fertility occur and the oldest authors on hybrids have pointed out that a very definite relation exists between the differences of the parents and the degree of sterility, both of the cross and of the hybrid offspring. In a broad sense these two factors are proportionate to each other, the sterility being the greater, the lesser the affinity between the parents. Many writers have [262] tried to trace this rule in the single cases, but have met with nearly unsurmountable difficulties, owing chiefly to our ignorance of the units which form the differences between the parents in the observed cases.
     In the case of Oenothera muricata x biennis the differentiating units reduce the fertility to a low degree, threatening the offspring with almost complete infertility and extinction. But then we do not know whether these characters are really units, or perhaps only seemingly so and are in reality composed of smaller entities which as yet we are not able to segregate. And as long as we are devoid of empirical means of deciding such questions, it seems useless to go farther into the details of the question of the sterility. It should be stated here however, that pure varietal crosses, when not accompanied by unbalanced characters, have never showed any tendency to diminished fertility. Hence there can be little doubt that the unpaired units are the cause of this decrease in reproductive power.
     The genus Oenothera is to a large degree devoid of varietal characteristics, especially in the subgenus Onagra, to which biennis, muricata, lamarckiana and some others belong. On the other hand it seems to be rich in elementary species, but an adequate study of [263] them has as yet not been made. Unfortunately many of the better systematists are in the habit of throwing all these interesting forms together, and of omitting their descriptive study. I have made a large number of crosses between such undescribed types and as a rule got constant hybrid races. Only one or two exceptions could be quoted, as for instance the Oenothera brevistylis, which in its crosses always behaves as a pure retrogressive variety. Instead of giving an exhaustive survey of hybrids, I simply cite my crosses between lamarckiana and biennis, as having nearly the aspect of the last named species, and remaining true to this in the second generation without any sign of reversion or of splitting. I have crossed another elementary species, the Oenothera hirtella with some of my new and with some older Linnean species, and got several constant hybrid races. Among these the offspring of a cross between muricata and hirtella is still in cultivation. The cross was made in the summer of 1897 and last year (1903) I grew the fourth generation of the hybrids. These had the characters of the muricata in their narrow leaves, but the elongated spikes and relatively large flowers of the hirtella parent, and remained true to this type, showing only slight fluctuations and never reverting or segregating [264] the mixed characters. Both parents bear large capsules with an abundance of seed, but in the hybrids the capsules remain narrow and weak, ripening not more than one-tenth the usual quantity of seed. Both parents are easily cultivated in annual generations and the same holds good for the hybrid. But whereas the hybrid of muricata and biennis is a stout plant, this type is weak with badly developed foliage, and very long strict spikes. Perhaps it was not able to withstand the bad weather of the last few years.
     A goodly number of constant hybrids are described in literature, or cultivated in fields and gardens. In such cases the essential question is not whether they are now constant, but whether they have been so from the beginning, or whether they prove to be constant whenever the original cross is repeated. For constant hybrids may also be the issue of incipient splittings, as we shall soon see.
     Among other examples we may begin with the hybrid alfalfa or hybrid lucerne (Medicago media). It often originates spontaneously between the common purple lucerne or alfalfa and its wild ally with yellow flowers and procumbent stems, the Medicago falcata. This hybrid is cultivated in some parts of Germany on a large scale, as it is more productive than [265] the ordinary lucerne. It always comes true from seed and may be seen in a wild state in parks and on lawns. It is one of the oldest hybrids with a pure and known lineage. The original cross has been repeated by Urban, who found the hybrid race to be constant from the beginning.
     Another very notorious constant hybrid race is the Aegilops speltaeformis. It has been cultivated in botanic gardens for more than half a century, mostly in annual or biennial generations. It is sufficiently fertile and always comes true. Numerous records have been made of it, since formerly it was believed by Fabre and others to be a spontaneous transition from some wild species of grass to the ordinary wheat, not a cross. Godron, however, showed that it can be produced artificially, and how it has probably sprung into existence wherever it is found wild. The hybrid between Aegilops ovata, a small weed, and the common wheat is of itself sterile, producing no good pollen. But it may be fertilized by the pollen of wheat and then gives rise to a secondary hybrid, which is no other than the Aegilops speltaeformis. This remained constant in Godron’s experiments during a number of generations, and has been constant up to the present time.
     [266] Constant hybrids have been raised by Millardet between several species of strawberries. He combined the old cultivated forms with newly discovered types from American localities. They ordinarily showed only the characteristics of one of their parents and did not exhibit any new combination of qualities, but they came true to this type in the second and later generations.
     In the genus Anemone, Janczewski obtained the same results. Some characters of course may split, but others remain constant, and when only such are present, hybrid races result with new combinations of characters, which are as constant as the best species of the same genus. The hybrids of Janczewski were quite fertile, and he points out that there is no good reason why they should not be considered as good new species. If they had not been produced artificially, but found in the wild state, their origin would have been unknown, and there can be no doubt that they would have been described by the best systematists as species of the same value as their parents. Such is especially the case with a hybrid between Anemone magellanica and the common Anemone sylvestris.
     Starting from similar considerations Kerner von Marilaun pointed out the fact long ago that many so-called species, of rare occurrence, [267] standing between two allied types, may be considered to have originated by a cross. Surely a wide field for abuse is opened by such an assertion, and it is quite a common habit to consider intermediate forms as hybrids, on the grounds afforded by their external characters alone, and without any exact knowledge of their real origin and often without knowing anything as to their constancy from seed. All such apparent explanations are now slowly becoming antiquated and obsolete, but the cases adduced by Kerner seem to stand this test.
     Kerner designates a willow, Salix ehrhartiana as a constant hybrid between Salix alba and S. pentandra. Rhododendron intermedium is an intermediate form between the hairy and the rusty species from the Swiss Alps, R. hirsutum and R. ferrugineum, the former growing on chalky, and the other on silicious soils. Wherever both these types of soil occur in the same valley and these two species approach one another, the hybrid R. intermedium is produced, and is often seen to be propagating itself abundantly. As is indicated by the name, it combines the essential characters of both parents.
Linaria italica is a hybrid toad-flax between L. genistifolia and L. vulgaris, a cross which I have repeated in my garden. Drosera obovata [268] is a hybrid sundew between D. anglica and D. rotundifolia. Primula variabilis is a hybrid between the two common primroses, P. officinalis and P. grandiflora. The willow-herb (Epilobium), the self-heal (Brunella) and the yellow pond-lilies (Nuphar) afford other instances of constant wild hybrids.
     Macfarlane has discovered a natural hybrid between two species of sundew in the swamps near Atco, N.J. The parents, D. intermedia and D. filiformis, were growing abundantly all around, but of the hybrid only a group of eleven plants was found. A detailed comparison of the hybrid with its parents demonstrated a minute blending of the anatomical peculiarities of the parental species.
