Are the Effects of Use and Disuse Inherited?

ÆSTHETIC FACULTIES

The modern development of music and harmony (p. 19) is undeniable, but why could it only have been brought about by the help of the inheritance of the effects of use? Why are we to suppose that "minor traits" such as the "æsthetic perceptions" cannot have been evolved by natural selection (p. 20) or by sexual selection? Darwin holds that our musical faculties were developed by sexual preference long before the acquisition of speech. He believes that the "rhythms and cadences of oratory are derived from previously developed musical powers" – a conclusion "exactly opposite" to that arrived at by Mr. Spencer.12 The emotional susceptibility to music, and the delicate perceptions needed for the higher branches of art, were apparently the work of natural and sexual selection in the long past. Civilization, with its leisure and wealth and accumulated knowledge, perfects human faculties by artificial cultivation, develops and combines means of enjoyment, and discovers unsuspected sources of interest and pleasure. The sense of harmony, modern as it seems to be, must have been a latent and indirect consequence of the development of the sense of hearing and of melody. Use, at least, could never have called it into existence. Nature favours and develops enjoyments to a certain extent, for they subserve self-preservation and sexual and social preference in innumerable ways. But modern æsthetic advance seems to be almost entirely due to the culture of latent abilities, the formation of complex associations, the selection and encouragement of talent, and the wide diffusion and imitation of the accumulated products of the well-cultivated genius of favourably varying individuals. The fact that uneducated persons do not enjoy the higher tastes, and the rapidity with which such tastes are acquired or professed, ought to be sufficient proof that modern culture is brought about by far swifter and more potent influences than use-inheritance. Neither would this hypothetical factor of evolution materially aid in explaining the many other rapid changes of habit brought about by education, custom, and the changed conditions of civilization generally. Powerful tastes – as is incontestably shown in the cases of alcohol and tobacco – lie latent for ages, and suddenly become manifest when suitable conditions arise. Every discovery, and each step in social and moral evolution, produces its wide-spreading train of consequences. I see no reason why use-inheritance need be credited with any share in the cumulative results of the invention of printing and the steam-engine and gunpowder, or of freedom and security under representative government, or of science and art and the partial emancipation of the mind of man from superstition, or of the innumerable other improvements or changes that take place under modern civilization.

Mr. Spencer suggests an inquiry whether the greater powers possessed by eminent musicians were not mainly due to the inherited effect of the musical practice of their fathers (p. 19). But these great musicians inherited far more than their parents possessed. The excess of their powers beyond their parents' must surely be attributed to spontaneous variation; and who shall say that the rest was in any way due to use-inheritance? If, too, the superiority of geniuses proves use-inheritance, why should not the inferiority of the sons of geniuses prove the existence of a tendency which is the exact opposite of use-inheritance? But nobody collects facts concerning the degenerate branches of musical families. Only the favourably varying branches are noticed, and a general impression of rapid evolution of talent is thus produced. Such cases might be explained, too, by the facts that musical faculty is strong in both sexes, that musical families associate together, and that the more gifted members may intermarry. Great musicians are often astonishingly precocious. Meyerbeer "played brilliantly" at the age of six. Mozart played beautifully at four. Are we to suppose that the effect of the adult practice of parents was inherited at this early age? If use-inheritance was not necessary in the case of Handel, whose father was a surgeon, why is it needed to account for Bach?

LACK OF EVIDENCE

The "direct proofs" of use-inheritance are not as plentiful as might be desired, it appears (pp. 24-28). This acknowledged "lack of recognized evidence" is indeed the weakest feature in the case, though Mr. Spencer would fain attribute this lack of direct proof to insufficient investigation and to the inconspicuous nature of the inheritance of the modification. But there is an almost endless abundance of conspicuous examples of the effects of use and disuse in the individual. How is it that the subsequent inheritance of these effects has not been more satisfactorily observed and investigated? Horse-breeders and others could profit by such a tendency, and one cannot help suspecting that the reason they ignore it must be its practical inefficacy, arising probably from its weakness, its obscurity and uncertainty or its non-existence.

