Individual and Transmissible Type Not Modified Alike by the Direct Effect of Changed Habits or Conditions
Mr. Spencer finds it hard to believe that the modifications conveyed to offspring are not identical in tendency with the changes effected in the parent by altered use or habit (pp. 23-25, 34). But it is perfectly certain that the two sets of effects do not necessarily correspond. The effect of changed habits or conditions on the individual is often very far from coinciding with the effects on the reproductive elements or the transmissible type. The reproductive system is “extremely sensitive” to very slight changes, and is often powerfully affected by circumstances which otherwise have little effect on the individual (Origin of Species, p. 7). Various animals and plants become sterile when domesticated or supplied with too much nourishment. The native Tasmanians have already become extinct from sterility caused by greatly changed diet and habits. If, as Mr. Spencer teaches, continued culture and brain-work will in time produce lessened fertility or comparative sterility, we may yet have to be careful that intellectual development does not become a species of suicide, and that the culture of the race does not mean its extinction—or at least the extinction of those most susceptible of culture.
The reproductive elements are also disturbed and modified in innumerable minor ways. Changed conditions or habits tend to produce a general “plasticity” of type, the “indefinite variability” thus caused being apparently irrelevant to the change, if any, in the individual.14 A vast number of variations of structure have certainly arisen independently of similar parental modification as the preliminary. Whatever first caused these “spontaneous” congenital variations affected the reproductive elements quite differently from the individual. “When a new peculiarity first appears we can never predict whether it will be inherited.” Many varieties of plants only keep true from shoots, and not from seed, which is by no means acted on in the same way as the individual plant. Seeing that such plants have two reproductive types, both constant, it is evident that these cannot both be modified in the same way as the parent is modified. Many parental modifications of structure and habit are certainly not conveyed to neuter ants and bees; other modifications, which are not seen in the parents, being conveyed instead. Many other circumstances tend to show that the individual and the transmissible type are independent of each other so far as modifications of parts are concerned.
It may seem natural to expect the transmission of an enlarged muscle or a cultivated brain, but, on the other hand, why should it be unreasonable to expect that a modification which was non-congenital in origin should still remain non-congenital? Why should the non-transmission of that which was not transmitted be surprising?
Mr. Spencer thinks that the non-transmission of acquired modifications is incongruous with the great fact of atavism. But the great law of the inheritance of that which is a development of the transmissible type does not necessarily imply the inheritance of modifications acquired by the individual. Because English children may inherit blue eyes and flaxen hair from their Anglo-Saxon ancestors, it by no means follows that an Englishman must inherit his father’s sunburnt complexion or smooth-shaven face. Of course atavism ultimately adopts many instances of revolt against its sway. But to assume that these changes of type follow the personal change rather than cause it, is to assume the whole question at issue. That like begets like is true as a broad principle, but it has many exceptions, and the non-heredity of acquired characters may be one of them.
Footnotes
2 Principles of Biology, § 166, footnote. The English jaws are somewhat lighter than the Australian jaws, though I could not undertake to affirm that they are really shorter and smaller. In the typical skulls depicted on p. 68 of the official guide to the mammalian galleries at South Kensington, the typical Caucasian jaw is very much larger than the Tasmanian jaw, although the repulsively obtrusive teeth of the latter convey the contrary idea to the imagination. Mr. Spencer’s assumption that the ancient Britons had large jaws appears to me erroneous. (See Professor Rolleston’s Scientific Papers and Addresses, i. p. 250.)
3 Romanes, Galton, and Weismann have made great use of this principle in explaining the diminution of disused organs. Weismann has given it the name of Panmixia,—all individuals being equally free to survive and commingle their variations, and not merely selected or favoured individuals. See his Essays on Heredity, &c., p. 90 (Clarendon Press).
