Weakness of Use-Inheritance
Use-inheritance is normally so weak that it appears to be quite helpless when opposed to any other factor of evolution. Natural selection evolves and maintains the instincts of ants and termites in spite of use-inheritance to a more wonderful degree than it evolves the instincts of almost any other animal with the fullest help of use-inheritance. It develops seldom-used horns or natural armour just as readily as constantly-used hoofs or teeth. Sexual selection evolves elaborate structures like the peacock’s tail in spite of disuse and natural selection combined. Artificial selection appears to enlarge or diminish used parts or disused parts with equal facility. The assistance of use-inheritance seems to be as unnecessary as its opposition is ineffective.
The alleged inheritance of the effects of use and disuse in our domestic animals must be very slow and slight.51 Darwin tells us that “there is no good evidence that this ever follows in the course of a single generation.” “Several generations must be subjected to changed habits for any appreciable result.”52 What does this mean? One of two things. Either the tendency is very weak, or it is non-existent. If it is so weak that we cannot detect its alleged effects till several generations have elapsed, during which time the more powerful agency of selection has been at work, how are we to distinguish the effects of the minor factor from that of the major? Are we to conclude that use-inheritance plus selection will modify races, just as Voltaire firmly held that incantations, together with sufficient arsenic, would destroy flocks of sheep? Is it not a significant fact that the alleged instances of use-inheritance so often prove to be self-conflicting in their details?
For satisfactory proof of the prevalence of a law of use-inheritance we require normal instances where selection is clearly inadequate to produce the change, or where it is scarcely allowed time or opportunity to act, as in the immediate offspring of the modified individual. Of the first kind of cases there seems to be a plentiful lack. Of the latter kind, according to Darwin, there appears to be none—a circumstance which contrasts strangely and suspiciously with the many decisive cases in which variation from unknown causes has been inherited most strikingly in the immediate offspring. It must be expected, indeed, that among these innumerable cases some will accidentally mimic the alleged effects of use-inheritance.
If Darwin had felt certain that the effects of habit or use tended in any marked degree to be conveyed directly and cumulatively to succeeding generations, he could hardly have given us such cautious, half-hearted encouragement of good habits as the following:—“It is not improbable that after long practice virtuous tendencies may be inherited.” “Habits, moreover followed during many generations probably tend to be inherited.”53 This is probable, independently of use-inheritance. The “many generations” specified or implied, will allow time for the play of selective as well as of cumulatively-educative influences. There must apparently be a constitutional or inheritable predisposition or fitness for the habits spoken of, which otherwise would scarcely be continued for many generations, except by the favourably-varying branches of a family: which again is selection rather than use-inheritance.
Where is the necessity for even the remains of the Lamarckian doctrine of inherited habit? Seeing how powerful the general principle of selection has shown itself in cases where use-inheritance could have given no aid or must even have offered its most strenuous opposition, why should it not equally be able to develop used organs or repress disused organs or faculties without the assistance of a relatively weak ally? Selection evolved the remarkable protective coverings of the armadillo, turtle, crocodile, porcupine, hedgehog, &c.; it formed alike the rose and its thorn, the nut and its shell; it developed the peacock’s tail and the deer’s antlers, the protective mimicry of various insects and butterflies, and the wonderful instincts of the white ants; it gave the serpent its deadly poison and the violet its grateful odour; it painted the gorgeous plumage of the Impeyan pheasant and the beautiful colours and decorations of countless birds and insects and flowers. These, and a thousand other achievements, it has evidently accomplished without the help of use-inheritance. Why should it be thought incapable of reducing a pigeon’s wing or enlarging a duck’s leg? Why should it be credited with the help of an officious ally in effecting comparatively slight changes, when great and striking modifications are effected without any such aid?
Footnotes
15 Weismann’s Essays on Heredity, &c. Clarendon Press, 1889.
16 Life and Letters, i. p. 16. Darwin’s reverence for his father “was boundless and most touching. He would have wished to judge everything else in the world dispassionately, but anything his father had said was received with almost implicit faith;... he hoped none of his sons would ever believe anything because he said it, unless they were themselves convinced of its truth—a feeling in striking contrast with his own manner of faith” (Life and Letters, i. pp. 10, 11).
17 Ibid., i. p. 38.
18 Life and Letters, ii. p. 14.
19 Origin of Species, pp. 117, 118.
20 Ibid., p. 180.
21 Contemporary Review, December, 1875, pp. 89, 93.
22 Variation of Animals and Plants under Domestication, i. 292.
23 Variation of Animals and Plants under Domestication, i. 299-301.
24 To keep pace with this lateral increase in weight, the leg-bones should have lengthened considerably so that their total deficiency in proportional length is 17 per cent.,—a changed proportion which being linear is more excessive than the increase of weight by 28 per cent. So marked is the effect of the combined thickening and shortening that in the Aylesbury breed—which is the most typically representative one—the leg-bones have become 70 per cent. heavier than they should be if their thickness had continued to be proportional to their length.
