APPENDIX C.
THE INHERITANCE OF FUNCTIONALLY-WROUGHT MODIFICATIONS: A SUMMARY.
The assertion that changes of structure caused by changes of function are transmitted to descendants is continually met by the question—Where is the evidence? When some facts are assigned in proof, they are pooh-poohed as insufficient. If after a time the question is raised afresh and other facts are named, there is a like supercilious treatment of them. Successively rejected in this way, the evidences do not accumulate in the minds of opponents; and hence produce little or no effect. When they are brought together, however, it turns out that they are numerous and weighty. We will group them into negative and positive.
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Negative evidence is furnished by those cases in which traits otherwise inexplicable are explained if the structural effects of use and disuse are transmitted. In the foregoing chapters and appendices three have been given.
(1) Co-adaptation of co-operative parts comes first. This has been exemplified by the case of enlarged horns in a stag, by the case of an animal led into the habit of leaping, and in the case of the giraffe (cited in "The Factors of Organic Evolution"); and it has been shown that the implied co-adaptations of parts cannot possibly have been effected by natural selection.
(2) The possession of unlike powers of discrimination by different parts of the human skin, was named as a problem to be solved on the hypothesis of natural selection or the hypothesis of panmixia; and it was shown that neither of these can by any twisting yield a solution. But the facts harmonize with the hypothesis that the effects of use are inherited.
(3) Then come the cases of those rudimentary organs which, like the hind limbs of the whale, have nearly disappeared. Dwindling by natural selection is here out of the question; and dwindling by panmixia, even were its assumptions valid, would be incredible. But as a sequence of disuse the change is clearly explained.
Failure to solve any one of these three problems would, I think, alone prove the Neo-Darwinian doctrines untenable; and the fact that we have three unsolved problems seems to me fatal.
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From this negative evidence, turn now to the positive evidence. This falls into several groups.
There are first the facts collected by Mr. Darwin, implying functionally-altered structures in domestic animals. The hypothesis of panmixia is, as we have seen, out of court; and therefore Mr. Darwin's groups of evidences are reinstated. There is the changed ratio of wing-bones and leg-bones in the duck; there are the drooping ears of cats in China, of horses in Russia, of sheep in Italy, of guinea-pigs in Germany, of goats and cattle in India, of rabbits, pigs, and dogs in all long-civilized countries. Though artificial selection has come into play where drooping has become a curious trait (as in rabbits), and has probably caused the greater size of ears which has in some cases gone along with diminished muscular power over them; yet it could not have been the initiator, and has not been operative on animals bred for profit. Again there are the changes produced by climate; as instance, among plants, the several varieties of maize established in Germany and transformed in the course of a few generations.
Facts of another class are yielded by the blind inhabitants of caverns. One who studies the memoir by Mr. Packard on The Cave Fauna of North America, &c., will be astonished at the variety of types in which degeneration or loss of the eyes has become a concomitant of life passed in darkness. A great increase in the force of this evidence will be recognized on learning that absence or extreme imperfection of visual organs is found also in creatures living in perpetual night at the bottoms of deep oceans. Endeavours to account for these facts otherwise than by the effects of disuse we have seen to be futile.
Kindred evidence is yielded by decrease of the jaws in those races which have had diminished use of them—mankind and certain domestic animals. Relative smallness in the jaws of civilized men, manifest enough on comparison, has been proved by direct measurement. In pet dogs—pugs, household spaniels—we find associated the same cause with the same effect. Though there has been artificial selection, yet this did not operate until the diminution had become manifest. Moreover there has been diminution of the other structures concerned in biting: there are smaller muscles, feeble zygomata, and diminished areas for insertion of muscles—traits which cannot have resulted from selection, since they are invisible in the living animal.
