Are the Effects of Use and Disuse Inherited?

Here we perceive a difficulty which will equally trouble those who affirm use-inheritance and those who deny. Broadly speaking, the adaptive effects ascribed to use-inheritance coincide with the effects of natural selection. The individual adaptability (as shown in the thickening of skin, fur, muscle, &c., under the stimulus of friction, cold, use, &c.) is identical in kind and direction with the racial adaptability under natural selection. Consequently the alleged inheritance of the advantageous effects of use and disuse cannot readily be distinguished from the similarly beneficial effects of natural selection. The indisputable fact that natural selection imitates or simulates the beneficial effects ascribed to use-inheritance may be the chief source and explanation of a belief which may prove to be thoroughly fallacious. A similar simulation of course occurs under domestication, where natural selection is partly replaced by artificial selection of the best adapted and therefore most flourishing animals, while in disused parts panmixia or the comparative cessation of selection will aid or replace "economy of growth" in causing diminution.[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.]

INFERIORITY OF SENSES IN EUROPEANS

"The inferiority of Europeans, in comparison with savages, in eyesight and in the other senses," is attributed to "the accumulated and transmitted effect of lessened use during many generations."[46 - Descent of Man, p. 33.] But why may we not attribute it to the slackened and diverted action of the natural selection which keeps the senses so keen in some savage races?

SHORT-SIGHT IN WATCHMAKERS AND ENGRAVERS

Darwin notices that watchmakers and engravers are liable to be short-sighted, and that short-sight and long-sight certainly tend to be inherited.[47 - Descent of Man, p. 33.] But we must be careful not to beg the question at issue by assuming that the frequent heredity of short sight necessarily covers the heredity of artificially-produced short-sight. Elsewhere, however, Darwin states more decisively that "there is ground for believing that it may often originate in causes acting on the individual affected, and may thence-forward become transmissible."[48 - Variation of Animals and Plants under Domestication, i., 453.] This impression may arise (1) from the facts of ordinary heredity – the ancestral liability being excited in father and son by similar artificial habits, such as reading, and viewing objects closely as among watchmakers and engravers – or by constitutional deterioration from indoor life, &c., acting upon a constitutional liability of the eye to the "something like inflammation of the coats, under which they yield" and so cause shortness of sight by altering the spherical shape of the eye-ball. (2) Panmixia, or the suspension of natural selection, together with altered habits, will account for an increase of short-sight among the population generally. (3) Long-sighted people could not work at watchmaking and engraving so comfortably and advantageously as at other occupations, and hence would be less likely to take to such callings.

LARGER HANDS OF LABOURERS' INFANTS.[49 - Descent of Man, p. 33.]

These are best explained as the result of natural selection and of the diminution of the hand by sexual selection in the gentry. If the larger hands of labourers' infants are really due to the inherited effects of ancestral use, why does the development occur so early in life, instead of only at a corresponding period, as is the rule? During the first few years of its life, at least, the labourer's infant does no more work than the gentleman's child. Why are not the effects of this disuse inherited by the labourer's infant? If the enlargement of the infant's hand illustrates the transference of a character gained later in life, it is evident that the transference must take place in spite of the inherited effects of disuse.

THICKENED SOLE IN INFANTS

Darwin also attributes the thickened sole in infants, "long before birth," to "the inherited effects of pressure during a long series of generations."[50 - Descent of Man, p. 33.] But disuse should make the infant's sole thin, and it is this thinness that should be inherited. If we suppose the inheritance of the thickened soles of later life to be transferred to an earlier period, we have the anomaly of the inherited effects of disuse at that earlier period being overpowered by the untimely inheritance of the effects of use at another. On the other hand, it is clear that natural selection would favour thickened soles for walking on, and might also promote an early development which would ensure their being ready in good time for actual use; for variations in the direction of delay would be cut off, while variations in the other direction would be preserved. Anyhow, the mere transference of a character to an earlier period is no proof of use-inheritance. The real question is whether the thickened sole was gained by natural selection or by the inherited effects of pressure, and the mere transference or hastened appearance of the thickening does not in any degree solve this question. It merely excludes the effect of disuse during lifetime, and thus presents a fallacious appearance of being decisive. The thickened sole of the unborn infant, however, like the lanugo or hairy covering, is probably a result of the direct inheritance of ancestral stages of evolution, of which the embryo presents a condensed epitome. While the relative thinness of the infant's sole might be pointed to as the effect of disuse during a long series of generations, its thickness is rather an illustration of atavism still resisting the effects of long-continued disuse. There is nothing to show that the inheritable portion of the full original thickness was not gained by natural selection rather than by the directly inherited effect of use; and the latter, being cumulative and indiscriminative in its action, would apparently have made the sole very much thicker and harder than it is. If natural selection were not supreme in such cases, how could we account for the effects of pressure resulting in hard hoofs in some cases and only soft pads in others?