     Luther Burbank of Santa Rosa, California, has produced a great many hybrid brambles, the qualities of which in many respects surpass those of the wild species. Most of them are only propagated by cuttings and layers, not being stable from seed. But some crosses between the blackberry and the raspberry (R. fruticosus and R. idaeus) which bear good fruit and have become quite popular, are so fixed in their type as to reproduce their composite characters from seed with as much regularity as the species of Rubus found in nature. Among them are the “Phenomenal” and the [269] “Primus.” The latter is a cross between the Californian dewberry and the Siberian raspberry and is certainly to be regarded as a good stable species, artificially produced. Bell Salter crossed the willow-herbs Epilobium tetragonum and E. montanum, and secured intermediate hybrids which remained true to their type during four successive generations.
     Other instances might be given. Many of them are to be found in horticultural and botanical journals which describe their systematic and anatomical details. The question of stability is generally dealt with in an incidental manner, and in many cases it is difficult to reach conclusions from the facts given. Especially disturbing is the circumstance that from a horticultural point of view it is quite sufficient that a new type should repeat itself in some of its offspring to be called stable, and that for this reason absolute constancy is rarely proved.
     The range of constant hybrids would be larger by far were it not for two facts. The first is the absolute sterility of so many beautiful hybrids, and the second is the common occurrence of retrogressive characters among cultivated plants. To describe the importance of both these groups of facts would take too much [270] time, and therefore it seems best to give some illustrative examples instead.
     Among the species of Ribes or currant, which are cultivated in our gardens, the most beautiful are without doubt the Californian and the Missouri currant, or Ribes sanguineum and R. aureum. A third form, often met with, is “Gordon’s currant,” which is considered to be a hybrid between the two. It has some peculiarities of both parents. The leaves have the general form of the Californian parent, but are as smooth as the Missouri species. The racemes or flower-spikes are densely flowered as in the red species, but the flowers themselves are of a yellow tinge, with only a flesh-red hue on the outer side of the calyx. It grows vigorously and is easily multiplied by cuttings, but it never bears any fruit. Whether it would be constant, if fertile, is therefore impossible to decide. Berberis ilicifolia is considered as a hybrid between the European barberry (B. vulgaris) and the cultivated shrub Mahonia aquifolia. The latter has pinnate leaves, the former undivided ones. The hybrid has undivided leaves which are more spiny than those of the European parent, and which are not deciduous like them, but persist during the winter, a peculiarity inherited from the Mahonia. As far as I [271] have been able to ascertain, this hybrid never produces seed.
     Another instance of an absolutely sterile hybrid is the often quoted Cytisus adami. It is a cross between the common laburnum (Cytisus Laburnum) and another species of the same genus, C. purpureus, and has some traits of both. But since the number of differentiating marks is very great in this case, most of the organs have become intermediate. It is absolutely sterile. But it has the curious peculiarity of splitting in a vegetative way. It has been multiplied on a large scale by grafting and was widely found in the parks and gardens of Europe during the last century. Nearly all these specimens reverted from time to time to the presumable parents. Not rarely a bud of Adam’s laburnum assumed all the qualities of the common laburnum, its larger leaves, richer flowered racemes, large and brightly yellow flowers and its complete fertility. Other buds on the same tree reverted to the purple parent, with its solitary small flowers, its dense shrublike branches and very small leaves. These too are fertile, though not producing their seeds as abundantly as the C. Laburnum reversions. Many a botanist has sown the seeds of the latter and obtained only pure common C. Laburnum plants. I had a lot of nearly a hundred seedlings [272] myself, many of which have already flowered, bearing the leaves and flowers of the common species. Seeds of the purple reversions have also been sown, and also yielded the parental type only.
     Why this most curious hybrid sports so regularly and why others always remain true to their type is as yet an open question.
     But recalling our former consideration of this subject the supposition seems allowable that the tendency to revert is not connected with the type of the hybrid, but is apt to occur in some rare individuals of every type. But since most of the sterile hybrids are only known to us in a single individual and its vegetative offspring, this surmise offers an explanation of the rare occurrence of sports.
     Finally, we must consider some of the so called hybrid races or strains of garden-plants. Dahlia, Gladiolus, Amaryllis, Fuchsia, Pelargonium and many other common flowers afford the best known instances. Immeasurable variability seems here to be the result of crossing. But on a closer inspection the range of characters is not so very much wider in these hybrid races than in the groups of parent species which have contributed to the origin of the hybrids. Our tuberous begonias owe their variability to at least seven original parent species, [273] and to the almost incredible number of combinations which are possible between their characters. The first of these crosses was made in the nursery of Veitch and Sons near London by Seden, and the first hybrid is accordingly known as Begonia sedeni and is still to be met with. It has been superseded by subsequent crosses between the sedeni itself and the Veitchi and rosiflora, the davisii, the clarkii and others. Each of them contributed its advantageous qualities, such as round flowers, rosy color, erect flower stalks, elevation of the flowers above the foliage and others. New crosses are being made continuously, partly between the already existing hybrids and partly with newly introduced wild species. Only rarely is it possible to get pure seeds, and I have not yet been able to ascertain whether the hybrids would come true from seed. Specific and varietal characters may occur together in many of the several forms, but nothing is as yet accurately known as to their behavior in pure fertilizations. Constancy and segregation are thrown together in such a manner that extreme variability results, and numerous beautiful types may be had, and others may be expected from further crosses. For a scientific analysis, however, the large range of recorded facts and the written history, which at first sight [274] seems to have no lacunae, are not sufficient. Most of the questions remain open and need investigation. It would be a capital idea to try to repeat the history of the begonias or any other hybrid race, making all the described crosses and then recording the results in a manner requisite for complete and careful scientific investigations.
     Many large genera of hybrid garden-flowers owe their origin to species rich in varieties or in elementary subspecies. Such is the case with the gladiolus and the tulips. In other cases the original types have not been obtained from the wild state but from the cultures of other countries.
     The dahlias were cultivated in Mexico when first discovered by Europeans, and the chrysanthemums have been introduced from the old gardens of Japan. Both of them consisted of various types, which afterwards have been increased chiefly by repeated intercrossing.
     The history of many hybrid races is obscure, or recorded by different authorities in a different way. Some have derived their evidence from one nursery, some from another, and the crosses evidently may have been different in different places. The early history of the gladiolus is an instance. The first crosses are recorded to have been made between Gladiolus [275] psittacinus and G. cardinalis, and between their hybrid, which is still known under the name of gandavensis and the purpureo-auratus. But other authors give other lines of descent. So it is with Amaryllis, which is said by De Graaff to owe its stripes to A. vittata, its fine form to A. brasiliensis, the large petals to A. psittacina, the giant flowers to A. leopoldi, and the piebald patterns to A. pardina. But here, too, other authors give other derivations.
     Summarizing the results of our inquiry we see in the first place how very much remains to be done. Many old crosses must be repeated and studied anew, taking care of the purity of the cross as well as of the harvesting of the seeds. Many supposed facts will be shown to be of doubtful validity. New facts have to be gathered, and in doing so the distinction between specific and varietal marks must be taken strictly into account. The first have originated as progressive mutations; they give unbalanced crosses with a constant offspring, as far as experience now goes. The second are chiefly due to retrograde modifications, and will be the subject of the next lecture.

