The Theory of Natural Selection
We have already anticipated, to some extent, Darwin’s conclusion in regard to the outcome of the competition of animals and plants. This result is supposed to lead to the survival of the fittest. The competition is carried out by nature, who is personified as selecting those forms for further experiments that have won in the struggle for existence.
“Can the principle of selection, which we have seen is so potent in the hands of man, apply under Nature? I think we shall see that it can act most efficiently. Let the endless number of slight variations and individual differences occurring in our domestic productions, and, in a lesser degree, in those under Nature, be borne in mind; as well as the strength of the hereditary tendency. Can it, then, be thought improbable, seeing that variations useful to man have undoubtedly occurred, that other variations useful in some way to each being in the great and complex battle for life, should occur in the course of many successive generations? If such do occur can we doubt (remembering how many more individuals are born than can possibly survive) that individuals having any advantage, however slight, over others, would have the best chance of surviving and of procreating their kind? On the other hand, we may feel sure that any variation in the least degree injurious would be rigidly destroyed.”
The process of natural selection is defined as follows, “The preservation of favorable individual differences and variations and the destruction of those that are injurious I have called Natural Selection or the Survival of the Fittest.” And immediately there follows the significant statement, that, “Variations neither useful nor injurious would not be affected by natural selection, and would be left either a fluctuating element, as perhaps we see in certain polymorphic species, or would ultimately become fixed, owing to the nature of the organism and the nature of the conditions.” It will be seen from this quotation, as well as from others already given, that Darwin leaves many structures outside of the pale of natural selection, and uses his theory to explain only those cases that are of sufficient use to be decisive in the life and death struggle of the individuals with each other and with the surrounding conditions.
Darwin states that we can best understand “the probable course of natural selection by taking the case of a country undergoing some slight physical change, for instance, of climate. The proportional numbers of its inhabitants will almost immediately undergo a change, and some species will probably become extinct. We may conclude, from what we have seen of the intimate and complex manner in which the inhabitants of each country are bound together, that any change in the numerical proportions of the inhabitants, independency of the change of climate itself, would seriously affect the others.... In such cases, slight modifications, which in any way favored the individuals of any species, by better adapting them to their altered conditions, would tend to be preserved; and natural selection would have free scope for the work of improvement.”
The first half of the first of these two quotations seems so plausible, that without further thought we may be tempted to give a ready assent to the second, yet the whole issue is contained in this statement. In the abstract, it undoubtedly appears true that any slightly useful modification might tend to be preserved. Whether it will, in reality, be preserved must depend on many things that should be taken into account. This question will come up later for further consideration; but it should be pointed out here, that, even assuming that one or more individuals happen to possess a favorable variation, it by no means follows that natural selection would have free scope for the work of improvement, because the question of the inheritance of this variation, and of its accumulation and building up through successive generations, must be determined before we can be expected to give assent to this argument, that appears so attractive when stated in an abstract and vague way.
Darwin again makes the statement that under the term variation it must never be forgotten that mere individual differences are meant. “As a man can produce a great result with his domestic animals and plants by adding up in any given direction individual differences, so could natural selection, but far more easily from having incomparably longer time for action.” Too much emphasis cannot be laid on the fact that Darwin believed that selection takes place amongst the small individual differences that we find in animals and plants. Some of his followers, as we shall see, are apt to put into the background this fundamental conception of Darwin’s view. His constant comparison between the results of artificial and natural selection leaves no room for doubt as to his meaning. Darwin himself seems, at times, not unconscious of the weakness of this comparison. He says: “How fleeting are the wishes and efforts of man! how short his time! and consequently how poor will be his results, compared with those accumulated by Nature during whole geological periods. Can we wonder then that Nature’s productions should be far ‘truer’ in character than man’s productions; that they should be infinitely better adapted to the most complex conditions of life, and should plainly bear the stamp of far higher workmanship?” We should not lose sight of the fact that even after the most rigorous selective process has been brought to bear on organisms, namely, by isolation under domestication, we do not apparently find ourselves gradually approaching nearer and nearer to the formation of new species, but we find, on the contrary, that we have produced something quite different. In the light of this truth, the relation between the two selective theories may appear quite different from the interpretation that Darwin gives of it. We may well doubt whether nature does select so much better than does man, and whether she has ever made new species in this way.
