Evolution and Adaptation

One of the most striking and peculiar characteristics of living things is that through use a part is able to carry out a particular function better than before, and in some cases the use of the part leads to its increase in size. Conversely, disuse leads to the decrease of a part in size. We are perfectly familiar with this process in ourselves as applied to our nervous system and muscles.

It is not surprising that the idea should have arisen that, if the results of the use of a part are inherited by the next generation, the adaptation of organisms might be explained in this way. The presence of the organs of touch, in those parts of the body that are more likely to come into contact with foreign bodies, offers a striking parallel to the perfecting of the sensation of touch that can be brought about through the use of any part. The development of eyes only on the exposed parts of the body, as on the tentacles of the sedentary annelids, or along the margin of the mantle of a bivalve mollusk, suggests that there may be some direct connection between their presence in these regions and the effect of light on the parts. In fact, ever since the time of Lamarck, there have been many zoologists who have claimed that many of the adaptations of organisms have arisen in this way, that is, through the inheritance of the characters acquired through use. In general this theory is summed up in the phrase, “the inheritance of acquired characters.”

This view is prominently associated with the name of Lamarck, who held, however, a different view in regard to the origin of some of the other structures of the organism. Moreover, Erasmus Darwin, even before Lamarck, had suggested the principle of the inheritance of acquired characters.

As has just been said, Lamarck held that the inheritance of acquired characters was only one of the ways in which animals have become changed, and he clearly stated that in the case of all plants and of some of the lower animals the change (evolution) which he supposed them to undergo was due to the general influence of the environment. Since plants and the lower animals (as he supposed) have no central nervous system, or at least no such well-defined nervous system as have the higher animals, Lamarck thought that they could not have evolved in the same way as have the higher animals. We now know that, so far as the lower animals, at least, are concerned, there was no need for such a distinction, since many of their responses are like those of the higher animals. This distinction that Lamarck made is responsible, no doubt, for a misconception that was long held in regard to a part of his views. It is often stated that he supposed the desire of the animal for a particular part has led to the development of that part; while in reality he only maintained the desire to use a particular organ to fulfil some want led to its better development through exercise, and the result was inherited. Lamarck also supposed that the decrease in use of a part which leads to its decrease in size accounts for the degeneration of organs.

Lamarck first advanced his theory in 1801, when he cited the following examples in its favor. A bird, driven through want to the water to find its food, will separate its toes when they strike the water. The skin uniting the bases of the toes will be stretched in consequence, and in this way the broad membrane between the toes of ducks and geese has been acquired. The toes of a bird that is in the habit of perching on a tree become elongated in consequence of becoming stretched, hence has arisen the foot with the long toes characteristic of arboreal birds.

Shore-birds, “which do not care to swim,” but must approach the water in order to obtain food, will be in danger of sinking into the mud, “but, wishing to act so that their body shall not fall into the liquid, they will contract the habit of extending and lengthening their legs.” Hence have arisen the stiltlike legs of shore-birds.

These ideas were more fully elaborated in the following year. He added the further examples: Our dray-horses have arisen through the use to which they have been put, and the race-horse also, which has been used in a different way. Cultivated plants, on the contrary, are the result of the new environment to which they have been subjected.

In the “Philosophic Zoologique,” published in 1809, Lamarck has much more fully developed his theory. Here he combats strenuously the idea that species are fixed. His point of view may be judged by the following propositions, which he believes can be established:—

1. That all organized bodies of our globe are veritable productions of nature, which she has successively produced in the course of a long time.

2. That in her progress nature began, and begins still every day, to produce the simplest organisms, and that she still produces directly the same primitive kinds of organizations. This process has been called spontaneous generation.

3. That the first beginning of animals and of plants takes place in favorable localities and under favorable circumstances. An organic movement having once established their production, they have of necessity gradually developed their organs, and have become diversified in the course of time.

4. That the power of growth of each part of the body being inherited as a consequence of the first effect of life, different modes of multiplication and of regeneration have arisen, and these have been conserved.

5. That with the aid of sufficient time and of favorable circumstances the changes that have taken place on the surface of the globe have called forth new structures and new habits, and in consequence have modified the organs of the body, and made animals and plants such as we see them at the present day.