[276]

LECTURE X

MENDEL’S LAW OF BALANCED CROSSES

In the scientific study of the result of crosses, the most essential point is the distinction of the several characters of the parents in their combination in the hybrids and their offspring. From a theoretical point of view it would be best to choose parents which would differ only in a single point. The behavior of the differentiating character might then easily be seen.
     Unfortunately, such simple cases do not readily occur. Most species, and even many elementary species are distinguished by more than one quality. Varieties deviating only in one unit-character from the species, are more common. But a closer inspection often reveals some secondary characters which may be overlooked in comparative or descriptive studies, but which reassume their importance in experimental crossings.
     In a former lecture we have dealt with the qualities which must be considered as being due to the acquisition of new characters. If we [277] compare the new form in this case with the type from which it has originated, it may be seen that the new character does not find its mate, or its opposite, and it will be unpaired in the hybrid.
     In the case of retrogressive changes the visible modification is due, at least in the best known instances, to the reduction of an active quality to a state of inactivity or latency. Now if we make a cross between a species and its variety, the differentiating character will be due to the same internal unit, with no other difference than that it is active in the species and latent in the variety. In the hybrid these two corresponding units will make a pair. But while all other pairs in the same hybrid individuals consist of like antagonists, only this pair consists of slightly unlike opponents.
     This conception of varietal crosses leads to three assertions, which seem justifiable by actual experience.
     First, there is no reason for a diminution of the fertility, as all characters are paired in the hybrid, and no disturbance whatever ensues in its internal structure. Secondly, it is quite indifferent, how the two types are combined, or which of them is chosen as pistillate and which as staminate parent. The deviating pair will have the same constitution in both cases, being [278] built up of one active and one dormant unit. Thirdly this deviating pair will exhibit the active unit which it contains, and the hybrid will show the aspect of the parent in which the character was active and not that of the parent in which it was dormant. Now the active quality was that of the species, and its latent state was found in the variety. Hence the inference that hybrids between a species and its retrograde variety will bear the aspect of the species. This attribute may be fully developed, and then the hybrid will not be distinguishable from the pure species in its outer appearance. Or the character may be incompletely evolved, owing to the failure of cooperation of the dormant unit. In this case the hybrid will be in some sense intermediate between its parents, but these instances are more rare than the alternate ones, though presumably they may play an important part in the variability of many hybrid garden-flowers.
     All of these three rules are supported by a large amount of evidence. The complete fertility of varietal hybrids is so universally acknowledged that it is not worth while to give special instances. With many prominent systematists it has become a test between species and varieties, and from our present point of view this assumption is correct. Only the test is of little use in practice, as fertility may be diminished [279] in unbalanced unions in all possible degrees, according to the amount of difference between the parents. If this amount is slight, if for instance, only one unit-character causes the difference, the injury to fertility may, be so small as to be practically nothing. Hence we see that this test would not enable us to judge of the doubtful cases, although it is quite sufficient as a proof in cases of wider differences.
     Our second assertion related to the reciprocal crosses. This is the name given to two sexual combinations between the same parents, but with interchanged places as to which furnishes the pollen. In unbalanced crosses of the genus Oenothera the hybrids of such reciprocal unions are often different, as we have previously shown. Sometimes both resemble the pollen parent more, in other instances the pistil-parent. In varietal crosses no such divergence is as yet known. It would be quite superfluous to adduce single cases as proofs for this rule, which was formerly conceived to hold good for hybrids in general. The work of the older hybridists, such as Koelreuter and Gaertner affords numerous instances.
     Our third rule is of a wholly different nature. Formerly the distinction between elementary species and varieties was not insisted upon, and the principle which stamps retrograde changes [280] as the true character of varieties is a new one. Therefore it is necessary to cite a considerable amount of evidence in order to prove the assertion that a hybrid bears the active character of its parent-species and not the inactive character of the variety chosen for the cross.
     We may put this assertion in a briefer form, stating that the active character prevails in the hybrid over its dormant antagonist. Or as it is equally often put, the one dominates and the other is recessive. In this terminology the character of the species is dominant in the hybrid while that of the variety is recessive. Hence it follows that in the hybrid the latent or dormant unit is recessive, but it does not follow that these three terms have the same meaning, as we shall see presently. The term recessive only applies to the peculiar state into which the latent character has come in the hybrid by its pairing with the antagonistic active unit.
     In the first place it is of the highest importance to consider crosses between varieties of recorded origin and the species from which they have sprung. When dealing with mutations of celandine we shall see that the laciniated form originated from the common celandine in a garden at Heidelberg about the year 1590. Among my Oenotheras one of the eldest of the recent productions is the O. brevistylis or short [281] styled species which was seen for the first time in the year 1889. The third example offered is a hairless variety of the evening campion, Lychnis vespertina, found the same year, which hitherto had not been observed.
     For these three cases I have made the crosses of the variety with the parent-species, and in each case the hybrid was like the species, and not like the variety. Nor was it intermediate. Here it is proved that the older character dominates the younger one.
     In most cases of wild, and of garden-varieties, the relation between them and the parent-species rests upon comparative evidence. Often the variety is known to be younger, in other cases it may be only of local occurrence, but ordinarily the historic facts about its origin have never been known or have long since been forgotten.