We come now to a point that touches the theory of natural selection in a very vital spot.
“It may be well here to remark that with all beings there must be much fortuitous destruction, which can have little or no influence on the course of natural selection. For instance, a vast number of eggs or seeds are annually devoured, and these could be modified through natural selection only if they varied in some manner which protected them from their enemies. Yet many of these eggs or seeds would perhaps, if not destroyed, have yielded individuals better adapted to their conditions of life than any of those which happened to survive. So again a vast number of mature animals and plants, whether or not they be the best adapted to their conditions, must be annually destroyed by accidental causes, which would not be in the least degree mitigated by certain changes of structure or constitution which would in other ways be beneficial to the species. But let the destruction of the adults be ever so heavy, if the number which can exist in any district be not wholly kept down by such causes,—or again let the destruction of eggs or seeds be so great that only a hundredth or a thousandth part are developed,—yet of those which do survive, the best adapted individuals, supposing that there is any variability in a favorable direction, will tend to propagate their kind in larger numbers than the less well adapted. If the numbers be wholly kept down by the causes just indicated, as will often have been the case, natural selection will be powerless in certain beneficial directions; but this is no valid objection to its efficiency at other times and in other ways; for we are far from having any reason to suppose that many species ever undergo modification and improvement at the same time in the same area.”
Some of the admissions made in this paragraph have an important bearing on the theory of natural selection. Far from supposing that fortuitous destruction would have no influence on the course of natural selection, it can be shown that it would have a most disastrous effect. In many cases the destruction comes in the form of a catastrophe to the individuals, so that small differences in structure, whether advantageous or not, are utterly unavailing. Our experience shows us that a destruction of this sort is going on around us all the time, and accounts in large part for the way in which the majority of animals and plants are destroyed. Unless, for example, a seed happen to fall on a place suitable for its growth, it will perish without respect to a slight advantage it may have over other seeds of its kind. Of the thousands of eggs laid by one starfish, chance alone will decide whether one or another embryo is destroyed by larger animals, or if they escape this danger, the majority of them may be carried out to sea, where it will not be of the least avail if one individual has a slight advantage over the others. Darwin admits this, but adds that, if only a thousandth part is developed, yet of those that do survive the best adapted individuals will tend to propagate their kind in larger numbers than the less well adapted. The argument is not, however, so simple as it appears to be on the surface. I pass over, for the present, the apparent inconsequence in this statement that the best adapted individuals will tend to propagate their kind in larger numbers. It is not by any means certain that this is the case. Darwin’s meaning is, however, fairly clear, and can be interpreted to mean this: after the fortuitous destruction has finished, there will be a further competition of the survivors amongst themselves and with the surrounding conditions. In this higher competition, which is less severe, small individual differences suffice to determine the survival of certain individuals. These are, therefore, selected.
In this argument it is assumed that a second competition takes place after the first destruction of individuals has occurred, and this presupposes that more individuals reach maturity than there is room for in the economy of nature. But we do not know to what extent this takes place. If only as many mature as can survive, then the second competition does not take place. If, on the other hand, fewer mature than there is room for, then again competition does not take place. And if at all times selection is not rigorously carried out, everything may be lost that has been so laboriously gained. We see then that the result that Darwin imagines would take place, can be carried out only when more individuals reach maturity than there is room for (if it is a case of competition with one another), or that escape their enemies (if it is a question of competition with other forms).
It is instructive to consider some of the examples that Darwin has given to illustrate how the process of natural selection is carried out. The first example is the imaginary case of a species of wolf, the individuals of which secure their prey sometimes by craft, sometimes by strength, and sometimes by fleetness. If the prey captured by the first two methods should fail, then all the wolves would be obliged to capture their food by fleetness, and consequently the fleetest alone would survive. “I can see no more reason to doubt that this would be the result than that man should improve the fleetness of his greyhounds.” But even if the fleetness of the race could be kept up in this way, it does not follow that a new species of wolf would be formed in consequence, as Darwin implies. His own comment on this illustration is, perhaps, the best criticism that can be made.