6. Finally, as a result of these changes that living bodies have been forced to undergo, species have been formed, but these species have only a relative constancy, and are not as ancient as is nature herself. If the environment remains the same, species also remain the same, as is exemplified by the animals living at present in Egypt, which are exactly like those living there in ancient times.

Lamarck concludes that the appearance of stability is always mistaken by the layman for the reality, because, in general, every one judges things relatively to himself. In fact, species are not absolutely constant, but are so only temporarily. “The influence of the environment is continuous and always active, but its effects may only be recognized after a long time.” The irregularity and the complexity of the organization of animals is the outcome of the infinitely diversified circumstances to which they have been subjected. These changes, Lamarck claims, do not directly cause modifications in the form of animals,^[17] but bring about changes in their needs, and changes in their needs bring about changes in their actions. If the needs remain the same, the acquired actions become habits. These habitual actions lead to the use of certain parts in preference to others, and this in turn to an alteration in form and structure. The individuals so changed breed together and leave descendants that inherit the acquired modification.

Curiously enough, Lamarck follows up this argument by citing some cases amongst plants that have been changed directly by the action of the environment. He says that since plants have no motions they have consequently no habits, but they are developed by changes in their nutrition, etc., and this brings about the superiority of some of the vital movements over others.

Amongst domestic animals Lamarck cites the case of the dog, that has come from a wild form like the wolf, but having been carried into different countries has acquired different and new habits, and this has led to the formation of new races, such as the bulldog, greyhound, pug-dog, spaniel, etc.

Lamarck’s argument shifts so often back and forth from animals to plants, that it is clear that in his own mind he did not see any important difference between the action of the environment on plants, and the use of the organs of the animal. He gives in this same connection his oft-quoted summary of what he calls the two laws of nature “which observation always establishes.”

First Law. In every animal, that has not passed beyond the term of its development, the frequent and sustained use of any organ strengthens it, develops it, increases its size, and gives it strength proportionate to the length of time of its employment. On the other hand, the continued lack of use of the same organ sensibly weakens it; it deteriorates, and its faculties diminish progressively until at last it disappears.

Second Law. Nature preserves everything that she has caused the individual to acquire or to lose by the influence of the circumstances to which the race has been for a long time exposed, and consequently by the influence of the predominant use of certain organs (or in consequence of its continued disuse). She does this by the generation of new individuals which are produced with the newly acquired organs. This occurs, provided that the acquired changes were common to the two sexes, or to the individuals that produced the new forms.

These laws are, Lamarck says, fundamental truths which cannot be misunderstood except by those who have never observed or followed nature in her operations. He insists that it is a mistake to suppose that the parts are responsible for the functions, for it is easy to demonstrate that it is the needs and uses of the organs that have caused the parts to develop.

If it is supposed, he continues, that these laws are hypothetical, they may be demonstrated by the following facts: The adult baleen whale is without teeth, although in the fœtus teeth are present, concealed in the jaws. The loss of the teeth is the result of the whale swallowing its food without first masticating it. The ant-eater is also without teeth, and has also the habit of swallowing its food without chewing it. The mole has very small eyes, and this is the result of its having made very little use of them, since its habits are subterranean. Another animal, the aspalax, has only the rudiments of eyes, and has almost completely lost the power of sight. This animal also lives underground like the mole.

Proteus, an aquatic salamander living in deep caves, has only rudimentary eyes. In these latter cases it is the disuse of the eye that has led to its degeneration. This is proven, Lamarck adds, by the fact that the organs of hearing are never in this condition, because sound vibrations penetrate everywhere, even into the densest bodies.

It is a part of the plan of organization of the reptiles that they have four legs; but the snakes, although belonging to this group, have no legs. This absence of legs is explained by their having acquired the habit of gliding over the ground, and of concealing themselves in the grass. Owing to their repeated effort to elongate themselves, in order to pass through narrow spaces, their bodies have become drawn out. Under these circumstances legs would be useless, since long ones would interfere with their motion, and short ones could not move their long bodies. Since the plan of organization limits the snakes to only four legs, and since this number would be useless, they have disappeared.

Many insects are destitute of wings, although wings are a part of the plan of organization of this group. They are absent only in those forms whose habits render wings useless, consequently they have disappeared through disuse.