     The easiest and most widely known varietal crosses are those between varieties with white flowers and the red- or blue-flowered species. Here the color prevails in the hybrid over the lack of pigment, and as a rule the hybrid is as deeply tinted as the species itself, and cannot be distinguished from it, without an investigation of its hereditary qualities. Instances may be cited of the white varieties of the snapdragon, of the red clover, the long-spurred violet (Viola [282] cornuta) the sea-shore aster (Aster Tripolium), corn-rose (Agrostemma Githago), the Sweet William (Silene Armeria), and many garden flowers, as for instance, the Clarkia pulchella, the Polemonium coeruleum, the Veronica longifolia, the gloxinias and others. If the red hue is combined with a yellow ground-color in the species, the variety will be yellow and the hybrid will have the red and yellow mixture of the species as for instance, in the genus Geum. The toad-flax has an orange-colored palate, and a variety occurs in which the palate is of the same yellow tinge as the remaining parts of the corolla. The hybrid between them is in all respects like the parent-species.
     Other instances could be given. In berries the same rule prevails. The black nightshade has a variety with yellow berries, and the black color returns in the hybrid. Even the foliage of some garden-plants may afford instances, as for instance, the purplish amaranth (Amaranthus caudatus). It has a green variety, but the hybrid between the two has the red foliage of the species.
     Special marks in leaves and in flowers follow the same rule. Some varieties of the opium poppy have large black patches at the basal end of the petals, while in others this pattern is entirely white. In crossing two such varieties, [283] for instance, the dark “Mephisto” with the white-hearted “Danebrog,” the hybrid shows the active character of the dark pattern.
     Hairy species crossed with their smooth varieties produce hairy hybrids, as in some wheats, in the campion (Lychnis), in Biscutella and others. The same holds good for the crosses between spiny species and their unarmed derivatives, as in the thorn-apple, the corn-crowfoot (Ranunculus arvensis) and others.
     Lack of starch in seeds is observed in some varieties of corn and of peas. When such derivatives are crossed with ordinary starch-producing types, the starch prevails in the hybrid.
     It would take too much time to give further examples. But there is still one point which should be insisted upon. It is not the systematic relation of the two parents of a cross, that is decisive, but only the occurrence of the same quality, in the one in an active, and in the other in an inactive condition. Hence, whenever this relation occurs between the parents of a cross, the active quality prevails in the hybrid, even when the parents differ from each other in other respects so as to be distinguished as systematic species. The white and red campions give a red hybrid, the black and pale henbane (Hyoscyamus niger and H. pallidus) give a hybrid [284] with the purple veins and center in the corolla of the former, the white and blue thornapple produce a blue hybrid, and so on. Instances of this sort are common in cultivated plants.
     Having given this long list of examples of the rule of the dominancy of the active character over the opposite dormant unit, the question naturally arises as to how the antagonistic units are combined in the hybrid. This question is of paramount importance in the consideration of the offspring of the hybrids. But before taking it up it is as well to learn the real signification of recessiveness in the hybrids themselves.
     Recessive characters are shown by those rare cases, in which hybrids revert to the varietal parent in the vegetative way. In other words by bud-variations or sports, analogous to the splitting of Adam’s laburnum into its parents, by means of bud-variation already described. But here the wide range of differentiating characters of the parents of this most curious hybrid fail. The illustrative examples are extremely simple, and are limited to the active and inactive condition of only one quality.
     An instance is given by the long-leaved veronica (Veronica longifolia), which has bluish flowers in long spikes. The hybrid between [285] this species and its white variety has a blue corolla. But occasionally it produces some purely white flowers, showing its power of separating the parental heritages, combined in its internal structures. This reversion is not common, but in thousands of flowering spikes one may expect to find at least one of them. Sometimes it is a whole stem springing from the underground system and bearing only white flowers on all its spikes. In other instances it is only a side branch which reverts and forms white flowers on a stem, the other spikes of which remain bluish. Sometimes a spike even differentiates longitudinally, bearing on one side blue and on the other white corollas, and the white stripe running over the spike may be seen to be long and large, or narrow and short in various degrees. In such cases it is evident that the heritages of the parents remain uninfluenced by each other during the whole life of the hybrid, working side by side, but the active element always prevails over its latent opponent which is ready to break free whenever an opportunity is offered.
     It is now generally assumed that this incomplete mixture of the parental qualities in a hybrid, this uncertain and limited combination is the true cause of the many deviations, exhibited by varietal hybrids when compared with their [286] parents. Partial departures are rare in the hybrids themselves, but in their offspring the divergence becomes the rule.
     Segregation seems to be a very difficult process in the vegetative way, but it must be very easy in sexual reproduction, indeed so easy as to show itself in nearly every single instance.
     Leaving this first generation, the original hybrids, we now come to a discussion of their offspring. Hybrids should be fertilized either by their own pollen, or by that of other individuals born from the same cross. Only in this case can the offspring be considered as a means of arriving at a decision as to the internal nature of the hybrids themselves. Breeders generally prefer to fertilize hybrids with the pollen of their parents. But this operation is to be considered as a new cross, and consequently is wholly excluded from our present discussion. Hence it follows that a clear insight into the heredity of hybrids may be expected only from scientific experiments. Furthermore some of the diversity observed as a result of ordinary crosses, may be due to the instability of the parents themselves or at least of one of them, since breeders ordinarily choose for their crosses some already very variable strain. Combining such a strain with the desirable qualities of some newly imported species, a new strain may [287] result, having the new attribute in addition to all the variability of the old types. In scientific experiments made for the purpose of investigating the general laws of hybridity, such complex cases are therefore to be wholly excluded. The hereditary purity of the parents must be considered as one of the first conditions of success.
     Moreover the progeny must be numerous, since neither constancy, nor the exact proportions in the case of instability, can be determined with a small lot of plants.