“It should be observed that, in the above illustration, I speak of the slimmest individual wolves, and not of any single strongly marked variation having been preserved. In former editions of this work I sometimes spoke as if this latter alternative had frequently occurred. I saw the great importance of individual differences, and this led me fully to discuss the results of unconscious selection by man, which depends on the preservation of all the more or less valuable individuals, and on the destruction of the worst. I saw, also, that the preservation in a state of nature of any occasional deviation of structure, such as a monstrosity, would be a rare event; and that, if at first preserved, it would generally be lost by subsequent intercrossing with ordinary individuals. Nevertheless, until reading an able and valuable article in the North British Review (1867), I did not appreciate how rarely single variations, whether slight or strongly marked, could be perpetuated. The author takes the case of a pair of animals, producing during their lifetime two hundred offspring, of which, from various causes of destruction, only two on an average survive to procreate their kind. This is rather an extreme estimate for most of the higher animals, but by no means so for many of the lower organisms. He then shows that if a single individual were born, which varied in some manner, giving it twice as good a chance of life as that of the other individuals, yet the chances would be strongly against its survival. Supposing it to survive and to breed, and that half its young inherited the favourable variation; still, as the reviewer goes on to show, the young would have only a slightly better chance of surviving and breeding; and this chance would go on decreasing in the succeeding generations. The justice of these remarks cannot, I think, be disputed. If, for instance, a bird of some kind could procure its food more easily by having its beak curved, and if one were born with its beak strongly curved, and which consequently flourished, nevertheless there would be a very poor chance of this one individual perpetuating its kind to the exclusion of the common form; but there can hardly be a doubt, judging by what we see taking place under domestication, that this result would follow from the preservation during many generations of a large number of individuals with more or less strongly curved beaks, and from the destruction of a still larger number with the straightest beaks.”
There then follows what, I believe, is one of the most significant admissions in the “Origin of Species”:—
“It should not, however, be overlooked that certain rather strongly marked variations, which no one would rank as mere individual differences, frequently recur owing to a similar organization being similarly acted on—of which fact numerous instances could be given with our domestic productions. In such cases, if the varying individual did not actually transmit to its offspring its newly acquired character, it would undoubtedly transmit to them, as long as the existing conditions remained the same, a still stronger tendency to vary in the same manner. There can also be little doubt that the tendency to vary in the same manner has often been so strong that all the individuals of the same species have been similarly modified without the aid of any form of selection. Or only a third, fifth, or tenth part of the individuals may have been thus affected, of which fact several instances could be given. Thus Graba estimates that about one-fifth of the guillemots in the Faroe Islands consist of a variety so well marked, that it was formerly ranked as a distinct species under the name of Uria lacrymans. In cases of this kind, if the variation were of a beneficial nature, the original form would soon be supplanted by the modified form, through the survival of the fittest.”
Do not the admissions in this paragraph almost amount to a withdrawal of much that has preceded in regard to the survival of fluctuating, individual differences? In the last edition, from which we have just quoted, Darwin, in response to the criticisms which his book met, inserted here and there statements that are in many ways in contradiction to the statements in the first edition, and yet the earlier statements have been allowed to stand for the most part.
The next example is also worthy of careful examination, since it appears to prove too much:—
“It may be worth while to give another and more complex illustration of the action of natural selection. Certain plants excrete sweet juice, apparently for the sake of eliminating something injurious from the sap: this is effected, for instance, by glands at the base of the stipules in some Leguminosæ, and at the backs of the leaves of the common laurel. This juice, though small in quantity, is greedily sought by insects; but their visits do not in any way benefit the plant. Now, let us suppose that the juice or nectar was excreted from the inside of the flowers of a certain number of plants of any species. Insects in seeking the nectar would get dusted with pollen, and would often transport it from one flower to another. The flowers of two distinct individuals of the same species would thus get crossed; the act of crossing, as can be fully proved, gives rise to vigorous seedlings, which consequently would have the best chance of flourishing and surviving. The plants which produced flowers with the largest glands or nectaries, excreting most nectar, would oftenest be visited by insects, and would oftenest be crossed; and so in the long run would gain the upper hand and form a local variety.”