The preceding cases are those in which the disuse of an organ has led to its degeneration. The following cases are cited to show that by use an organ increases in size. The formation of the web in the feet of water-birds has already been given as a character which Lamarck supposes to have been acquired through use; also the case of shore-birds, which, by an effort to elongate their legs, have actually made them so in the course of time. The necks of water-birds are also long on account of their having been stretched in the efforts to catch fish. The long tongues of the ant-eater, of the woodpecker, and of humming-birds are the result of use, and the long, forked tongue of serpents has come from their using their tongue to feel objects in front of them.

Fishes that have acquired the habit of living in shallow water, flounders, soles, etc., have been forced to swim on their sides in order to approach nearer to the shore. Since more light comes from above than from below, the eye on the under side, straining to turn to the light, has finally migrated to the upper side.

The habit of eating great quantities of food, which distends the digestive organs, has caused the bodies of herbivorous quadrupeds to become large, as seen in the elephant, the rhinoceros, oxen, horses, and buffaloes. The habit of standing for a long time on their feet has caused some animals to develop hard, thick hoofs. Herbivorous animals, that inhabit countries where they are constantly subjected to attack, as deer and antelopes for example, are forced to escape by rapid flight, and in consequence their bodies have become slenderer and their legs thinner. The horns, antlers, and protuberances that many of these animals possess are the results of their butting each other when angered.

“The long neck and the form of the giraffe offer a curious case. We know that the giraffe is the tallest of all animals. It inhabits the centre of Africa, living in those localities where the earth is nearly always dry and without herbage. It is obliged to browse on the foliage of trees, and this leads to its stretching continually upwards. As a result of this habit, carried on for a long time, in all the individuals of the race, the anterior limbs have become longer than the posterior, and its neck has also lengthened, so that the giraffe without rising on its hind-legs stretches up its neck and can reach to the height of six metres.”

The curved claws of the carnivora have arisen from the necessity of grasping their prey. The power of retracting the claws has also been acquired by the effort to draw them in when running over hard ground. The abdominal pouch of the kangaroo, in which the young are carried, opens anteriorly, and this has led to the animal standing erect so that its young are not injured. In consequence, the fore-legs have become shorter through disuse, and the hind-legs have become stronger through use. The tail, which is also used as a support, has become enormously thick at its base.

The sloth has been compelled to seek refuge in the trees, and has taken up its abode permanently there, feeding on leaves. Its movements are limited to those involved in crawling along the limbs in order to reach the leaves. After feeding it remains inactive and sluggish, these habits being provoked by the heat of the climate. The results of its mode of life have been to cause the arms to become elongated due to the habit of the sloth of grasping the limbs of the tree; the claws of the fingers and toes have also become long and hooked in order to retain their hold. The digits that do not make any individual movements have lost the power to do so, and have become fused, and can only be bent in and straightened out. The thighs, being bent out to clasp the larger branches, have caused the pelvis to widen, and, in consequence, the cotyloid cavities have become directed backward. Many of the bones of the skeleton have become fused, as a result of the immobility of the animal.

Lamarck says, that “Nature, in producing, successively, all the species of animals, beginning with the most imperfect, or the most simple, and terminating with the most perfect, has gradually complicated their organization. These animals becoming scattered throughout the habitable regions of the globe each species has received from the influences of its surroundings its present habits, and the modifications of the parts the use of which we recognize.”