     Finally, and in order to come to a definite choice of research material, we should keep in mind that the chief object is to ascertain the relation of the offspring to their parents. Now in nearly all cases the seeds are separated from the fruits and from one another, before it becomes possible to judge of their qualities. One may open a fruit and count the seeds, but ordinarily nothing is noted as to their characters. In this respect no other plant equals the corn or maize, as the kernels remain together on the spike, and as it has more than one variety characterized by the color, or constitution, or other qualities of the grains. A corn-grain, however, is not a seed, but a fruit containing a seed. Hence the outer parts pertain to the parent plant and only the innermost ones to the [288] seedling and therefore to the following generation. Fruit-characters thus do not offer the qualities we need, only the qualities resulting from fertilizations are characteristic of the new generation. Such attributes are afforded in some cases by the color, in others by the chemical constitution.
     We will choose the latter, and take the sugarcorn in comparison with the ordinary or starch producing forms for our starting point. Both sugar- and starch-corns have smooth fruits when ripening. No difference is to be seen in the young ripe spikes. Only the taste, or a direct chemical analysis might reveal the dissimilarity. But as soon as the spikes are dried, a diversity is apparent. The starchy grains remain smooth, but the sugary kernels lose so much water that they become wrinkled. The former becomes opaque, the latter more or less transparent. Every single kernel may instantly be recognized as belonging to either of the types in question, even if but a single grain of the opposite quality might be met with on a spike. Kernels can be counted on the spike, and since ordinary spikes may bear from 300-500 grains and often more, the numerical relation of the different types may be deduced with great accuracy.
     Coming now to our experiment, both starchy [289] and sugary varieties are in this respect wholly constant, when cultivated separately. No change is to be seen in the spikes. Furthermore it is very easy to make the crosses. The best way is to cultivate both types in alternate rows and to cut off the staminate panicles a few days before they open their first flowers. If this operation is done on all the individuals of one variety, sparing all the panicles of the other, it is manifest that all the plants will become fertilized by the latter, and hence that the castrated plants will only bear hybrid seeds.
     The experiment may be made in two ways; by castrating the sugary or the starchy variety. In both cases the hybrid kernels are the same. As to their composition they repeat the active character of the starchy variety. The sugar is only accumulated as a result of an incapacity of changing it into starch, and the lack of this capacity is to be considered as a retrogressive varietal mark. The starch-producing unit character, which is active in the ordinary sorts of corns, is therefore latent in sugar-corn.
     In order to obtain the second generation, the hybrid grains are sown under ordinary conditions, but sufficiently distant from any other variety of corn to insure pure fertilization. The several individuals may be left to pollinate [290] each other, or they may be artificially pollinated with their own pollen.
      The outcome of the experiments is shown by the spikes, as soon as they dry. Each spike bears two sorts of kernels irregularly dispersed over its surface. In this point all the spikes are alike. On each of them one may see on the first inspection that the majority of the kernels are starch-containing seeds, while a minor part becomes wrinkled and transparent according to the rule for sugary seeds. This fact shows at once that the hybrid race is not stable, but has differentiated the parental characters, bringing those of the varietal parent to perfect purity and isolation. Whether the same holds good for the starchy parent, it is impossible to judge from the inspection of the spikes, since it has been seen in the first generation that the hybrid kernels are not visibly distinguished from those of the pure starch-producing grains.
     It is very easy to count the number of both sorts of grains in the spike of such a hybrid. In doing so we find, that the proportion is nearly the same on all the spikes, and only slight variations would be found in hundreds of them. One-fourth of the seeds are wrinkled and three-fourths are always smooth. The number may vary in single instances and be a little more or a little less than 25%, ranging, for [291] instance, from 20 to 27%, but as a rule, the average is found nearly equal to 25%.
     The sugary kernels, when separated from the hybrid spikes and sown separately, give rise to pure sugary race, in no degree inferior in purity to the original variety. But the starchy kernels are of different types, some of them being internally like the hybrids of the first generation and others like the original parent. To decide between these two possibilities, it is necessary to examine their progeny.
     For the study of this third hybrid generation we will now take another example, the opium poppies. They usually have a dark center in the flowers, the inferior parts of the four petals being stained a deep purple, or often nearly black. Many varieties exhibit this mark as a large black cross in the center of the flower. In other varieties the pigment is wanting, the cross being of a pure white. Obviously it is only reduced to a latent condition, as in so many other cases of loss of color, since it reappears in a hybrid with the parent-species.
     For my crosses I have taken the dark-centered “Mephisto” and the “Danebrog,” or Danish flag, with a white cross on a red field. The second year the hybrids were all true to the type of “Mephisto.” From the seeds of each artificially self-fertilized capsule, one-fourth (22.5%) [292] in each instance reverted to the varietal mark of the white cross, and three-fourths (77.5%) retained the dark heart. Once more the flowers were self-pollinated and the visits of insects excluded. The recessives now gave only recessives, and hence we may conclude that the varietal marks had returned to stability. The dark hearted or dominants behaved in two different ways. Some of them remained true to their type, all their offspring being dark-hearted. Evidently they had returned to the parent with the active mark, and had reassumed this type as purely as the recessives had reached theirs. But others kept true to the hybrid character of the former generation, repeating in their progeny exactly the same mixture as their parents, the hybrids of the first generation, had given.
     This third generation therefore gives evidence, that the second though apparently showing only two types, really consists of three different groups. Two of them have reassumed the stability of their original grandparents, and the third has retained the instability of the hybrid parents.
     The question now arises as to the numerical relation of these groups. Our experiments gave the following results: [293]