The reader will notice that the sweet juice or nectar secreted by certain plants is supposed to have first appeared independently of the action of natural selection. Why then account for its presence in flowers as the outcome of an entirely different process? If the nectar is eagerly sought for by insects, without the plant benefiting in any way by their visitations, why give a different explanation of its origin in flowers where it is of benefit to the plant?
Darwin carries his illustration further: “When our plant, by the above process long continued, had been rendered highly attractive to insects, they would unintentionally, on their part, regularly carry pollen from flower to flower; and that they do this effectually, I could easily show by many striking facts. I will give only one, as likewise illustrating one step in the separation of the sexes of plants.... As soon as the plant had been rendered so highly attractive to insects that pollen was regularly carried from flower to flower, another process might commence. No naturalist doubts the advantage of what has been called the ‘physiological division of labour’; hence we may believe that it would be advantageous to a plant to produce stamens alone in one flower or on one whole plant, and pistils alone in another flower or on another plant. In plants under culture and placed under new conditions of life, sometimes the male organs and sometimes the female organs become more or less impotent; now if we suppose this to occur in ever so slight a degree under nature, then, as pollen is already carried regularly from flower to flower, and as a more complete separation of the sexes of our plant would be advantageous on the principle of the division of labour, individuals with this tendency more and more increased would be continually favoured or selected, until at last a complete separation of the sexes might be effected. It would take up too much space to show the various steps, through dimorphism and other means, by which the separation of the sexes in plants of various kinds is apparently now in progress; but I may add that some of the species of holly in North America are, according to Asa Gray, in an exactly intermediate condition, or, as he expresses it, are more or less diœciously polygamous.”
From this it will be seen that Darwin supposes that the separation of the sexes in some of the higher plants has been brought about by natural selection. Despite the supposed advantage of the so-called “division of labor,” one may, I venture to suggest, be sceptical as to whether the separation of the sexes can be explained in this way. The whole case is largely supposititious, since in most of the higher hermaphroditic plants and in nearly all hermaphroditic animals the sexual products ripen at different times in the same individual. Hence there is no basis for the assumption that unless the sexes are separated there will be self-fertilization. Shall we assume that this difference in time of ripening of the two kinds of sex-cells is also the outcome of natural selection, and that there has existed an earlier stage in all animals and plants, that now have different times for the ripening of their sexual elements, a time when these products ripened simultaneously? I doubt if even a Darwinian would give such loose rein to his fancy.
But this is not yet the whole story that Darwin has made out in this connection, for he continues:—
“Let us now turn to the nectar-feeding insects; we may suppose the plant, of which we have been slowly increasing the nectar by continued selection, to be a common plant; and that certain insects depended in main part on its nectar for food. I could give many facts showing how anxious bees are to save time: for instance, their habit of cutting holes and sucking the nectar at the bases of certain flowers, which with a very little more trouble, they can enter by the mouth. Bearing such facts in mind, it may be believed that under certain circumstances individual differences in the curvature or length of the proboscis, etc., too slight to be appreciated by us, might profit a bee or other insect, so that certain individuals would be able to obtain their food more quickly than others; and thus the communities to which they belonged would flourish and throw off many swarms inheriting the same peculiarities.”
Aside from the general criticism that will suggest itself here also, it should be pointed out that even if “certain individuals” of the bees had slightly longer proboscides, this would, in the case of the hive-bees at least, be of no avail, since they do not reproduce, and hence leave no descendants with longer mouth-parts. Of course, it may be replied that those colonies in which the queens produce more of the long-proboscis kind of worker would have an advantage over other colonies not having so many individuals of this sort. It would then be a competition of one colony with another, as Darwin supposes to take place in colonial forms. But whether slight differences of this sort would lead to the elimination of the least well-endowed colonies is entirely a matter of speculation. Since there are flowers with corolla-tubes of all lengths, we can readily suppose that if one kind of flower excluded individuals of certain colonies, they would search elsewhere for their nectar rather than perish. While different races might arise in this way, the process would not be the survival of the fittest, but a process of adaptation to a new environment.