Such are Lamarck’s views and a fairly complete statement of the facts from which he draws his conclusions. His illustrations appear naïve, and often not a little ludicrous, but it must be admitted that, despite their absurdities, his theory appears in some cases to account wonderfully well for the facts. The long legs of wading birds, the long neck and disproportionately long fore-legs of the giraffe, the structure of the sloth, and particularly the degeneration of the eyes of animals living in the dark, seem to find a simple explanation in the principle of the inheritance of acquired characters. But the crucial point of the entire theory is passed over in silence, or rather is taken for granted by Lamarck, namely, the inheritance in the offspring of the characters acquired through use or disuse in the parent. He does not even discuss this topic, but in several places states unreservedly that the increase or decrease of a part reappears in the next generation. It is here that Lamarck’s theory has been attacked in more modern times, for as soon as experimental proof was demanded to show that the results of use or of disuse of an organ is inherited, no such proof was forthcoming. Yet the theory is one that has the great merit of being capable of experimental test, and it is astonishing to find that, with the immense amount that has been written by his followers, so few attempts have been made to give the theory a thorough test. The few results that have been obtained are not, however, favorable to the theory, but almost the only attempts at experiment that have been made in this direction have been those of mutilating certain parts; and were it not for popular belief to the effect that such mutilations are inherited, one would least expect to get evidence for or against the theory in this direction. Lamarck himself believed that the changes were slowly acquired, and I think modern Lamarckians are justified in claiming that the validity of the theory can only be tested by experiments in which the organism is subjected to influences extending over a considerable period, although Lamarck appears to have believed that the first results may appear quite soon. Before expressing any opinion in regard to the probability of the theory, let us examine what the followers of Lamarck have contributed in the way of evidence to the theory, rather than the applications that they have made of the theory. We shall also find it profitable to consider some of the modern criticism, to which the theory has been subjected.

Despite the contempt with which Darwin referred to Lamarck’s theory, he himself, as we have seen, often made use of the principle of the inheritance of acquired characters, and even employed the same illustrations cited by Lamarck. Darwin seems to have misunderstood Lamarck’s view, and to have accepted the current opinion that Lamarck supposed an animal acquired a new organ by desiring or needing it. Darwin says, “Heaven forefend me from Lamarck’s nonsense of a tendency to progressive adaptation from the slow willing of the animals.” Darwin speaks of Lamarck as stating that animals will that the egg shall be a particular form so as to become attached to particular objects. Lamarck’s latest biographer, Packard, says he is unable to find any statements of this sort in Lamarck’s writings.

The following cases that Darwin tried to explain through the inheritance of acquired characters are exactly like those to which Lamarck applied his theory. The bones of the wing of the domestic duck weigh less than those of the wild duck, and the bones of the leg more. Darwin believes this is due to the effects of the inheritance of acquired characters. The drooping ears of many domestic mammals are also explained by him as a result of disuse—“the animals being seldom much alarmed.” In speaking of the male of the beetle, Onites apelles, Darwin quotes Kirby to the effect that the tarsi are so habitually lost that the species has been described without this part of the foot. In the sacred beetle of Egypt the tarsus is totally absent. Hence he concludes that the absence of tarsi in the sacred beetle, and the rudimentary condition of the tarsus in others, is probably the result of disuse, rather than a case of inheritance of a mutilation. Darwin grants that “the evidence that accidental mutilations can be inherited is at present not decisive, but the remarkable case observed by Brown-Séquard in guinea-pigs of the inherited effects of operations should make us cautious in denying this tendency.”

The wingless condition of several insects inhabiting oceanic islands has come about, Darwin thinks, through disuse. The ostrich also, owing to its increase in size, made less use of its wings and more use of its legs, with the result that its wings degenerated and its legs got stronger. The rudimentary condition of the eyes of the mole is the result of disuse, “aided perhaps by natural selection.” Many of the animals inhabiting the caves of Kentucky and of Carniola are blind, and this is ascribed to disuse. “As it is difficult to imagine that the eyes, though useless, could be in any way injurious to animals living in darkness, their loss may be attributed to disuse.” The long neck of the giraffe Darwin attributes partly to natural selection and partly to use.

These references will suffice to show that Darwin is in full accord with the main argument of Lamarck. In fact, the curious hypothesis of pangenesis that Darwin advanced was invented partly to account for the inheritance of acquired characters. Despite the hesitancy that Darwin himself felt in advancing this view, and contrary to Huxley’s advice, he at last published his provisional hypothesis of pangenesis in the twenty-seventh chapter of his “Animals and Plants under Domestication.”

