General Criticism of the Theory of Sexual Selection
1. Some of the objections that apply to the theory of natural selection apply also with equal force to the theory of sexual selection in so far as the results in both cases are supposed to be the outcome of the selection of individual, or fluctuating, variations. If these variations appear in only a few individuals, their perpetuation is not possible, since they will soon disappear through crossing. It would be, of course, preposterous to suppose that at any one time only those few individuals pair and leave descendants that have the secondary sexual characters developed to the highest point, but if something of this sort does not occur, the extreme of fluctuating variations cannot be maintained. Even if half of the individuals are selected in each generation, the accumulation of a variation in a given direction could not go very far. The assumption, however, that only half of all the individuals that reach maturity breed, and that all of these are chosen on account of the special development of their secondary sexual characters, seems preposterous. Furthermore, if it is assumed that the high development of the new character appears in a large number of individuals, then it is not improbable that its continued appearance might be accounted for without bringing in, at all, the hypothesis of sexual selection.
2. But even supposing that the females select the most beautiful males, then, since in the vast majority of higher animals the males and the females are in equal numbers, the others will also be able to unite with each other in pairs after this first selection has taken place. Nothing will therefore be gained in the next generation. It is interesting to see how Darwin attempts to meet this argument. He tries to show in the case of birds, that there are always unpaired individuals, but since the few facts that he has been able to collect show that there are as many additional females as males, the argument proves too much. A few species are polygamous, one male having a number of female birds; but on this basis we can only account, at best, for the development through competition of the organs of offence and defence used to keep away the weaker males. Yet it is just amongst these birds that we often find the ornamental characters well developed. In fact, since all the females in such cases are selected, and since they will transmit the characters of all the males, it is evident that the secondary sexual characters could not be formed in the way imagined.
3. If the female fails to select only the more ornamental males, no result will follow. It has not been shown that she is capable of making such a choice, and in the lower forms particularly, it does not seem probable that this is done. The argument that Darwin often employs, namely, that unless she does select, the display of the males before her is meaningless, is not to the point. So far as we can detect the “cause” of the display of the male, it appears to be due to his own excitement; and even if we go so far as to admit that the “purpose” is to attract the other sex, it still does not in the least follow that the most ornamental male is selected, and unless this occurs the display has no bearing on the hypothesis of sexual selection.
4. The two forms of sexual selection, namely, competition of the males with one another (really one form of natural selection), and the selection of the most ornamental or gifted individuals, are both used by Darwin to explain secondary sexual characters, the one for organs of offence and defence, and the other for ornamental characters. If we fully appreciate the difficulties that any theory of selection meets with, we shall realize how extraordinarily complex the action must be, when two such processes are carried out at the same time, or even during alternating periods.
5. It has been objected to Darwin’s theory of sexual selection, that he suddenly reverses its mode of action to explain those cases in which the female is the stronger and more ornamented sex; but if, as Darwin shows, the instincts of the male have also changed, and have become more like those of the female, I can see no inherent difficulty in this way of applying the theory. A much more serious objection, it seems to me, is that the male is supposed to select the female for one set of characteristics, and the female to select the male for another set. It sounds a little strange to suppose that women have caused the beard of man to develop by selecting the best-bearded individuals, and the compliment has been returned by the males selecting the females that have the least amount of beard. It is also assumed that the results of the selection are transmitted to one sex only. Unless, in fact, the character in question were from the beginning peculiar to only one sex as to its inheritance, the two sexes might go on forever selecting at cross-purposes, and the result would be nothing.
6. The development, or the presence, of the æsthetic feeling in the selecting sex is not accounted for on the theory. There is just as much need to explain why the females are gifted with an appreciation of the beautiful, as that the beautiful colors develop in the males. Shall we assume that still another process of selection is going on, as a result of which those females are selected by the males that appreciate their unusual beauty, or that those females whose taste has soared a little higher than that of the average (a variation of this sort having appeared) select males to correspond, and thus the two continue heaping up the ornaments on one side and the appreciation of these ornaments on the other? No doubt an interesting fiction could be built up along these lines, but would any one believe it, and, if he did, could he prove it?
Darwin assumes that the appreciation on the part of the female is always present, and he thus simplifies, in appearance, the problem, but he leaves half of it unexplained.
7. It has been pointed out, that it is important to distinguish between the possible excitement of the female by the display or antics of the male, and the selection of the more beautiful or agile performer. Darwin himself records a few cases, which plainly show that the more beautiful is not always the more successful. It has also been suggested that the battles of the males are sometimes sham performances, and even when they are real, if the less vigorous do not remain to be destroyed but run away, they live to find mates of their own. In fact, the conduct of the males at the breeding season appears to be much more the outcome of their own excitement than an attempt to attract the females.
8. There is another side to the question, the importance of which is so great, that it is surprising that Darwin has not taken any notice of it. If, in order to bring about, or even maintain, the results of sexual selection, such a tremendous elimination of individuals must take place, it is surprising that natural selection would not counteract this by destroying those species in which a process, so useless for the welfare of the species, is going on. It is curious that this has not been realized by those who believe in both of these two hypotheses.
9. What has just been said applies also with almost equal force to the development of such structures as the horns of deer, bison, antelopes, and the brilliant colors of many insects and birds. If in nature, competition between species takes place on the scale that the Darwinian theory of natural selection postulates, such forms, if they are much exposed, would be needlessly reduced in numbers in the process of acquiring these structures. So many individuals would have been at such a disadvantage in breeding, that if competition is as severe as the theory of natural selection postulates, these species could hardly be expected to compete successfully with other species in which sexual selection was not taking place.
