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).