We come now to a topic on which Darwin lays much stress: the divergence of character. He tries to show how the “lesser differences between the varieties become augmented into the greater differences between species.”
“Mere chance, as we may call it, might cause one variety to differ in some character from its parents, and the offspring of this variety again to differ from its parent in the very same character and in a greater degree; but this alone would never account for so habitual and large a degree of difference as that between the species of the same genus. As has always been my practice, I have sought light on this head from our domestic productions.”
Then, after pointing out that under domestication two different races, the race-horse and the dray-horse, for instance, might arise by selecting different sorts of variations, Darwin inquires:—
“But how, it may be asked, can any analogous principle apply in nature? I believe it can and does apply most efficiently (though it was a long time before I saw how), from the simple circumstance that the more diversified the descendants from any one species become in structure, constitution, and habits, by so much will they be better enabled to seize on many and widely diversified places in the polity of nature, and so be enabled to increase in numbers.”
Here we touch on one of the fundamental principles of the doctrine of evolution. It is intimated that the new form of animal or plant first appears (without regard to any kind of selection), and then finds that place in nature where it can remain in existence and propagate its kind. Darwin refers here, of course, only to the less extensive variations, the individual or fluctuating kind; but as we shall discuss at greater length in another place, this same process, if extended to other kinds of variation, may give us an explanation of evolution without competition, or selection, or destruction of the individuals of the same kind taking place at all.
CHAPTER V
THE THEORY OF NATURAL SELECTION (Continued)
Objections to the Theory of Natural Selection
Although in the preceding chapter a number of criticisms have been made of the special parts of the theory of natural selection, there still remain to be considered some further objections that have been made since the first publication of the theory. It is a fortunate circumstance from every point of view that Darwin himself was able in the later editions of the “Origin of Species” to reply to those criticisms that he thought of sufficient importance. He says:—
“Long before the reader has arrived at this part of my work, a crowd of difficulties will have occurred to him. Some of them are so serious that to this day I can hardly reflect on them without being in some degree staggered; but, to the best of my judgment, the greater number are only apparent, and those that are real are not, I think, fatal to the theory.”
The first difficulty is this: “Why, if species have descended from other species by fine gradations, do we not everywhere see innumerable transitional forms? Why is not all nature in confusion, instead of the species being, as we see them, well defined?”
The answer that Darwin gives is, that by competition the new form will crowd out its own less-improved parent form, and other less-favored forms. But is this a sufficient or satisfactory answer? If we recall what Darwin has said on the advantage that those forms will have, in which a great number of new variations appear to fit them to the great diversity of natural conditions, and if we recall the gradations that exist in external conditions, I think we shall find that Darwin’s reply fails to give a satisfactory answer to the question.
It is well known, and Darwin himself has commented on it, that the same species often remains constant under very diverse external conditions, both inorganic and organic. Hence I think the explanation fails, in so far as it is based on the accumulation by selection of small individual variations that are supposed to give the individuals some slight advantage under each set of external conditions. Darwin admits that “this difficulty for a long time quite confounded me. But I think it can be in large part explained.” The first explanation that is offered is that areas now continuous may not have been so in the past. This may be true in places, but the great continents have had continuous areas for a long time, and Darwin frankly acknowledges that he “will pass over this way of explaining the difficulty.” The second attempt is based on the supposed narrowness of the area, where two species, descended from a common parent, overlap. In this region the change is often very abrupt, and Darwin adds:—
“To those who look at climate and the physical conditions of life as the all-important elements of distribution, these facts ought to cause surprise, as climate and height or depth graduate away insensibly. But when we bear in mind that almost every species, even in its metropolis, would increase immensely in numbers, were it not for other competing species; that nearly all either prey on or serve as prey for others; in short, that each organic being is either directly or indirectly related in the most important manner to other organic beings,—we see that the range of the inhabitants of any country by no means exclusively depends on insensibly changing physical conditions, but in a large part on the presence of other species, on which it lives, or by which it is destroyed, or with which it comes into competition; and as these species are already defined objects, not blending one into another by insensible gradations, the range of any one species, depending as it does on the range of others, will tend to be sharply defined.”