INHERITED EPILEPSY IN GUINEA-PIGS

Brown-Séquard's discovery that an epileptic tendency artificially produced by mutilating the nervous system of a guinea-pig is occasionally inherited may be a fact of "considerable weight," or on the other hand it may be entirely irrelevant. Cases of this kind strike one as peculiar exceptions rather than as examples of a general rule or law. They seem to show that certain morbid conditions may occasionally affect both the individual and the reproductive elements or transmissible type in a similar manner; but then we also know that such prompt and complete transmission of an artificial modification is widely different from the usual rule. Exceptional cases require exceptional explanations, and are scarcely good examples of the effect of a general tendency which in almost all other cases is so inconspicuous in its immediate effects. Further remarks on this inherited epilepsy can be most conveniently introduced later on in connection with Darwin's explanation of the inherited mutilation which it usually accompanies, but which Mr. Spencer does not mention.

INHERITED INSANITY AND NERVOUS DISORDERS

Mr. Spencer infers that, because insanity is usually hereditary, and insanity can be artificially produced by various excesses, therefore this artificially-produced insanity must also be hereditary (p. 28). Direct evidence of this conclusion would be better than a mere inference which may beg the very question at issue. That the liability to insanity commonly runs in families is no proof that strictly non-inherited insanity will subsequently become hereditary. I think that theories should be based on facts rather than facts on theories, especially when those facts are to be the basis or proof of a further theory.

Mr. Spencer also points out that he finds among physicians "the belief that nervous disorders of a less severe kind are inheritable" – a general belief which does not necessarily include the transmission of purely artificially-produced disorders, and so misses the point which is really at issue. He proceeds, however, to state more definitely that "men who have prostrated their nervous systems by prolonged overwork or in some other way, have children more or less prone to nervousness." The following observations will, I think, warrant at least a suspension of judgment concerning this particular form of use-inheritance.

(1) The nervousness is seen in the children at an early age, although the nervous prostration from which it is supposed to be derived obviously occurs in the parent at a much later period of life. This change in time is contrary to the rule of inheritance at corresponding periods; and, together with the unusual promptness and comparative completeness of the inheritance, it may indicate a special injury or deterioration of the reproductive elements rather than true inheritance. The healthy brain of early life has failed to transmit its robust condition. Is use-inheritance, then, only effective for evil? Does it only transfer the newly-acquired weakness, and not the previous long-continued vigour?

(2) Members of nervous families would be liable to suffer from nervous prostration, and by the ordinary law of heredity alone would transmit nervousness to their children.

(3) The shattered nerves or insanity resulting from alcoholic and other excesses, or from overwork or trouble, are evidently signs of a grave constitutional injury which may react upon the reproductive elements nourished and developed in that ruined constitution. The deterioration in parent and child may often display itself in the same organs – those probably which are hereditarily weakest. Acquired diseases or disorders thus appear to be transmitted, when all that was conveyed to the offspring was the exciting cause of a lowered vitality or disordered action, together with the ancestral liability to such diseases under such conditions.

(4) Francis Galton says that "it is hard to find evidence of the power of the personal structure to react upon the sexual elements, that is not open to serious objection." Some of the cases of apparent inheritance he regards as coincidence of effect. Thus "the fact that a drunkard will often have imbecile children, although his offspring previous to his taking to drink were healthy," is an "instance of simultaneous action," and not of true inheritance. "The alcohol pervades his tissues, and, of course, affects the germinal matter in the sexual elements as much as it does that in his own structural cells, which have led to an alteration in the quality of his own nerves. Exactly the same must occur in the case of many constitutional diseases that have been acquired by long-continued irregular habits."13

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.

