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 subst.i.tutes 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 pa.s.sed 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 cla.s.ses 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.
MARKED EFFECTS OF USE AND DISUSE ON THE INDIVIDUAL.
These are, to some extent, an argument against the c.u.mulative inheritance of such effects. When a nerve atrophies from disuse, or a duct shrivels, or bone is absorbed, or a muscle becomes small or flabby, it proves, so far, that the average effect of use through enormous ages is _not_ transmitted. When the fibula of a dog"s leg thickens by 400 per cent. to a size "equal to or greater than" that of the removed tibia which previously did the work,[69] it shows that in spite of disuse for countless generations, the "almost filiform" bone has retained a potentiality of development which is fully equal to that possessed by the larger one which has been constantly used. When, after being reared on the ailanthus, the caterpillars of the _Bombyx hesperus_ die of hunger rather than return to their natural food, the inherited effect of ancestral habit does not seem to be particularly strong. Neither is there any strongly-inherited effect of long-continued ancestral wildness in many animals which are easily tamed.
WOULD NATURAL SELECTION FAVOUR USE-INHERITANCE?
If use-inheritance is really one of the factors of evolution, it is certainly a subordinate one, and an utterly helpless one, whenever it comes into conflict with the great ruling principle of Selection. Would this dominant cause of evolution have favoured a tendency to use-inheritance if such had appeared, or would it have discouraged and destroyed it? We have already seen that use-inheritance is unnecessary, since natural selection will be far more effective in bringing about advantageous modifications; and if it can be shown that use-inheritance would often be an evil, it then becomes probable that on the whole natural selection would more strongly discourage and eliminate it as a hostile factor than it might occasionally favour such a tendency as a totally unnecessary aid.
USE-INHERITANCE AN EVIL.
Use-inheritance would crudely and indiscriminately proportion parts to actual work done--or rather to the varying _nourishment and growth_ resulting from a multiplicity of causes--and this in its various details would often conflict most seriously with the real necessities of the case, such as occasional pa.s.sive strength, or appropriate shape, lightness and general adaptation. If its acc.u.mulated effects were not corrected by natural or s.e.xual selection, horns and antlers would disappear in favour of enlarged hoofs. The elephant"s tusks would become smaller than its teeth. Men would have callosities for sitting on, like certain monkeys, and huge corns or hoofs for walking on. Bones would often be modified disastrously. Thus the condyle of the human jaw would become larger than the body of the jaw, because as the fulcrum of the lever it receives more pressure. Some organs (like the heart, which is always at work) would become inconveniently or unnecessarily large.
Other absolutely indispensable organs, which are comparatively pa.s.sive or are very seldom used, would dwindle until their weakness caused the ruin of the individual or the extinction of the species. In eliminating various evil results of use-inheritance, natural selection would be eliminating use-inheritance itself. The displacement of Lamarck"s theory by Darwin"s shows that the effects of use-inheritance often differ from those required by natural selection; and it is clear that the latter factor must at least have reduced use-inheritance to the very minor position of comparative feebleness and harmlessness a.s.signed to it by Darwin.
Use-inheritance would be ruinous through causing unequal variation in co-operative parts--of which Mr. Spencer may accept his own instances of the jaws and teeth, and the cave-crab"s lost eyes and persistent eye-stalks, as typical examples. That the variation would be unequal seems almost self-evident from the varying rapidity and extent of the effects of use and disuse on different tissues and on different parts of the general structure. The optic nerve may atrophy in a few months from disuse consequent on the loss of the eye. Some of the bones of the rudimentary hind legs of the whale are still in existence after disuse for an enormous period. Evidently use-inheritance could not equally modify the turtle and its sh.e.l.l, or the brain and its skull; and in minor matters there would be the same incongruity of effect. Thus, if the molar teeth lengthened from extra use the incisors could not meet.
Unequal and indiscriminate variation would throw the machinery of the organism out of gear in innumerable ways.
Use-inheritance would perpetuate various evils. We are taught, for instance, that it perpetuates short-sight, inferior senses, epilepsy, insanity, nervous disorders, and so forth. It would apparently transmit the evil effects of over-exertion, disuse, hardship, exposure, disease and accident, as well as the defects of age or immaturity.
Would it not be better on the whole if each individual took a fresh start as far as possible on the advantageous typical lines laid down by natural selection? Through the long stages of evolution from primaeval protoplasm upwards, such species as were least affected by use-inheritance would be most free to develop necessary but seldom-used organs, protective coverings such as sh.e.l.ls or skulls, and natural weapons, defences, ornaments, special adaptations, and so forth; and this would be an advantage--for survival would obviously depend on the _importance_ of a structure or faculty in deciding the struggle for existence and reproduction, and not on the total amount of its using or nourishment. If natural selection had on the whole favoured this officious ally and frequent enemy, surely we should find better evidence of its existence.