Here we have a petitio principii. The sharp definition of species, that we started out to account for, is explained by the sharp definition of other species!
A third part of the explanation is that, owing to the relative fewness of individuals at the confines of the range during the fluctuations of their enemies, or of their prey, or in the nature of the seasons, they would be extremely liable to utter extermination. If this were really the case, then new species themselves which, on the theory, are at first few in numbers ought to be exterminated. On the whole, then, it does not appear that Darwin has been very successful in his attempt to meet this objection to the theory.
Darwin tries to meet the objection, that organs of extreme perfection and complication cannot be accounted for by natural selection, as follows:—
“To suppose that the eye with all its inimitable contrivances for adjusting the focus to different distances, for admitting different amounts of light, and for the correction of spherical and chromatic aberration, could have been formed by natural selection, seems, I freely confess, absurd in the highest degree.”
The following sketch that Darwin gives to show how he imagined the vertebrate eye to have been formed is very instructive, as illustrating how he supposed that natural selection acts:—
“If we must compare the eye to an optical instrument, we ought in imagination to take a thick layer of transparent tissue, with spaces filled with fluid, and with a nerve sensitive to light beneath, and then suppose every part of this layer to be continually changing slowly in density, so as to separate into layers of different densities and thicknesses, placed at different distances from each other, and with the surfaces of each layer slowly changing in form. Further we must suppose that there is a power, represented by natural selection or the survival of the fittest, always intently watching each slight alteration in the transparent layers; and carefully preserving each which, under varied circumstances, in any way or in any degree, tends to produce a distincter image. We must suppose each new state of the instrument to be multiplied by the million; each to be preserved until a better one is produced, and then the old ones to be all destroyed. In living bodies, variation will cause the slight alterations, generation will multiply them almost infinitely, and natural selection will pick out with unerring skill each improvement. Let this process go on for millions of years; and during each year on millions of individuals of many kinds; and may we not believe that a living optical instrument might thus be formed as superior to one of glass, as the works of the Creator are to those of man.”
We may conclude in Darwin’s own words:—
“To arrive, however, at a just conclusion regarding the formation of the eye, with all its marvellous yet not absolutely perfect characters, it is indispensable that the reason should conquer the imagination; but I have felt the difficulty far too keenly to be surprised at others hesitating to extend the principle of natural selection to so startling a length.”
The electric organs, present in several fish, offer a case of special difficulty to the selection theory. When well developed, as in the Torpedo and in Gymnotus, it is conceivable that it may serve as an organ of defence, but in other forms the shock is so weak that it is not to be supposed that it can have any such function. Romanes, who in many ways was one of the stanchest followers of Darwin, admits that, so far as he can see, the evolution of the electric organs cannot be explained by the selection theory. Darwin offers no explanation, but bases his defence on the grounds that we do not know of what use this organ can be to the animal.
Darwin also refers to the phosphorescent, or luminous, organs as a supposed case of difficulty for his theory.
“The luminous organs which occur in a few insects, belonging to widely different families, and which are situated in different parts of the body, offer, under our present state of ignorance, a difficulty almost exactly parallel with that of the electric organs.”
In this case also, as in that of the electric organs, the structures appear in entirely different parts of the body of the insect in different species, so that their occurrence in this group cannot be accounted for on a common descent. In whatever way they have arisen, they must have evolved independently in different species. Darwin advances no explanation of the origin of the luminous organs, but states that they “offer under our present state of ignorance a difficulty almost exactly parallel with that of the electric organs.” It will be noticed that the difficulty referred to rests on the assumption that since the organs are well developed they must have some important use!
