Are the Effects of Use and Disuse Inherited?

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

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1

Which originally appeared in the Nineteenth Century for April and May, 1886.

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).