Part 5

[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.

[46] _Descent of Man_, p. 33.

[47] _Descent of Man_, p. 33.

[48] _Variation of Animals and Plants under Domestication_, i., 453.

[49] _Descent of Man_, p. 33.

[50] _Descent of Man_, p. 33.

[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).

[52] _Variation of Animals and Plants under Domestication_, ii. 287-289.

[53] _Descent of Man_, pp. 612, 131.

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]

Darwin's most striking case--and to my mind the only case of any importance--is that of Brown-Sequard's epileptic guinea-pigs, which inherited the mutilated condition of parents who had gnawed off their own gangrenous toes when anaesthetic through the sciatic nerve having been divided.[55] 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]--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] 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] but the results are so anomalous[59] that Brown-Sequard'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 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] 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] But Lucas was somewhat credulous. One of his cases is that many girls were born in London without mammae 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 "hyperaesthesia" 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]--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] 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]

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] 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-Sequard's guinea-pigs are only exceptional cases of quasi-inheritance, and are not necessarily indicative of any general rule affecting true inheritance.

FOOTNOTES:

[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.

[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-Sequard does not say that the defective feet were on the same side as in the parents (_Lancet_, Jan., 1875, pp. 7, 8).

[56] _Variation of Animals and Plants under Domestication_, ii. 57.

[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.

[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).

[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.

[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.

[61] Natural History Museum, central hall, third recess on the left.

[62] _Traite de l'Heredite_, 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?

[63] Compare the three cases of crooked fingers given in _Variation of Animals and Plants under Domestication_, ii. 55, 240.

[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).

[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.

[66] _Variation of Animals and Plants under Domestication_, ii. 290; i. 454.

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] 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] 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.