Without laying undue stress upon the evil effects of use-inheritance, a careful examination of them in detail may at least serve to counter-balance the optimistic _a priori_ arguments for belief in that plausible but unproven factor of evolution.
The benefits derivable from use-inheritance are largely illusory. The effects of _use_, indeed, are generally beneficial up to a certain point; for natural selection has sanctioned or evolved organs which possess the property or potentiality of developing to the right extent under the stimulus of use or nourishment. But use-_inheritance_ would c.u.mulatively alter this individual adaptability, and would tend to fix the size of organs by the average amount of ancestral use or disuse rather than by the actual requirements of the individual. Of course under changed conditions involving increased or lessened use of parts it might become advantageous; but even here it may prove a decided hindrance to adaptive evolution in some respects as well as an unnecessary aid in others. Thus in the case of animals becoming heavier, or walking more, it would _lengthen_ the legs although natural selection might require them to be shortened. In the Aylesbury duck and the Call duck, if use-inheritance has increased the dimensions of the bones and tendons of the leg, natural selection has had to counteract this increase so far as length is concerned, and to effect 8 per cent. of shortening besides. If use-inheritance thickens bones without proportionally lengthening them, it would hinder rather than help the evolution of such structures as the long light wings of birds, or the long legs and neck of the giraffe or crane.
VARIED EFFECTS OF USE AND DISUSE.
The changes which we somewhat roughly and empirically group together as the effects of "use and disuse" are of widely diverse character. Thus bone, as the physiological fact, thickens under _alternations_ of pressure (and the consequent increased flow of nourishment), but atrophies under a steadily continued pressure; so that if the use of a bone involved continuous pressure, the effect of such use would be a partial or total absorption of that bone. Darwin shows that bone lengthens as well as thickens from carrying a greater weight, while tension (as seen in sailors" arms, which are used in pulling) appears to have an equally marked effect in shortening bones (_Descent of Man_, p.
32). Thus different kinds of use may produce opposite results. The c.u.mulative inheritance of such effects would often be mischievous. The limbs of the sloth and the prehensile tail of the spider monkey would continually grow shorter, while the legs of the evolving elephant or rhinoceros might lengthen to an undesirable extent. Such c.u.mulative tendencies of use-inheritance, if they exist, are obviously well kept under by natural selection.
Although the ultimate effect of use is generally growth or enlargement through increased flow of blood, the first effect usually is a loss of substance, and a consequent diminution of size and strength. When the loss exceeds the growth, use will diminish or deteriorate the part used, while disuse would enlarge or perfect it. Teeth, claws, nails, skin, hair, hoofs, feathers, &c., may thus be worn away faster than they can renew themselves. But this wearing away usually stimulates the repairing process, and so increases the rate of growth; that is, it will increase the size produced, if not the size retained. Which effect of use does use-inheritance transmit in such cases--the increased rate of growth, or the dilapidation of the worn-out parts? We can hardly suppose that both these effects of use will be inherited. Would shaving destroy the beard in time or strengthen it? Will the continued shearing of sheep increase or lessen the growth of wool? What will be the ultimate effect of plucking geese"s quills, and of the eider duck"s abstraction of the down from her breast? If the mutilated parts grow stronger or more abundantly, why were the motmot"s feathers alleged to be narrowed by the inherited effects of ancestral nibbling?
The "use" or "work" or "function" of muscles, nerves, bones, teeth, skin, tendon, glands, ducts, eyes, blood corpuscles, cilia, and the other const.i.tuents of the organism, is as widely different as the various parts are from each other, and the effects of their use or disuse are equally varied and complicated.
USE-INHERITANCE IMPLIES PANGENESIS.
How could the transmission of these varied effects to offspring be accounted for? Is it possible to believe, with Mr. Spencer, that the effects of use and disuse on the parts of the personal structure are simultaneously registered in corresponding impressions on the seminal germs? Must we not feel, with Darwin apparently,[70] that the _only_ intelligible explanation of use-inheritance is the hypothesis of Pangenesis, according to which each modified cell, or physiological unit, throws off similarly-modified gemmules or parts of itself, which ultimately reproduce the change in offspring? If we reject pangenesis, it becomes difficult to see how use-inheritance can be possible.
PANGENESIS IMPROBABLE.