Darwin’s Hypothesis of Pangenesis

The study of bud variation, of the various forms of inheritance, and of reproduction and of the causes of variation, led him, Darwin says, to the belief that these subjects stand in some sort of relation to each other. He says: “I have been led, or rather forced, to form a view which to a certain extent connects these facts by a tangible method. Every one would wish to explain to himself, even in an imperfect manner, how it is possible for a character possessed by some remote ancestor suddenly to reappear in the offspring; how the effects of increased or decreased use of a limb can be transmitted to the child; how the male sexual element can act not solely on the ovules, but occasionally on the mother form; how a hybrid can be produced by the union of the cellular tissue of two plants independently of the organs of generation; how a limb can be reproduced on the exact line of amputation, with neither too much nor too little added; how the same organism may be produced by such widely different processes, as budding and true seminal generation; and, lastly, how of two allied forms, one passes in the course of its development through the most complex metamorphoses, and the other does not do so, though when mature both are alike in every detail of structure. I am aware that my view is merely a provisional hypothesis or speculation; but, until a better one be advanced, it will serve to bring together a multitude of facts which are at present left disconnected by any efficient cause.”

In presenting the hypothesis of pangenesis Darwin begins by enumerating the different kinds of sexual and asexual processes of reproduction, for which he hopes to offer a provisional explanation. Here we find mentioned various methods of budding and self-division, regeneration, parthenogenesis, sexual reproduction, and the inheritance of acquired characters. It is with the last only that we are here chiefly concerned; in fact, the need of an hypothesis of this sort to explain the other kinds of inheritance is by no means evident. There are, however, two other phenomena, besides that of the supposed inheritance of acquired characters, to which the hypothesis of pangenesis might appear to apply specially, namely, the effect of foreign pollen on the tissues of the mother plant, and the supposed influence of the union with the first male on the subsequent young (telegony). It is, however, far from being shown that any influence of this latter kind really occurs, despite the fact that it is generally believed in by breeders.

It is important to observe that Darwin proposes to explain on the hypothesis of pangenesis, not only the inheritance of characters acquired through use, but also the decrease of structures through disuse; and this applies, not only to the structure, but to function as well, as when the intelligence of the dog is explained through his association with man, and the tameness of the domestic rabbits through their long confinement. In the following quotation these points are referred to: “How can the use or disuse of a particular limb or of the brain affect a small aggregate of reproductive cells, seated in a distant part of the body, in such a manner that the being developed from these cells inherits the characters of either one or both parents? Even an imperfect answer to this question would be satisfactory.”

Coming now to the theory, we find that it consists of one chief assumption and several minor ones. “It is universally admitted that the cells or units of the body increase by self-division or proliferation, retaining the same nature, and that they ultimately become converted into the various tissues and substances of the body. But besides this means of increase I assume that the units throw off minute granules which are dispersed throughout the whole system; that these, when supplied with proper nutriment, multiply by self-division, and are ultimately developed into units like those from which they were originally derived. These granules may be called gemmules. They are collected from all parts of the system to constitute the sexual elements, and their development in the next generation forms a new being; but they are likewise capable of transmission in a dormant state to future generations, and may then be developed.... Gemmules are supposed to be thrown off by every unit, not only during the adult state, but during each stage of development of every organism; but not necessarily during the continued existence of the same unit. Lastly, I assume that the gemmules in their dormant state have a mutual affinity for each other, leading to their aggregation into buds, or into the sexual elements. Hence, it is not the reproductive organs, or buds, which generate new organisms, but the units of which each individual is composed. These assumptions constitute the provisional hypothesis which I have called Pangenesis.”

It will be noticed that the first assumption is that the cells throw off minute gemmules or granules. The second assumption is that these are collected in the reproductive organs, or in buds, or in regenerating parts; the third assumption is that the gemmules may lie dormant through several generations; the fourth, that the development of the reproductive cells is not so much the development of the cell itself, but of the gemmules that have collected in it. The fifth assumption is that the gemmules are thrown off at all stages of development; the sixth, that in their dormant state they have a mutual affinity for each other; the seventh, that there may be a sort of continual competition in the germ-cells between the original gemmules and the new ones, and, according to which win, the old or the new form develops. Thus we see on closer analysis that the pangenesis hypothesis is made up of a goodly number of different assumptions. At least half a dozen imaginary properties are ascribed to the imaginary gemmules, and these attributes are all essential to the working of the hypothesis.