Cross 1. Generation 2. Generation 3. Generation

Mephisto 4. 100% Mephisto
        | /
        | /
        | — 77.5 % Dom.—
        | / \
         \ / \
          >- All Mephisto 9. all hybrids with
         / \ 83-68% dominants and
        | \ 17-32% recessives.
        | \
Danebrog — 22.5 % Rec. 100% Danebrog.

Examining these figures we find one-fourth of constant recessives, as has already been said, further one-fourth of constant dominants, and the rest or one half as unstable hybrids. Both of the pure groups have therefore reappeared [293] in the same numbers. Calling A the specimens with the pure active mark, L those with the latent mark, and H the hybrids, these proportions may be expressed as follows:

1A+2H+1L.

This simple law for the constitution of the second generation of varietal hybrids with a single differentiating mark in their parents is called the law of Mendel. Mendel published it in 1865, but his paper remained nearly unknown to scientific hybridists. It is only of late years that it has assumed a high place in scientific literature, and attained the first rank as an investigation on fundamental questions of heredity. [294] Read in the light of modern ideas on unit characters it is now one of the most important works on heredity and has already widespread and abiding influence on the philosophy of hybridism in general.
     But from its very nature and from the choice of the material made by Mendel, it is restricted to balanced or varietal crosses. It assumes pairs of characters and calls the active unit of the pair dominant, and the latent recessive, without further investigations of the question of latency. It was worked out by Mendel for a large group of varieties of peas, but it holds good, with only apparent exceptions, for a wide range of cases of crosses of varietal characters. Recently many instances have been tested, and even in many cases third and later generations have been counted, and whenever the evidence was complete enough to be trusted, Mendel’s prophecy has been found to be right.
     According to this law of Mendel’s the pairs of antagonistic characters in the hybrid split up in their progeny, some individuals reverting to the pure parental types, some crossing with each other anew, and so giving rise to a new generation of hybrids. Mendel has given a very suggestive and simple explanation of his formula. Putting this in the terminology of to-day, and limiting it to the occurrence of only [295] one differential unit in the parents, we may give it in the following manner. In fertilization, the characters of both parents are not uniformly mixed, but remain separated though most intimately combined in the hybrid throughout life. They are so combined as to work together nearly always, and to have nearly equal influence on all the processes of the whole individual evolution. But when the time arrives to produce progeny, or rather to produce the sexual cells through the combination of which the offspring arises, the two parental characters leave each other, and enter separately into the sexual cells. From this it may be seen that one-half of the pollen-cells will have the quality of one parent, and the other the quality of the other. And the same holds good for [296] the egg-cells. Obviously the qualities lie latent in the pollen and in the egg, but ready to be evolved after fertilization has taken place.
     Granting these premises, we may now ask as to the results of the fertilization of hybrids, when this is brought about by their own pollen. We assume that numerous pollen grains fertilize numerous egg cells. This assumption at once allows of applying the law of probability, and to infer that of each kind of pollen grains one-half will reach egg-cells with the same quality [297] and the other half ovules with the opposite character.
     Calling P pollen and O ovules, and representing the active mark by P and O, the latent qualities by P’ and O’, they would combine as follows:

P + 0	giving uniform pairs with the active mark,
P + 0’	P + 0’ giving unequal pairs,
P’ + 0	P’ + 0 giving unequal pairs,
P’ + 0’	P’ + 0’ giving uniform pairs with the latent mark.