DARWIN'S EXAMPLES

The most formidable cases brought forward by Mr. Spencer are from Darwin. I shall endeavour to show, however, that Darwin was probably wrong in retaining the older explanation of these facts, and that the remains of the Lamarckian theory of use-inheritance need not any longer encumber the great explanation which has superseded that fallacious and unproven theory and has rendered it totally unnecessary. Meanwhile I think it is an excellent sign that Mr. Spencer has to complain that "Nowadays most naturalists are more Darwinian than Mr. Darwin himself" – inasmuch as they are inclined to say that there is "no proof" that the effects of use and disuse are inherited. Other excellent signs are the recent issue of a translation of Weismann's important essays on this and kindred subjects,15 the strong support given to his views by Wallace in his Darwinism, and their adoption by Ray Lankester in his article on Zoology in the latest edition of the Encyclopædia Britannica. So sound and cautious an investigator as Francis Galton had also in 1875 concluded that "acquired modifications are barely, if at all, inherited, in the correct sense of that word."

Darwin's belief in the inheritance of acquired characters was more or less hereditary in the family. His grandfather, Erasmus Darwin, anticipated Lamarck's views in his Zoonomia, which Darwin at one time "greatly admired." His father was "convinced" of the "inherited evil effects of alcohol," and to this extent at least he strongly impressed the belief in the inheritance of acquired characters upon his children's minds.16 Darwin must also have been imbued with Lamarckian ideas from other sources, although Dr. Grant's enthusiastic advocacy entirely failed to convert him to a belief in evolution.17 "Nevertheless," he says, "it is probable that the hearing rather early in life such views maintained and praised may have favoured my upholding them under a different form in my Origin of Species" – a remark which refers to Lamarck's views on the general doctrine of evolution, but might also prove equally true if applied to Darwin's partial retention of the Lamarckian explanation of that evolution. Professor Huxley has pointed out that in Darwin's earlier sketch of his theory of evolution (1844) he attached more weight to the inheritance of acquired habits than he does in his Origin of Species published fifteen years later.18 He appears to have acquired the belief in early life without first questioning and rigorously testing it as he would have done had it originated with himself. In later life it appeared to assist his theory of evolution in minor points, and in particular it appeared absolutely indispensable to him as the only explanation of the diminution of disused parts in cases where, as in domestic animals, economy of growth seemed to be practically powerless. He failed to adequately notice the effect of panmixia, or the withdrawal of selection, in causing or allowing degeneracy and dwindling under disuse; and he hardly attached sufficient importance to the fact that rudimentary organs and other supposed effects of use or disuse are quite as marked features in neuter insects which cannot transmit the effects of use and disuse as they are in the higher animals.

REDUCED WINGS OF BIRDS OF OCEANIC ISLANDS

Darwin himself has pointed out that the rudimentary wings of island beetles, at first thought to be due to disuse, are mainly brought about by natural selection – the best-winged beetles being most liable to be blown out to sea. But he says that in birds of the oceanic islands "not persecuted by any enemies, the reduction of their wings has probably been caused by disuse." This explanation may be as fallacious as it is acknowledged to have been in the case of the island beetles. According to Darwin's own views, natural selection must at least have played an important part in reducing the wings; for he holds that "natural selection is continually trying to economize every part of the organization." He says: "If under changed conditions of life a structure, before useful, becomes less useful, its diminution will be favoured, for it will profit the individual not to have its nutriment wasted in building up an useless structure… Thus, as I believe, natural selection will tend in the long run to reduce any part of the organization, as soon as it becomes, through changed habits, superfluous."19 If, as Darwin powerfully urges (and he here ignores his usual explanation), ostriches' wings are insufficient for flight in consequence of the economy enforced by natural selection,20 why may not the reduced wings of the dodo, or the penguin, or the apteryx, or of the Cursores generally, be wholly attributed to natural selection in favour of economy of material and adaptation of parts to changed conditions? The great principle of economy is continually at work shaping organisms, as sculptors shape statues, by removing the superfluous parts; and a mere glance at the forms of animals in general will show that it is well-nigh as dominant and universal a principle as is that of the positive development of useful parts. Other causes, moreover besides actual economy, would favour shorter and more convenient wings on oceanic islands. In the first place, birds that were somewhat weak on the wing would be most likely to settle on an island and stay there. Shortened wings would then become advantageous because they would restrain fatal migratory tendencies or useless and perilous flights in which the birds that flew furthest would be most often carried away by storms and adverse winds. Reduced wings would keep the birds near the shelter and the food afforded by the island and its neighbourhood, and in some cases would become adapted to act as fins or flappers for swimming under water in pursuit of fish.