A SOURCE OF MENTAL CONFUSION

Of course in a certain sense this thickening of the sole has resulted from use. In one sense or other, most – or perhaps all – of the results of natural selection are inherited effects of use or disuse. Natural selection preserves that which is of use and which is used, while it eliminates that which is useless and is not used. The most confident assertions of the effects of use and disuse in modifying the heritable type, appear to rest on this indefeasible basis. Darwin's statements concerning the effects of use and disuse in evolution can frequently be read in two senses. They often command assent as undeniable truisms as they stand, but are of course written in another and more debatable sense. Thus in the case of the shortened wings and thickened legs of the domestic duck, I believe equally with Darwin and Spencer that "no one will dispute that they have resulted from the lessened use of the wings and the increased use of the legs." "Use" is at bottom the determining circumstance in evolution generally. The trunk of the elephant, the fin of the fish, the wing of the bird, the cunning hand of man and his complicated brain – and, in short, all organs and faculties whatsoever – can only have been moulded and developed by use – by usefulness and by using – but not necessarily by use-inheritance, not necessarily by directly inherited effects of use or disuse of parts in the individual. So, too, reduced or rudimentary organs are due to disuse, but it by no means follows that the diminution is caused by any direct tendency to the inheritance of the effects of disuse in the individual. The effects of natural selection are commonly expressible as effects of use and disuse, just as adaptation in nature is expressible in the language of teleology. But use-inheritance is no more proven by one of these necessary coincidences than special design is by the other. The inevitable simulation of use-inheritance may be entirely deceptive.

Darwin thinks that "there can be no doubt that use in our domestic animals has strengthened and enlarged certain parts, and disuse diminished them; and that such modifications are inherited." Undoubtedly "such" or similar modifications have often been inherited, but how can Darwin possibly tell that they are not due to the simulation of use-inheritance by natural or artificial selection acting upon general variability? Of the inevitability of selection and of its generally adaptive tendencies "there can be no doubt," and panmixia would tend to reduce disused parts; so that there must always remain grave doubts of the alleged inheritance of the similar effects of use and disuse, unless we can accomplish the extremely difficult feat of excluding both natural and artificial selection as causes of enlargement, and panmixia and selection as causes of dwindling.

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 - 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).] 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 - Variation of Animals and Plants under Domestication, ii. 287-289.] 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 - Descent of Man, pp. 612, 131.] 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?

INHERITED INJURIES

INHERITED MUTILATIONS

The almost universal non-inheritance of mutilations seems to me a far more valid argument against a general law of modification-inheritance than the few doubtful or abnormal cases of such inheritance can furnish in its favour. No inherited effect has been produced by the docking of horses' tails for many generations, or by a well-known mutilation which has been practised by the Hebrew race from time immemorial. As lost or mutilated parts are reproduced in offspring independently of the existence of those parts in the parent, there is the less reason to suppose that the particular condition of parental parts transmits itself, or tends to transmit itself, to the offspring. So unsatisfactory is the argument derivable from inherited mutilations that Mr. Spencer does not mention them at all, and Darwin has to attribute them to a special cause which is independent of any general theory of use-inheritance.[54 - A very able anatomist of my acquaintance denies the inheritance of mutilations and injuries, although he strongly believes in the inheritance of the effects of use and disuse.]