In this combination the four groups are obviously of the same size, each containing one-fourth of the offspring. Manifestly they correspond exactly to the direct results of the experiments, P + O representing the individuals which reverted to the specific mark, P’ + O’ those who reassumed the varietal quality and P + O’ and P + O’ those who hybridized [298] for the second time. These considerations lead us to the following form of Mendel’s,

P + O	= 1/4 Active	or 1A,
P + O’ P’ + O	= 1/2 Hybrid	or 2 H,
P’ + O’	= 1/4 Latent	or 1 L

Which is evidently the same as Mendel’s empirical law given above.
     To give the proof of these assumptions Mendel has devised a very simple crossing experiment, [299] which he has effected with his varieties of peas. I have repeated it with the sugar-corn, which gives far better material for demonstration. It starts from the inference that if dissimilarity among the pollen grains is excluded, the diversity of the ovules must at once became manifest and vice versa. In other terms, if a hybrid of the first generation is not allowed to fertilize itself, but is pollinated by one of its parents, the result will be in accordance with the Mendelian formula.
     In order to see an effect on the spikes produced in this way, it is of course necessary to fertilize them with the pollen of the variety, and not with that of the specific type. The latter would give partly pure starchy grains and partly hybrid kernels, but these would assume the same type. But if we pollinate the hybrid with pollen of a pure sugar-corn, we may predict the result as follows.
     If the spike of the hybrid contains dormant paternal marks in one-half of its flowers and in the other half maternal latent qualities, the sugar-corn pollen will combine with one-half of the ovules to give hybrids, and with the other half so as to give pure sugar-grains. Hence we see that it will be possible to count out directly the two groups of ovules on inspecting the ripe and dry spikes. Experience teaches us [298] that both are present, and in nearly equal numbers; one-half of the grains remaining smooth, and the other half becoming wrinkled.
     The corresponding experiment could be made with plants of a pure sugar-race by pollination with hybrid pollen. The spikes would show exactly the same mixture as in the above case, but now this may be considered as conclusive proof that half the pollen-grains represent the quality of one parent and the other half the quality of the other.
     Another corollary of Mendel’s law is the following. In each generation two groups return to purity, and one-half remains hybrid. These last will repeat the same phenomenon of splitting in their progeny, and it is easily seen that the same rule will hold good for all succeeding generations. According to Mendel’s principle, in each year there is a new hybridization, differing in no respect from the first and original one. If the hybrids only are propagated, each year will show one-fourth of the offspring returning to the specific character, one-fourth assuming the type of the variety and one-half remaining hybrid. I have tested this with a hybrid between the ordinary nightshade with black berries, and its variety, Solanum nigrum chlorocarpum, with pale yellow fruits. Eight generations of the hybrids were cultivated, [299] disregarding always the reverting offspring. At the end I counted the progeny of the sixth and seventh generations and found figures for their three groups of descendants, which exactly correspond to Mendel’s formula.
     Until now we have limited ourselves to the consideration of single differentiating units. This discussion gives a clear insight into the fundamental phenomena of hybrid fertilization. It at once shows the correctness of the assumption of unit-characters, and of their pairing in the sexual combinations.
     But Mendel’s law is not at all restricted to these simple cases. Quite on the contrary, it explains the most intricate questions of hybridization, providing they do not transgress the limits of symmetrical unions. But in this realm nearly all results may be calculated beforehand, on the ground of the principle of probability. Only one more assumption need be discussed. The several pairs of antagonistic characters must be independent from, and uninfluenced by, one another. This premise seems to hold good in the vast majority of cases, though rare exceptions seem to be not entirely wanting. Hence the necessity of taking all predictions from Mendel’s law only as probabilities, which will prove true in most, but not necessarily in all cases. [300] But here we will limit ourselves to normal cases.
     The first example to be considered is obviously the assumption that the parents of a cross differ from each other in respect to two characters. A good illustrative example is afforded by the thorn-apple. I have crossed the blue flowered thorny form, usually known as Datura Tatula, with the white thornless type, designated as D. Stramonium inermis. Thorns and blue pigment are obviously active qualities, as they are dominant in the hybrids. In the second generation both pairs of characters are resolved into their constituents and paired anew according to Mendel’s law. After isolating my hybrids during the period of flowering, I counted among their progeny:

128	individuals with blue flowers and thorns
47	individuals with blue flowers and thorns
54	individuals with white flowers and thorns
21	individuals with white flowers and without thorns
——
250

The significance of these numbers may easily be seen, when we calculate what was to be expected on the assumption that both characters follow Mendel’s law, and that both are independent from each other. Then we would have three-fourths blue offspring and one-fourth individuals with white flowers. Each of these [301] two groups would consist of thorn-bearing and thornless plants, in the same numerical relation. Thus, we come to the four groups observed in our experiment, and are able to calculate their relative size in the following way:

		Proportion
Blue with thorns	3/4 X 3/4 = 9/16 = 56.25%	9
Blue, unarmed	3/4 X 1/4 = 3/16 = 18.75%	3
White with thorns	1/4 X 3/4 = 3/16 = 18.75%	3
White, unarmed	1/4 X 1/4 = 1/16 = 6.25%	1

In order to compare this inference from Mendel’s law and the assumption of independency, with the results of our experiments, we must calculate the figures of the latter in percentages. In this way we find:

	Found	Calculated
Blue with thorns	128=51%	56.25%
Blue, unarmed	47=19%	18.75
White with thorns	54=22%	18.75
White, unarmed	54=22%	6.25%

The agreement of the experimental and the theoretical figures is as close as might be expected.
     This experiment is to be considered only as an illustrative example of a rule of wide application. The rule obviously will hold good in all such cases as comply with the two conditions already premised, viz.: that each character agrees with Mendel’s law, and that both are wholly independent of each other. It is clear that our figures show the numerical composition [302] of the hybrid offspring for any single instance, irrespective of the morphological nature of the qualities involved.
     Mendel has proved the correctness of these deductions by his experiments with peas, and by combining their color (yellow or green) with the chemical composition (starch or sugar) and other pairs of characters. I will now give two further illustrations afforded by crosses of the ordinary campion. I used the red-flowered or day-campion, which is a perennial herb, and a smooth variety of the white evening-campion, which flowers as a rule in the first summer. The combination of flower-color and pubescence gave the following composition for the second hybrid generation:

	Number	%	Calculation
Hairy and red	70	44	56.25%
Hairy and white	23	14	18.75%
Smooth and red	46	23	18.75%
Smooth and white	19	12	6.25%