25 This excessive thickening under disuse appears to be due partly to a positive lateral enlargement or increase of proportional weight of about 7½ per cent., and partly to a shortening of about 15 per cent. Carefully calculated, the reduction of the weight of the wing-bones in this breed is only 8·3 per cent. relatively to the whole skeleton, or only 5 per cent. relatively to the skeleton minus legs and wings. The latter method is the more correct, since the excessive weight of the leg-bones increases the weight of the skeleton more than the diminished weight of the wing-bones reduces it.
26 Variation of Animals and Plants under Domestication, i. 284.
27 Variation of Animals and Plants under Domestication, i. 184, 185.
28 Ibid., i. 144, 145.
29 Ibid., i. 185.
30 Variation of Animals and Plants under Domestication, i. 175.
31 Variation of Animals and Plants under Domestication, i. 184. I suspect that Darwin was in poor health when he wrote this page. He nods at least four times in it. Twice he speaks of “twelve” breeds where he obviously should have said eleven.
32 If a prominent breast is admired and selected by fanciers, the sternum might shorten in assuming a more forward and vertical position. If the shortening of the sternum is entirely due to disuse, it seems strange that Darwin has not noticed any similar shortening in the sternum of the duck. But selection has not tended to make the duck elegant, or “pigeon-breasted”; it has enlarged the abdominal sack instead, besides allowing the addition of an extra rib in various cases.
33 Variation of Animals and Plants under Domestication, 144, 175.
34 Variation of Animals and Plants under Domestication, i. 179.
35 In the six largest breeds the shortening of the sternum is nearly twice as great as in the three smaller breeds which remain nearest the rock-pigeon in size. We can hardly suppose that use-inheritance especially affects the eight breeds that have varied most in size. If we exclude these, there is only a total shortening of 7 per cent. to be accounted for.
36 Variation of Animals and Plants under Domestication, i. 183, 186.
37 Variation of Animals and Plants under Domestication, i. 130, 135; ii. 288.
38 Encyclopædia Britannica, article “Zoology.”
39 Variation of Animals and Plants under Domestication, ii. 367.
40 Variation of Animals and Plants under Domestication, ii. 367. Why then does the cheetah inherit ancestral habits so inadequately that it is useless for the chase unless it has first learned to hunt for itself before being captured? (ii. 133).
41 Descent of Man, p. 33.
42 Origin of Species, pp. 210, 211.
43 E. S. Delamer on Pigeons and Rabbits, pp. 132, 103. For other points referred to, see pages 133, 102, 100, 95, 131.
44 Origin of Species, pp. 188, 110; Descent of Man, pp. 32-35; Variation of Animals and Plants under Domestication, ii. 289, 293. Use or disuse during lifetime of course co-operates, and in some cases, as in that of the canoe Indians, may be the principal or even perhaps the sole cause of the change.
45 For the importance of panmixia as invalidating Darwin’s strongest evidence for use-inheritance—namely, that drawn from the effects of disuse in highly-fed domestic animals where there is supposed to be no economy of growth—see Professor Romanes on Panmixia, Nature, April 3, 1890.
46 Descent of Man, p. 33.
47 Descent of Man, p. 33.
48 Variation of Animals and Plants under Domestication, i., 453.
49 Descent of Man, p. 33.
50 Descent of Man, p. 33.
51 Wallace shows that the changes in our domestic animals, if spread over the thousands of years since the animals were first tamed, must be extremely insignificant in each generation, and he concludes that such infinitesimal effects of use and disuse would be swallowed up by the far greater effects of variation and selection (Darwinism, p. 436). Professor Romanes has replied to him in the Contemporary Review (August 1889), showing that this is no disproof of the existence of the minor factor, inasmuch as slight changes in each generation need not necessarily be matters of life and death to the individual, although their cumulative development by use-inheritance might eventually become of much service. But selection would favour spontaneous variations of a similarly serviceable character. The slightest tendency to eliminate the extreme variations in either direction would proportionally modify the average in a breed. Use-inheritance appears to be so relatively weak a factor that probably neither proof nor disproof of its existence can ever be given, owing to the practical impossibility of disentangling its effects (if any) from the effects of admittedly far more powerful factors which often act in unsuspected ways. Thus wild ducklings, which can easily be reared by themselves, invariably “die off” if reared with tame ones (Variation, &c., i. 292, ii. 219). They cannot get their fair share in the competition for food, and are completely eliminated. Professor Romanes fully acknowledges that there is the “gravest possible doubt” as to the transmission of the effects of disuse (Letter on Panmixia, Nature, March 13, 1890).
52 Variation of Animals and Plants under Domestication, ii. 287-289.
53 Descent of Man, pp. 612, 131.