Darwin's most striking case – and to my mind the only case of any importance – is that of Brown-Séquard's epileptic guinea-pigs, which inherited the mutilated condition of parents who had gnawed off their own gangrenous toes when anæsthetic through the sciatic nerve having been divided.[55 - Variation of Animals and Plants under Domestication, i. 467-469. Lost toes were only seen by Dr. Dupuy in three young out of two hundred. Obersteiner found that most of the offspring of his epileptic guinea-pigs were injuriously affected, being weakly, small, paralysed in one or more limbs, and so forth. Only two were epileptic, and both were weakly and died early (Weismann's Essays, p. 311). A morbid condition of the spinal cord might affect the hind limbs especially (as in paraplegia) and might occasionally cause loss of toes in the embryo by preventing development or by ulceration. Brown-Séquard does not say that the defective feet were on the same side as in the parents (Lancet, Jan., 1875, pp. 7, 8).] Darwin also mentions a cow that lost a horn by accident, followed by suppuration, and subsequently produced three calves which had on the same side of the head, instead of a horn, a bony lump attached merely to the skin. Such cases may seem to prove that mutilation associated with morbid action is occasionally inherited or repeated with a promptitude and thoroughness that contrast most strikingly with the imperceptible nature of the immediate inheritance of the effects of use and disuse; but they by no means prove that mutilation in general is inheritable, and they are absolutely no proof whatever of a normal and non-pathological tendency to the inheritance of acquired characters. Those who accept Darwin's special explanation of the supposed inheritance of mutilations, ought to notice that his explanation applies equally well under a theory which is strongly adverse to use-inheritance – namely, Galton's idea of the sterilization and complete "using up" of otherwise reproductive matter in the growth and maintenance of the personal structure.

Darwin's explanation of inherited mutilations – which, as he notes, occur "especially or perhaps exclusively" when the injury has been followed by disease[56 - Variation of Animals and Plants under Domestication, ii. 57.]– is that all the representative gemmules which would develop or repair or reproduce the injured part are attracted to the diseased surface during the reparative process and are there destroyed by the morbid action.[57 - Ibid., ii. 392. Perhaps it might be better to suppose that the best gemmules were sacrificed in repairing the injured nerve, and hence only inferior substitutes were left to take their place, and could only imperfectly reproduce the injured part of the nervous system in offspring.] Hence they cannot reproduce the part in offspring. This explanation by no means implies that mutilation would usually affect the offspring. On the contrary, in all ordinary cases of mutilation the purely atavistic elements or gemmules would be set free from any modifying influence of the non-existent or mutilated part. The gemmules – as in Galton's theory of heredity and with neuter insects – might be perfectly independent of pangenesis and the normal inheritance of acquired characters. Such self-multiplying gemmules without pangenesis would enable us to understand both the excessive weakness or non-existence of normal use-inheritance, and the excessive strength and abruptness of the effect of their partial destruction under special pathological conditions.

The series of epileptic phenomena that can be excited by tickling a certain part of the cheek and neck of the adult guinea-pig during the growth and rejoining of the ends of the severed nerve, are said to be repeated with striking accuracy of detail in the young who inherit mutilated toes; but as epilepsy is often due to some one exciting cause or morbid condition, the single transmission of a highly morbid condition of the system might easily reproduce the whole chain of consequences and might also have caused the loss of toes.

The particulars of the guinea-pig cases are very inadequately recorded,[58 - Hence perhaps Mr. Spencer's error in representing the epileptic liability as permanent and as coming on after healing (Factors of Organic Evolution, p. 27).] but the results are so anomalous[59 - It is not claimed that the imperfect foot was on the same side of the body as in the parent, and where parents had lost all the toes of a foot, or the whole foot, the few offspring affected usually had lost only two toes out of the three, or only a part of one or two or three toes. Sometimes the offspring had toes missing on both hind feet, although the parent was only affected in one. One diseased ear and eye in the parent was "generally" or "always" succeeded by two equally affected ears and eyes in the offspring (cf. Pop. Science Monthly, New York, xi. 334). The important law of inheritance at corresponding periods was also set aside. Gangrene or inflammation commenced in both ears and both eyes soon after birth (pointing possibly to infection of some kind); the epileptic period commenced "perhaps two months or more after birth," while the loss of toes had occurred before birth. In no case, as Weismann points out, is the original mutilation of the nervous system ever transmitted. Even where an extirpated ganglion was never regenerated in the parent, the offspring always regained the part in an apparently perfect condition. On the whole the conflicting results ought to be as puzzling to those who may attribute them to a universal tendency to inherit the exact condition of parents as they are to those who, like myself, are sceptical as to the existence of such a law or tendency.] that Brown-Séquard's own conclusion is that the epilepsy and the inherited injuries are not directly transmitted, but that "what is transmitted is the morbid state of the nervous system." He thinks that the missing toes may "possibly" be exceptions to this conclusion, "but the other facts only imply the transmission of a morbid state of the sympathetic or sciatic nerve or of a part of the medulla oblongata." Until we can tell what is transmitted, we are not in a position to determine whether there is any true inheritance or only an exaggerated simulation of it under peculiar circumstances. When the actual observers believe that the mutilations and epilepsy are not the cause of their own repetition, and when these observers guard themselves by such phrases as, "if any conclusion can at present be drawn from those facts," we who have only incomplete reports to guide us may well be excused if we preserve an even more pronounced attitude of caution and reserve.[60 - The various results need to be fully and impartially recorded, and they should also be well tested and confirmed in proportion as they appear improbable and contrary to general experience. Professor Romanes has been carrying out the necessary experiments for some time past.] The morbid state of the system may be wholly due to general injury of the germs rather than to specific inheritance.