In abnormal vision produced by abnormal use of the eyes we have evidence of another kind. That the Germans, among whom congenital short sight is notoriously prevalent, have been made shortsighted by inheritance of modifications due to continual reading of print requiring close attention, is by some disputed. It is strange, however, that if there exists no causal connexion between them, neither trait occurs without the other elsewhere. But for the belief that there is a causal connexion we have the verifying testimony of oculists. From Dr. Lindsay Johnson I have cited cases within his professional experience of functionally-produced myopia transmitted to children; and he asserts that other oculists have had like experiences.
Development of the musical faculty in the successive members of families from which the great composers have come, as well as in the civilized races at large, is not to be explained by natural selection. Even when it is great, the musical faculty has not a life-saving efficiency as compared with the average of faculties; for the most highly gifted have commonly passed less prosperous lives and left fewer offspring than have those possessed of ordinary abilities. Still less can it be said that the musical faculty in mankind at large has been developed by survival of the fittest. No one will assert that men in general have been enabled to survive and propagate in proportion as their musical appreciation was great.
The transmission of nervous peculiarities functionally produced is alleged by the highest authorities—Dr. Savage, president of the Neurological Society, and Dr. Hughlings Jackson. The evidence they assign confirms, and is confirmed by, that which the development of the musical faculty above named supplies.
Here, then, we have sundry groups of facts directly supporting the belief that functionally-wrought modifications descend from parents to offspring.
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Now let us consider the position of those Darwinians who dissent from Darwin, and who make light of all this evidence. We might naturally suppose that their own hypothesis is unassailable. Yet, strange to say, they admit that there is no direct proof that any species has been established by natural selection. The proof is inferential only.
The certainty of an axiom does not give certainty to the deductions drawn from it. That natural selection is, and always has been, operative is incontestable. Obviously I should be the last person to deny that survival of the fittest is a necessity: its negation is inconceivable. The Neo-Darwinians, however, judging from their attitude, apparently assume that firmness of the basis implies firmness of the superstructure. But however high may be the probability of some of the conclusions drawn, none of them can have more than probability; while some of them remain, and are likely to remain, very questionable. Observe the difficulties.
(1) The general argument proceeds upon the analogy between natural selection and artificial selection. Yet all know that the first cannot do what the last does. Natural selection can do nothing more than preserve those of which the aggregate characters are most favourable to life. It cannot pick out those possessed of one particular favourable character, unless this is of extreme importance.
(2) In many cases a structure is of no service until it has reached a certain development; and it remains to account for that increase of it by natural selection which must be supposed to take place before it reaches the stage of usefulness.
(3) Advantageous variations, not preserved in nature as they are by the breeder, are liable to be swamped by crossing or to disappear by atavism.
Now whatever replies are made, their component propositions cannot be necessary truths. So that the conclusion in each case, however reasonable, cannot claim certainty: the fabric can have no stability like that of its foundation.
When to uncertainties in the arguments supporting the hypothesis we add its inability to explain facts of cardinal significance, as proved above, there is I think ground for asserting that natural selection is less clearly shown to be a factor in the origination of species than is the inheritance of functionally-wrought changes.
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If, finally, it is said that the mode in which functionally-wrought changes, especially in small parts, so affect the reproductive elements as to repeat themselves in offspring, cannot be imagined—if it be held inconceivable that those minute changes in the organs of vision which cause myopia can be transmitted through the appropriately-modified sperm-cells or germ-cells; then the reply is that the opposed hypothesis presents a corresponding inconceivability. Grant that the habit of a pointer was produced by selection of those in which an appropriate variation in the nervous system had occurred; it is impossible to imagine how a slightly-different arrangement of a few nerve-cells and fibres could be conveyed by a spermatozoon. So too it is impossible to imagine how in a spermatozoon there can be conveyed the 480,000 independent variables required for the construction of a single peacock's feather, each having a proclivity towards its proper place. Clearly the ultimate process by which inheritance is effected in either case passes comprehension; and in this respect neither hypothesis has an advantage over the other.