10. Darwin admits that, in certain cases, external conditions may have acted directly to produce the colors in certain forms, and if these were not injurious he thinks they might have become constant. Such cases are left unexplained in the sense that they are not supposed to be adaptations to anything in particular. That colors produced in this way might afterward be found useful, irrespective of how they arose, is admitted as one of the ways in which sexual differences may have arisen.
11. It is baffling to find Darwin resorting to the Lamarckian explanation in those cases in which the improbability of the hypothesis of sexual selection is manifest. If either principle is true, we should expect it to apply to all phenomena of the same sort; yet Darwin makes use of the Lamarckian principle, in the hypothesis of sexual selection, only when difficulties arise.
12. In attempting to explain the development of the musical sense in man, it is clear that the hypothesis of sexual selection fails to give a satisfactory explanation. To suppose that the genius of a Beethoven or of a Mozart could have been the result of a process of sexual selection is too absurd to discuss. Neither the power of appreciation nor of expression in music could possibly have been the outcome of such a process, and it does not materially help the problem to refer it back to a troop of monkeys making the woods hideous with their cries.
We come now to some of the special cases to which Darwin’s hypothesis has been applied.
13. In one case at least, it is stated that a bird living on the ground might have acquired the color of the upper surface of the body through natural selection, while the under surface of the males of the same species might have become ornamented through the action of sexual selection. Thus in one and the same individual the two processes are supposed to have been at work, and it does not lessen the difficulty very much by supposing the two processes to have been carried out at different times, because it is evident that what had been gained at one time by one process might become lost while the color of certain parts was being acquired through the other process.
14. Darwin points out that “the plumage of certain birds goes on increasing in beauty during many years after they are fully mature,” as in the peacock, and in some of the birds of paradise, and with the plumes and crests of some herons. This is explained as possibly merely the result of “continued growth.” The improbability of selection is manifest in these cases, but if “continued growth” can accomplish this much, why may not the whole process be also the outcome of such growth? At any rate, whatever the explanation is, it is important to find a case of a secondary sexual character that the hypothesis obviously is insufficient to explain.
15. It is admitted in a number of cases, as in the stag for instance, that, although the larynx of the male is enlarged, this is not, in all probability, the outcome of sexual selection, but in other forms this same enlargement is ascribed to the selection process.
16. It is admitted that in none of the highly colored British moths is there much difference according to sex, although when a difference of color is found in butterflies this is put down to the action of sexual selection. If such wonderful colors as those of moths can arise without the action of selection, why make a special explanation for those cases in which this difference is associated with sex?
17. It is well known that birds sing at other times of the year than at the breeding season, and an attempt is made to account for this in that birds take pleasure in practising those instincts that they make use of at other times, as the cat plays with the captive mouse. Does not this suggest that, if they had certain instincts, they would be more likely to employ them at the times when their vitality or excitement is at its highest without regard to the way in which they have come by them?
18. The color of the iris of the eyes of many species of hornbills is said to be an intense crimson in the males, and white in the females. In the male condor the eye is yellowish brown, and in the female a bright red. Darwin admits that it is doubtful if this difference is the result of sexual selection, since in the latter case the lining of the mouth is black in the males, and flesh-colored in the females, which does not affect the external beauty. Yet if these colors were more extensive and on the exterior, there can be little doubt that they would have been explained as due to sexual selection.
19. When the females in certain species of birds differ more from each other than they do from their respective males, the case is compared to “those inexplicable ones, which occur independently of man’s selection in certain sub-breeds of the game-fowl, in which the females are very different, whilst the males can hardly be distinguished.” Here then is a case of difference in color associated with sex, but not the outcome of sexual selection.
20. The long hairs on the throat of the stag are said possibly to be of use to him when hunted, since the dogs generally seize him by the throat, “but it is not probable that the hairs were specially developed for this purpose; otherwise the young and the females would have been equally protected.” Here also is a sexual difference that can scarcely be ascribed to selection.
Some cases of differences in color between the sexes “may be the result of variations confined to one sex, and transmitted to the same sex without any good being gained, and, therefore, without the aid of selection. We have instances of this with our domesticated animals, as in the males of certain cats being rusty-red while the females are tortoise-shell colored. Analogous cases occur in nature: Mr. Bartlett has seen many black varieties of the jaguar, leopard, vulpine phalanger, and wombat; and he is certain that all or nearly all of these animals were males.” If changes of this sort occur, associated with one sex, why is there any need of a special explanation in other cases of difference?
In the light of the many difficulties that the theory of sexual selection meets with, I think we shall be justified in rejecting it as an explanation of the secondary sexual differences amongst animals. Other attempts to explain these differences have been equally unsuccessful. Thus Wallace accounts for them as due to the excessive vigor of the male, but Darwin’s reply to Wallace appears to show that this is not the cause of the difference. He points out that, while the hypothesis might appear plausible in the case of color, it is not so evident in the case of other secondary sexual characters, such, for instance, as the musical apparatus of the males of certain insects, and the difference in the size of the larynx of certain birds and mammals.
Darwin’s theory served to draw attention to a large number of most interesting differences between the sexes, and, even if it prove to be a fiction, it has done much good in bringing before us an array of important facts in regard to differences in secondary sexual characters. More than this I do not believe it has done. The theory meets with fatal objections at every turn.
In a later chapter the question will be more fully discussed as to the sense in which these secondary sexual differences may be looked upon as adaptations.
CHAPTER VII
THE INHERITANCE OF ACQUIRED CHARACTERS AS A FACTOR IN EVOLUTION
Lamarck’s Theory
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.
[17]. This is clearly meant to be applied only in the case of higher animals.
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.”