Weismann suggests that the morbid condition of the nervous system may be due to some infection such as might arise from microbes, which find a home in the mutilated and disordered nervous system in the parent, and subsequently transmit themselves to the offspring through the reproductive elements, as the infections of various diseases appear to do – the muscardine silkworm disease in particular being known to be conveyed to offspring in this manner.

But whether we can discover the true explanation or not, inherited mutilations can hardly be accounted for as the result of a general tendency to inherit acquired modifications. How could a factor which seems to be totally inoperative in cases of ordinary mutilation, and only infinitesimally operative in transmitting the normal effects of use and disuse, suddenly become so powerful as to completely overthrow atavism, and its own tendency to transmit the non-mutilated type of one of the parents and of the non-mutilated type presented by the injured parent in earlier life? Does not so striking and abrupt an intensification of its usually insignificant power demand an explanation widely different from that which might account for the extremely slow and slight inheritance of the normal effects of use and disuse? Surely it would be better to suspend one's judgment as to the true explanation of highly exceptional and purely pathological cases rather than resort to an hypothesis that creates more difficulties than it solves.

THE MOTMOT'S TAIL

The narrowing of the long central tail feathers of the motmot is attributed to the inherited effects of habitual mutilation (Descent of Man, pp. 384, 603). But in the specimens at South Kensington[61 - Natural History Museum, central hall, third recess on the left.] the narrowness extends upwards much beyond the habitually denuded part, and the broadened end is the broadest part of the whole feather. If the inherited effect of an inch or two of denudation extends from three to six inches upwards, why has it not also extended two inches downwards so as to narrow the broadened end? The narrowness seems to be a mainly relative or negative effect produced by the broadening out of a long tapering feather at its end under the influence of sexual selection. Several other birds have similarly narrowed or spoon-shaped feathers and do not bite them. Is it not more feasible to suppose that this attractive peculiarity first suggested its artificial intensification, than to suppose that the bird began nibbling without any definite cause? Sexual selection would then encourage the habit. Anyhow, it is as impossible to show that the mutilation preceded the narrowing as it is to show that tonsure preceded baldness.

OTHER INHERITED INJURIES MENTIONED BY DARWIN

Darwin quotes some cases from Dr. Prosper Lucas's "long" but weak and unsatisfactory "list of inherited injuries."[62 - Traité de l'Hérédité, ii. 489; Variation of Animals and Plants under Domestication, i. 469. If injuries are inherited, why has the repeated rupture of the hymen produced no inherited effect?] But Lucas was somewhat credulous. One of his cases is that many girls were born in London without mammæ through the injurious effect of certain corsets on the mothers. He also gives a long account of a Jew who could read through the thick covers of a book, and whose son inherited this "hyperæsthesia" of the sense of sight in a still more remarkable degree (i. 113-119). Evidently Lucas's cases cannot be accepted without some amount of reserve.

The cases of the three calves which inherited the one-horned condition of the cow, the two sons who inherited a father's crooked finger, and the two sons who were microphthalmic on the same side as their father had lost an eye, may be due to mere coincidence; or an inherited constitutional tendency or liability might lead to somewhat similar results in parent and offspring[63 - Compare the three cases of crooked fingers given in Variation of Animals and Plants under Domestication, ii. 55, 240.]– just as the tendency to certain fatal diseases or to suicide may produce similar results in father and son, although the artificially-produced hanging or apoplexy obviously cannot be directly transmitted. That more than one of the offspring was affected does not render the chances against coincidence "almost infinitely great," as Darwin mistakenly supposes. It "frequently occurs" that a man's sons or daughters may all exhibit either a latent or a newly-developed congenital peculiarity previously unknown;[64 - Ibid., i. 460. Thus, where two brothers married two sisters all the seven children were perfect albinos, although none of the parents or their relatives were albinos. In another case the nine children of two sound parents were all born blind (ii. 322).] and the coincidence may merely be that one of the parents accidentally suffered a similar kind of injury – a kind of coincidence which must of course occasionally occur, and which may have been partly caused by a latent tendency. The chances against coincidence are indeed great, but the cases appear to be correspondingly rare.