Some of the more obvious objections to the hypothesis have been stated by Darwin himself. Such, for instance, as our ignorance at what stage in their history the body-cells are capable of throwing off gemmules, and whether they collect only at certain times in the reproductive organs, as the increased flow of blood to these organs at certain seasons might seem to indicate. Nor have we any evidence that they are carried by the blood at all. The experiment of Galton, of transfusing the blood of one animal into another, and finding that this produced no effect on the young that were born later, might be interpreted to mean that gemmules are not transported by the blood; but this kind of experiment is inconclusive, especially in the light of recent results on the effect of the blood of one animal on that of another.

A part of the evidence on which Darwin relied to support his theory has been shown to be incorrect by later work. Thus the assumption that more than a single pollen grain, or more than one spermatozoon, is necessary in some cases for fertilization, is certainly wrong. In most cases, in fact, the entrance of more than one spermatozoon into the egg is disastrous to the development. The cases referred to by Darwin can probably be explained by the difficulty that some of the pollen grains, or spermatozoa, may have in penetrating the egg, or to the immaturity or impotence of some of the male germ-cells, and not to the need of more than one to accomplish the true fertilization.

Darwin’s idea that the small number of gemmules in the unfertilized egg may account for the lack of power of such eggs to develop until they are fertilized, has been shown to be incorrect by recent results in experimental embryology. We now know that many different kinds of stimuli have the power to start the development of the egg. Moreover, we also know that if a single spermatozoon is supplied with a piece of egg-protoplasm without a nucleus, it suffices to cause this piece of protoplasm to develop.

In the case of regeneration, which Darwin also tries to explain on the pangenesis hypothesis, we find that there is no need at all for an hypothesis of this sort; and there are a number of facts in connection with regeneration that are not in harmony with the hypothesis. For instance, when a part is cut off, the same part is regenerated; but under these circumstances it cannot be imagined that the part removed supplies the gemmules for the new part. Darwin tries to meet this objection by the assumption that every part of the body contains gemmules from every other part. But it has been shown that if a limb of the newt is completely extirpated, a new limb does not regenerate; and there is no reason why it should not do so on Darwin’s assumption that germs of the limb exist throughout the body.

The best-authenticated cases of the influence of the male on the tissues of the female are those in plants, where one species, or variety, is fertilized by another. Thus, if the orange is fertilized by the pollen of the lemon, the fruit may have the color and flavor of the lemon. Now the fruit is a product of the tissues of the ovary of the female, and not a part of the seedling that develops in the fruit from the cross-fertilized egg-cell. Analogous cases are recorded for the bean, whose pods may have their color influenced by fertilizing the flower with pollen of another variety having pods of a different color. In these cases we do not know whether the color of the fruit is influenced directly by the foreign pollen, or whether the influence is through the embryo that develops from the egg-cell. The action may appear to be the same, however, in either case; but because it seems probable here that there is some sort of influence of one tissue on another, let us not too readily conclude that this is brought about through any such imaginary bodies as gemmules. It may be directly caused, for instance, by some chemical substance produced in the young hybrid plant. If this is the case, the result would not be different in kind from that of certain flowers whose color may be influenced by certain chemical substances in the soil.

In the cases amongst animals, where the maternal tissues are believed to be influenced by a previous union with the male, as in the oft-cited case of Lord Morton’s mare, a reëxamination of the evidence by Ewart has shown that the case is not demonstrated, and not even probable. Several years ago I tried to test this view in the case of mice. A white mouse was first bred to a dark male house-mouse, and the next time to a white mouse, but none of the offspring from the second union showed any trace of black. If the spermatozoa of the dark mouse are hypodermically injected into the body-cavity of the female, the subsequent young from a white male show no evidence that the male cells have had any influence on the ovary.

The following facts, spoken of by Darwin himself, are not in favor of his hypothesis of pangenesis: “But it appears at first sight a fatal objection to our hypothesis that a part of an organ may be removed during several successive generations, and if the operation be not followed by disease, the lost part reappears in the offspring. Dogs and horses formerly had their tails docked during many generations without any inherited effect; although, as we have seen, there is some reason to believe that the tailless conditions of certain sheep-dogs is due to such inheritance.” The answer that Darwin gives is that the gemmules themselves, that were once derived from the part, are still present in other parts of the body, and it is from these that the organs in the next generation may be derived. But Darwin fails to point out that, if this were the case, it must also be true for those cases in which an organ is no longer used. Its decrease in size in successive generations cannot be due to its disuse, for the rest of the body would supply the necessary gemmules to keep it at its full state of development. Thus, in trying to meet an obvious objection to his hypothesis, Darwin brings forward a new view that is fatal to another part of his hypothesis.