The reduced size of the wings of these island birds is paralleled by the remarkable thinness, &c., of the shell of the "gigantic land-tortoise" of the Galapagos Islands. The changes seen in the carapace can hardly have been brought about by the inherited effects of special disuse. Why then should not the reduction of equally useless, more wasteful, and perhaps positively dangerous wings be also due to an economy which has become advantageous to bird and reptile alike through the absence of the mammalian rivals whose places they are evidently being modified to fill? The complete loss of the wings in neuter ants and termites can scarcely be due to the inherited effects of disuse; and as natural selection has abolished these wings in spite of the opposition of use-inheritance, it must clearly be fully competent to reduce wings without its aid. In considering the rudimentary wings of the apteryx, or of the moa, emu, ostrich, &c., we must not forget the frequent or occasional occurrence of hard seasons, and times of drought and famine, when Nature eliminates redundant, wasteful, and ill-adapted organisms in so severe and wholesale a fashion. Where enemies are absent there would be unrestrained multiplication, and this would greatly increase the severity of the competition for food, and so hasten the elimination of disused and useless parts.

DROOPING EARS AND DETERIORATED INSTINCTS

Mr. Galton has pointed out that existing races and existing organs are only kept at their present high pitch of organic excellence by the stringent and incessant action of natural or artificial selection; and the simple relaxation or withdrawal of such selective influences will almost necessarily result in a certain amount of deterioration, independently even of the principle of economy.21 I think that this cessation of a previous selective process will account for the drooping – but not diminished– ears of various domesticated animals (human preference and increased weight evidently aiding), and also for the inferior instincts seen in them and in artificially-fed caterpillars of the silk-moth, which now "often commit the strange mistake of devouring the base of the leaf on which they are feeding, and consequently fall down." Anyhow, I fail to see that anything is proved by this latter case, except that natural instinct may be perverted or aborted under unnatural conditions and a changed method of selection which abolishes the powerful corrective formerly supplied by natural selection.

WINGS AND LEGS OF DUCKS AND FOWLS

The reduced wings and enlarged legs of domesticated ducks and fowls are attributed by Darwin and Spencer to the inheritance of the effects of use and disuse. But the inference by no means follows. Natural selection would usually favour these adaptive changes, and they would also have been aided by an artificial selection which is often unconscious or indirect. Birds with diminished power of flight would be less difficult to keep and manage, and in preserving and multiplying such birds man would be unconsciously bringing about structural changes which would easily be regarded as effects of use and disuse. "About eighteen centuries ago Columella and Varro speak of the necessity of keeping ducks in netted enclosures like other wild fowl, so that at this period there was danger of their flying away."22 Is it not probable that the best fliers would escape most frequently, or would pine most if kept confined? On the other hand, birds with lessened powers of flight would not be eliminated as under natural conditions, but would be favoured; and natural selection, together with artificial selection of the most flourishing birds, would thicken and strengthen the legs to meet increased demands upon them.

The diminution of the duck's wing is not great even in the birds that "never fly," and from this we must deduct the direct effect of disuse on the individual during its lifetime. As Weismann suggests, the inherited portion of the change could only be ascertained by comparing the bones, &c., of wild and tame ducks similarly reared. If individual disuse diminished the weight of the duck's wing-bones by 9 per cent. there would be nothing left to account for.

I suspect that investigation would reveal anomalies inconsistent with the theory of use-inheritance. Thus according to Darwin's tables of comparative weights and measurements23 the leg-bones of the Penguin duck have slightly diminished in length, although they have increased 39 per cent. in weight. Relatively to the weight of the skeleton, the leg-bones have shortened in the tame breeds of ducks by over 5 per cent. (and in two breeds by over 8 per cent.) although they have increased more than 28 per cent. in proportional weight.24 How can increased use simultaneously shorten and thicken these bones? If the relative shortening is attributed to a heavier skeleton, then the apparently reduced weight of the wing-bones is fully accounted for by the same circumstance, and disuse has had no inherited effect.