Darwin acknowledges that many supposed instances of inherited mutilation may be due to coincidence; and there is apparently no more reason for attributing inherited scars, &c., to any special form of heredity than to the effect of the mother's imagination on the unborn babe – a popular but fallacious belief in corroboration of which far more alleged instances could be collected than of the inheritance of injuries.

As an instance of the coincidences that occur, I may mention that a friend of mine has a daughter who was born with a small hole in one ear, just as if it were already pierced for the earring which she has since worn in it. I suppose, however, that no one will venture to claim this as an instance of the inheritance of a mutilation practised by female ancestors, especially as such holes are not altogether unknown or inexplicable, though very rarely occurring low down in the lobe of the ear.[65 - See pp. 179-182, Evolution and Disease, by J. Bland Sutton, to whom and to our mutual friend Dr. D. Thurston I am indebted for information on various points.]

Many cases are known of the inheritance of mutilations or malformations arising congenitally from some abrupt variation in the reproductive elements. In such cases as the one-eared rabbits, the two-legged pigs, the three-legged dogs, the one-horned stags, hornless bulls, earless rabbits, lop-eared rabbits, tailless dogs, &c., if the father or the mother or the embryo had suffered from some accident or disease which might plausibly have been assigned as the cause of the original malformation, these transmitted defects would readily be cited as instances of the inheritance of an accidentally-produced modification.

The inheritance of exostoses on horses' legs may be the inheritance of a constitutional tendency rather than of the effect of the parents' hard travelling. Horses congenitally liable to such formations would transmit the liability,[66 - Variation of Animals and Plants under Domestication, ii. 290; i. 454.] and this might readily be mistaken for inheritance of the results of the liability. An apparent increase in this liability might arise from greater attention being now paid to it, or from increased use of harder roads; or a real increase might be due to panmixia and some obscure forms of correlation.

QUASI-INHERITANCE

Of course artificially-caused ill-health or weakness in parents will tend in a general way to injure the offspring. But deterioration thus caused is only a form of quasi-inheritance, as I should prefer to call it. Semi-starvation in a new-born babe is not truly inherited from its half-starved mother, but is the direct result of insufficient nourishment. The general welfare of germs – as of parasites – is necessarily bound up with that of the organism which feeds and shelters them, but this is not heredity, and is quite irrelevant to the question whether particular modifications are transmitted or not.

Another form of quasi-inheritance is seen in the communication of certain infections to offspring. Not being transmitted by the action of the organism so much as in defiance of it, such diseases are not truly hereditary, though for convenience' sake they are usually so described.

A perversion or prevention of true inheritance is also seen in the action of alcohol, or excessive overwork, or any other cause which by originating morbid conditions in individuals may also injure the reproductive elements.

These forms of quasi-inheritance are, of course, highly important so far as the improvement of the race is concerned. So, too, is the fact that improved or deteriorated habits and thoughts are transmitted by personal teaching and influence and are cumulative in their effect. But all this must not be confounded with the inheritance of acquired characters. Cases of quasi-inheritance may perhaps be most readily distinguished from cases of true inheritance by the time test. When a modification acquired in adult life is promptly communicated to the child in early life or from birth, it may rightly be suspected that the inheritance, like that of money or title, is not truly congenital, but is extraneous or even anti-congenital in its nature. Judged by such a standard, the inherited injuries in Brown-Séquard's guinea-pigs are only exceptional cases of quasi-inheritance, and are not necessarily indicative of any general rule affecting true inheritance.