The following cases, also given by Darwin, are admitted by him to be inexplicable on his hypothesis: “With respect to variations due to reversion, there is a similar difference between plants propagated from buds and seeds. Many varieties can be propagated securely by buds, but generally or invariably revert to their parent forms by seed. So, also, hybridized plants can be multiplied to any extent by buds, but are continually liable to reversion by seed,—that is, to the loss of their hybrid or intermediate character. I can offer no satisfactory explanation of these facts. Plants with variegated leaves, phloxes with striped flowers, barberries with seedless fruit, can all be securely propagated by buds taken from the stem or branches; but buds from the roots of these plants almost invariably lose their character and revert to their former condition. This latter fact is also inexplicable, unless buds developed from the roots are as distinct from those on the stem, as is one bud on the stem from another, and we know that these latter behave like independent organisms.” As Darwin here states, these facts appear to be directly contradictory to his hypothesis, and he makes no effort to account for them.

The entire question of the possibility of the inheritance of acquired characters is itself at present far from being on a satisfactory basis, as we shall try to show; and Darwin’s attempt at an explanation, in his chapter on pangenesis, does not put the matter in a much more satisfactory condition.

The Neo-Lamarckian School

Let us now turn our attention to a school that has grown up in modern times, the members of which call themselves Neo-Lamarckians. Let us see if they have supplied the essential evidence that is required to establish the Lamarckian view, namely, that characters acquired by the individual are transmitted to the offspring.

Lamarck’s views were adopted by Herbert Spencer, and play an important rôle in his “Principles of Biology” (1866-1871), and even a more conspicuous part in his later writings. In the former he cites, amongst other cases, that of “a puppy taken from its mother at six weeks old who, although never taught ‘to beg’ (an accomplishment his mother had been taught), spontaneously took to begging for everything he wanted when about seven or eight months old.” If tricks like this are inheritable is it not surprising that more puppies do not stand on their hind-legs?

The larger hands of the laboring classes in England are supposed to be inherited by their children, and the smaller hands of the leisure classes are supposed to be the result of the disuse of the hands by their ancestors; but even if these statements in regard to size are true, there are many other conceivable causes that may have led to this result.

Short-sightedness appears more often, it is said, in those classes of society that make most use of their eyes in reading and in writing; but if we ask for experimental evidence to show that this is due to inheritance, and not due to the children spoiling their eyes at school, there is none forthcoming. The problem is by no means so simple as the uninitiated may be led to believe.

Spencer thinks that “some of the best illustrations of functional heredity are furnished by mental characteristics.” He cites the musical faculty as one that could not have been acquired by natural selection, and must have arisen through the inheritance of acquired modifications. The explanation offered is “that the habitual association of certain cadences of speech with certain emotions has clearly established in the race an organized and inherited connection between such cadences and such emotions, ... and that by the continued hearing and practice of melody there has been gained and transmitted an increasing musical sensibility.” But a statement that the results have been acquired in this way does not supply the proof which the theory is in need of; neither does it follow that, because the results cannot be explained by the theory of natural selection, therefore, they must be explained by the Lamarckian theory.

The clearest proofs that Spencer finds of the inheritance of acquired characters are in the well-known experiments of Brown-Séquard. These experiments will be more fully discussed below. Amongst the other morbid processes that Spencer thinks furnish evidence in favor of this view, are cases of a tendency to gout, the occurrence of mental tricks, musical prodigies, liability to consumption, in all of which cases the fundamental distinction between the inheritance of an acquired character and the inherited tendency toward a particular malady is totally ignored.