For the combination of pubescence and the capacity of flowering in the first year I found:

	Number	%	Calculated
Hairy, flowering	286	52	56.25%
Hairy without stem	128	23	18.75%
Smooth and red	96	17	18.75
Smooth and white	42	8	6.25%

Many other cases have been tested by different writers and the general result is the [303] applicability of Mendel’s formula to all cases complying with the given conditions.
     Intentionally I have chosen for the last example two pairs of antagonisms, relating to the same pair of plants, and which may be tested in one experiment and combined in one calculation.
     For the latter we need only assume the same conditions as mentioned before, but now for three different qualities. It is easily seen that the third quality would split each of our four groups into two smaller ones in the proportion of 3/4: 1/4.
     We would then get eight groups of the following composition:

9/16 X 3/4 = 27/64	or	42.2%
9/16 X 1/4 = 9/64	″	14.1%
3/16 X 3/4 = 9/64	″	14.1%
3/16 X 1/4 = 3/64	″	4.7%
3/16 X 3/4 = 9/64	″	14.1%
3/16 X 1/4 = 3/64	″	4.7%
1/16 X 3/4 = 3/64	″	4.7%
1/16 X 1/4 = 1/64	″	1.6%

The characters chosen for our experiment include the absence of stem and flowers in the first year, and therefore would require a second year to determine the flower-color on the perennial specimens. Instead of doing so I have taken another character, shown by the teeth of the capsules when opening. These curve outwards [304] in the red campion, but lack this capacity in the evening-campion, diverging only until an upright position is reached. The combination of hairs, colors and teeth gives eight groups, and the counting of their respective numbers of individuals gave the following:

Hairs	Flowers	Teeth of capsules	Number	%	Calculated
Hairy	Red	curved	91	47	42.2%
Hairy	Red	straight	15	7.5	14.1%
Hairy	White	curved	23	12	14.1%
Hairy	White	straight	17	8.5	4.7%
Smooth	Red	curved	23	12	14.1%
Smooth	Red	straight	9	4.5	4.7%
Smooth	White	curved	5	2.5	4.7%
Smooth	White	straight	12	6	1.6%

The agreement is as comprehensive as might be expected from an experiment with about 200 plants, and there can be no doubt that a repetition on a larger scale would give still closer agreement.
     In the same way we might proceed to crosses with four or more differentiating characters. But each new character will double the number of the groups. Four characters will combine into 16 groups, five into 32, six into 64, seven into 128, etc. Hence it is easily seen that the size of the experiments must be made larger and larger in the same ratio, if we intend to expect numbers equally trustworthy. For [305] seven differentiating marks 16,384 individuals are required for a complete series. And in this set the group with the seven attributes all in a latent condition would contain only a single individual.
     Unfortunately the practical value of these calculations is not very great. They indicate the size of the cultures required to get all the possible combinations, and show that in ordinary cases many thousands of individuals have to be cultivated, in order to exhaust the whole range of possibilities. They further show that among all these thousands, only very few are constant in all their characters; in fact, it may easily be seen that with seven differentiating points among the 16,384 named above, only one individual will have all the seven qualities in pure active, and only one will have them all in a purely dormant condition. Then there will be some with some attributes active and others latent, but their numbers will also be very small. All others will split up in the succeeding generation in regard to one or more of their apparently active marks. And since only in very rare cases the stable hybrids can be distinguished by external characters from the unstable ones, the stability of each individual bearing a desired combination of characters would have to be established by experiment [306] after pure fertilization. Mendel’s law teaches us to predict the difficulties, but hardly shows any way to avoid them. It lays great stress on the old prescript of isolation and pure fertilization, but it will have to be worked out and applied to a large number of practical cases before it will gain a preeminent influence in horticultural practice.
     Or, as Bailey states it, we are only beginning to find a pathway through the bewildering maze of hybridization.
     This pathway is to be laid out with regard to the following considerations. We are not to cross species or varieties, or even accidental plants. We must cross unit-characters, and consider the plants only as the bearers of these units. We may assume that these units are represented in the hereditary substance of the cell-nucleus by definite bodies of too small a size to be seen, but constituting together the chromosomes. We may call these innermost representatives of the unit-characters pangenes, in accordance with Darwin’s hypothesis of pangenesis, or give them any other name, or we may even wholly abstain from such theoretical discussion, and limit ourselves to the conception of the visible character-units. These units then may be present, or lacking and in the first case active, or latent.
     [307] True elementary species differ from each other in a number of unit-characters, which do not contrast. They have arisen by progressive mutation. One species has one kind of unit, another species has another kind. On combining these, there can be no interchange. Mendelism assumes such an interchange between units of the same character, but in a different condition. Activity and latency are such conditions, and therefore Mendel’s law obviously applies to them. They require pairs of antagonistic qualities, and have no connection whatever with those qualities, which do not find an opponent in the other parent. Now, only pure varieties afford such pure conditions. When undergoing further modifications, some of them may be in the progressive line and others in the retrogressive. Progressive modifications give new units, which are not in contrast with any other, retrograde changes turn active units into the latent condition and so give rise to pairs. Ordinary species generally originate in this way, and hence differ from each other partly in specific, partly in varietal characters. As to the first, they give in their hybrids stable peculiarities, while as to the latter, they split up according to Mendel’s law.
     Unpaired or unbalanced characters lie side by side with paired or balanced qualities, and they [308] do so in nearly all the crosses made for practical purposes, and in very many scientific experiments. Even Mendel’s peas were not pure in this respect, much less do the campions noted above differ only in Mendelian characters.
     Comparative and systematic studies must be made to ascertain the true nature of every unit in every single plant, and crossing experiments must be based on these distinctions in order to decide what laws are applicable in any case.

[309]

D. EVER-SPORTING VARIETIES