4 Inclusive in each case of fixed strengthening wire weighing about a sixteenth of an ounce or less.
5 References of course are to Factors of Organic Evolution.
6 P. 13; and Nineteenth Century, February, 1888, p. 211.
7 Tomes’s Dental Surgery, pp. 273-275. Tomes observes that it is as yet uncertain in what way civilization predisposes to caries. But he shows that caries is caused by the lime salts in the teeth being attacked by acids from decomposing food in crevices, from artificial drink such as cyder, from sugar, from medicine, and from vitiated secretions of the mouth. It is evident that in civilized races natural selection cannot so rigorously insist on sound teeth, sound constitutions, and protective alkaline saliva. The reaction of the civilized mouth is often acid, especially when the system is disordered by dyspepsia or other diseases or forms of ill-health common under civilization. The main supply of saliva, which is poured from the cheeks opposite the upper molars, is often acid when in small quantities. But the submaxillary and sub-lingual saliva poured out at the foot of the lower incisors and held in the front part of the jaw as in a spoon, “differs from parotid saliva in being more alkaline” (Foster’s Text Book of Physiology, p. 238; Tomes, pp. 284, 685). One observer says that the reaction near the lower incisors is “never acid.” Hence (I conclude) the remarkable immunity of the lower incisors and canines from decay, an immunity which extends backwards in a lessening degree to the first and second bicuspids. The close packing of the lower incisors may assist by preventing the retention of decaying fragments of food. Sexual selection may promote caries by favouring white teeth, which are more prone to decay than yellow ones. Acid vitiation of the mucus might account both for caries and (possibly) for the strange infertility of some inferior races under civilization.
8 Origin of Species, pp. 198-9; Variation of Animals and Plants under Domestication, vol. ii. p. 328 footnote, also p. 206.
9 Mr. Spencer weakly argues that an advantageous attribute (such as swiftness, keen sight, courage, sagacity, strength, &c.) cannot be increased by natural selection unless it is “of greater importance, for the time being, than most of the other attributes”; and that natural selection cannot develop any one superiority when animals are equally preserved by “other superiorities.” But as natural selection will simultaneously eliminate tendencies to slowness, blindness, deafness, stupidity, &c., it must favour and improve many points simultaneously, although no one of them may be of greater importance than the rest. Of course the more complicated the evolution the slower it will be; but time is plentiful, and the amount of elimination is correspondingly vast.
10 I venture to coin this concise term to signify the direct inheritance of the effects of use and disuse in kind. Having a name for a thing is highly convenient; it facilitates clearness and accuracy in reasoning, and in this particular inquiry it may save some confusion of thought from double or incomplete meanings in the shortened phrases which would otherwise have to be employed to indicate this great but nameless factor of evolution.
11 Origin of Species, pp. 230-232; Bates’s Naturalist on the Amazons. Darwin is “surprised that no one has hitherto advanced the demonstrative case of neuter insects, against the well-known doctrine of inherited habit, as advanced by Lamarck.” As he justly observes, “it proves that with animals, as with plants, any amount of modification may be effected by the accumulation of numerous, slight, spontaneous variations, which are in any way profitable, without exercise or habit having been brought into play. For peculiar habits confined to the workers or sterile females, however long they might be followed, could not possibly affect the males and fertile females, which alone leave any descendants.” Some slight modification of these remarks, however, may possibly be needed to meet the case of “factitious queens,” who (probably through eating particles of the royal food) become capable of producing a few male eggs.
12 Descent of Man, pp. 573, 572, and footnote.
13 Contemporary Review, December, 1875, p. 92.
14 See Origin of Species, pp. 5-8. “Changed conditions induce an almost indefinite amount of fluctuating variability, by which the whole organization is rendered in some degree plastic” (Descent of Man, p. 30). It also appears that “the nature of the conditions is of subordinate importance in comparison with the nature of the organism in determining each particular form of variation;—perhaps of not more importance than the nature of the spark, by which a mass of combustible matter is ignited, has in determining the nature of the flames” (Origin of Species, p. 8).