Twenty-seven years later (in 1893) Spencer took up the open challenge of the anti-Lamarckian writers, and by bringing forward a number of new arguments attempted to reinstate the principle of the inheritance of acquired characters. His first illustration is drawn from the distribution of the sense of touch in different parts of our bodies. Weber’s experiments have shown that if the sharp points of a pair of compasses are applied to the tips of the forefingers, the sensation of two separate points is given when the points are only one-twelfth of an inch apart, and if the points are moved nearer together, they give the sensation of only one point. The inner surfaces of the second joints of the fingers can only distinguish two points when they are one-sixth of an inch apart. The innermost joints are less discriminating, and are about equal in the power of discrimination to the tip of the nose. The end of the big toe, the palm of the hand, and the cheek discriminate only about one-fifth as well as do the tips of the fingers. The back of the hand and the top of the head distinguish only about one-fifteenth as well as the finger-tips. The front of the thigh, near the knee, is somewhat less sensitive than the back of the hand. On the breast the points of the compasses must be separated by more than an inch and a half in order to give two sensations. In the middle of the back the points must be separated by two and a half inches, or more, in order to give two separate impressions.

What is the meaning of these differences, Spencer asks. If natural selection has brought about the result, then it must be shown that “these degrees of endowment have advantaged the possessor to such an extent that not infrequently life has been directly or indirectly preserved by it.” He asks if this, or anything approaching this, result could have occurred.

That the superior perceptiveness of the forefinger-tip might have arisen through selection is admitted by Spencer, but how could this have been the case, he asks, for the middle of the back, and for the face? The tip of the nose has three times more power of discrimination than the lower part of the forehead. Why should the front of the thigh near the knee be twice as perceptive as in the middle of the thigh; and why should the middle of the back and of the neck and the middle of the forearm and of the thigh stand at such low levels? Is it possible, Spencer asks again, that natural selection has determined these relations, and if not, how can they be explained? His reply is that the differences can all be accounted for on the theory of the inheritance of use, for it is evident that “these gradations in tactile perceptiveness correspond with the gradations in the tactual exercise of the parts.” Except from contact with the clothing the body receives hardly any touch sensations from outside, and this accounts for its small power of discrimination. The greater sensitiveness of the chest and abdomen, as compared with the back, is due to these regions being more frequently touched by the hands, and is also owing to inheritance from more remote ancestors, in which the lower surface of the body was more likely to have come in contact with foreign objects than was the back. The middle of the forearm and of the thigh are also less exposed than the knee and the hand, and have correspondingly the power of tactile discrimination less well developed.

Weber showed that the tip of the tongue is more sensitive than any other part of the body, for it can distinguish between two points only one twenty-fourth of an inch apart. Obviously, Spencer says, natural selection cannot account for such extreme delicacy of touch, because, even if it were useful for the tongue to distinguish objects by touch, this power could never be of vital importance to the animal. It cannot even be supposed that such delicacy is necessary for the power of speech.

The sensitiveness of the tongue can be accounted for, however, Spencer claims, as the result of the constant use of the tongue in exploring the cavity of the mouth. It is continually moving about, and touching now one part, and now another, of the mouth cavity. “No advantage is gained. It is simply that the tongue’s position renders perpetual exploration almost inevitable.” No other explanation of the facts seemed possible to Spencer.

Two questions will at once suggest themselves. First, can it be shown that the sensitiveness to touch in various parts of the body is the result of individual experience? Have we learned to discriminate in those parts of the body that are most often brought into contact with surrounding objects? Even the power of discrimination in the tips of the fingers can be improved, as Spencer himself has shown, in the case of the blind, and of skilled compositors. Can we account in this way for the power of discrimination in various parts of the body? In other words, if, beginning in infancy, the middle of the back constantly came into contact with surrounding objects, would this region become as sensitive as the tips of the fingers? The experiment has not, of course, been carried out, but it is not probable that it would succeed. I venture this opinion on the ground of the relative number of the nerves and of the organs of touch on the back, as compared with those of the finger-tips. But, it will be asked, will not the number of the sense-organs become greater if a part is continually used by the individual? It is improbable that much improvement could be brought about in this way. The improvement that takes place through experience is probably not so much the result of the development of more sense-organs, as of better discrimination in the sensation, because the increased power can be very quickly acquired.

An examination of the relative abundance of touch-spots in the skin shows that they are much more numerous in regions of greater sensitiveness. The following table, taken from Sherrington’s account of sense-organs in Schaefer’s “Textbook of Physiology,” gives the smallest distance that two points, simultaneously applied, can be recognized as such (and not simply as one impression) in different regions.