MISCELLANEOUS CONSIDERATIONS

TRUE RELATION OF PARENTS AND OFFSPRING

It is difficult to entirely free ourselves from the flattering and almost universal idea that parents are true originators or creators of copies of themselves. But the main truth, if not the whole truth, is that they are merely the transmitters of types of which they and their offspring are alike more or less similarly moulded resultants. A parent is a trustee. He transmits, not himself and his own modifications, but the stock, the type, the representative elements, of which he is a product and a custodian in one. It seems probable that he has no more definite or "particulate" influence over the reproductive elements within him than a mother over the embryo or a vessel over its cargo. Parent and offspring are like successive copies of books printed from the same "type." A battered letter in the "type" will display its effects in both earlier and later copies alike, but a purely extraneous or acquired flaw in the first copy is not necessarily repeated in subsequent copies. Unlike printer's type, however, the material source of heredity is of a fluctuating nature, consisting of competing elements derived from two parents and from innumerable ancestors.

Galton compares parent and child to successive pendants on the same chain. Weismann likens them to successive offshoots thrown up by a long underground root or sucker. Such comparisons indicate the improbability of acquired modifications being transmitted to offspring.

That parts are developed in offspring independently of those parts in parents is clear. Mutilated parents transmit parts which they do not possess. The offspring of young parents cannot inherit the later stages of life from parents who have not passed through them. Cases of remote reversion or atavism show that ancestral peculiarities can transmit themselves in a latent or undeveloped condition for hundreds or thousands of generations. Many obvious facts compelled Darwin to suppose that vast numbers of the reproductive gemmules in an individual are not thrown off by his own cells, but are the self-multiplying progeny of ancestral gemmules. Galton restricts the production of gemmules by the personal structure to a few exceptional cases, and would evidently like to dispense with pangenesis altogether, if he could only be sure that acquired characters are never inherited. Weismann entirely rejects pangenesis and the inheritance of acquired characters. This enables him to explain heredity by his theory of the "Continuity of the Germ-plasm."[67 - Essays on Heredity, p. 104. Weismann's theory is clear, simple and convenient, but incomplete; for, unlike Darwin's theory of pangenesis, it scarcely attempts any real explanation of the extremely complex potentialities possessed by the reproductive elements. Perhaps we might retain Darwin's self-multiplying gemmules without supposing them to be thrown off by the cells, which will no longer be credited with two modes of multiplication. These minute germs or gemmules may have been evolved by natural selection playing upon the sample germs that achieve development; and they may exist either separately, or (preferably but perhaps not invariably) in aggregates to form Weismann's germ-plasm.] Parent and offspring are alike successive products or offshoots of this persistent germ-substance, which obviously would not be correspondingly affected by modifications of parts in parents, and so would render the transmission of acquired characters impossible.

INVERSE INHERITANCE

Mr. Galton contends that the reproductive elements become sterile when used in forming and maintaining the individual, and that only a small proportion of them are so used.[68 - Contemporary Review, Dec., 1875, p. 88.] He holds that the next generation will be formed entirely, or almost entirely, from the residue of undeveloped germs, which, not having been employed in the structure and work of the individual, have been free to multiply and form the reproductive elements whence future individuals are derived. Hence the singular inferiority not infrequently displayed by the children of men of extraordinary genius, especially where the ancestry has been only of a mediocre ability. The valuable germs have been used up in the individual, and rendered sterile in the structure of his person. Hence, too, the "strong tendency to deterioration in the transmission of every exceptionally gifted race." Mr. Galton's hypothesis "explains the fact of certain diseases skipping one or more generations," and it "agrees singularly well with many classes of fact;" and it is strongly opposed to the theory of use-inheritance. The elements which are used die almost universally without germ progeny: the germs which are not used are the great source of posterity. Hence, when the germs or gemmules which achieve development are either better or worse than the residue, the qualities transmitted to offspring will be of an inverse character. If brain-work attracts, develops and sterilizes the best gemmules, the ultimate effect of education on the intellect of posterity may differ from its immediate effect.

EARLY ORIGIN OF THE OVA

As the ova are formed at as early a period as the rest of the maternal structure, Galton notices that it seems improbable that they would be correspondingly affected by subsequent modifications of parental structure. Of course it is not certain that this is a valid argument. We know that the paternal half of the reproductive elements does not enter the ovum till a comparatively late stage in its history, and it is quite possible that maternal elements or gemmules may also enter the ovum from without. If reproductive elements were confined to one special part or organ, we should be unable to explain the reproduction of lost limbs in salamanders, and the persistent effect of intercrossing on subsequent issue by the same mother, and the propagation of plants from shoots, or of the begonia from minute fragments of leaves, or the development of small pieces of water-worms into complete animals.