We may next consider “organs of little apparent importance as affected by natural selection.” Darwin says:—
“As natural selection acts by life and death,—by the survival of the fittest, and by the destruction of the less well-fitted individuals,—I have sometimes felt great difficulty in understanding the origin or formation of parts of little importance; almost as great, though of a very different kind, as in the case of the most perfect and complex organs.”
His answers to this difficulty are: (1) we are too ignorant “in regard to the whole economy of any one organic being to say what slight modifications would be of importance or not,”—thus such apparently trifling characters as the down on fruit, or the colors of the skin and hair of quadrupeds, which from being correlated with constitutional differences or from determining the attacks of insects might be acted on by natural selection; (2) organs now of trifling importance have in some cases been of high importance to an early progenitor; (3) the changed conditions of life may account for some of the useless organs; (4) reversion accounts for others; (5) the complex laws of growth account for still others, such as correlation, compensation of the pressure of one part on another, etc.; (6) the action of sexual selection is responsible for many characters not to be explained by natural selection. Admitting that there may be cases that can be accounted for on one or the other of these six possibilities, yet there can be no doubt that there are still a considerable number of specific characters that cannot be explained in any of these ways. I do not think that Darwin has by any means met this objection, even if all these six possibilities be admitted as generally valid.
Amongst the “miscellaneous objections” to his theory that Darwin considers we may select the most important cases. The following paragraph has been sometimes quoted by later writers to show that Darwin saw, to a certain extent, the insufficiency of fluctuating variations as a basis for selection. What he calls here “spontaneous variability” refers to sudden and extensive variations, or what we may call discontinuous variations. “In the earlier editions of this work I underrated, as it now seems probable, the frequency and importance of modifications due to spontaneous variability. But it is impossible to attribute to this cause the innumerable structures which are so well adapted to the habits of life of each species. I can no more believe in this, that the well-adapted form of a race-horse or greyhound, which before the principle of selection by man was well understood, excited so much surprise in the minds of the older naturalists, can thus be explained.”
Darwin appears to mean by the latter part of this statement, that he cannot believe that such sudden and great variations as have caused a peach tree to produce nectarines can account for the wonderful adaptations of organisms; but it is not really necessary to suppose that this would often occur, for the same result could be reached by several stages, even if the discontinuous variations had been small, and had appeared in many individuals simultaneously. After showing that in a number of flowers, especially of the Compositæ and Umbelliferæ, the individual flowers in the closely crowded heads are sometimes formed on a different type, Darwin concludes: “In these several cases, with the exception of that of the well-developed ray-florets, which are of service in making the flowers conspicuous to insects, natural selection cannot, as far as we can judge, have come into play, or only in a quite subordinate manner. All these modifications follow from the relative position and interaction of the parts; and it can hardly be doubted that if all the flowers and leaves on the same plant had been subjected to the same external and internal condition, as are the flowers and leaves in certain positions, all would have been modified in the same manner.”
Further on we meet with the following remarkable statement: “But when, from the nature of the organism and of the conditions, modifications have been induced which are unimportant for the welfare of the species, they may be, and apparently often have been, transmitted in nearly the same state to numerous, otherwise modified, descendants. It cannot have been of much importance to the greater number of mammals, birds, or reptiles, whether they were clothed with hair, feathers, or scales; yet hair has been transmitted to almost all mammals, feathers to all birds, and scales to all true reptiles. A structure, whatever it may be, which is common to many allied forms, is ranked by us as of high systematic importance, and consequently is often assumed to be of high vital importance to the species. Thus, as I am inclined to believe, morphological differences, which we consider as important,—such as the arrangement of the leaves, the divisions of the flower or of the ovarium, the position of the ovules, etc.,—first appeared in many cases as fluctuating variations, which sooner or later became constant through the nature of the organism and of the surrounding conditions, as well as through the intercrossing of distinct individuals, but not through natural selection; for as these morphological characters do not affect the welfare of the species, any slight deviations in them could not have been governed or accumulated through this latter agency. It is a strange result which we thus arrive at, namely, that characters of slight vital importance to the species are the most important to the systematist; but, as we shall hereafter see when we treat of the genetic principle of classification, this is by no means so paradoxical as it may at first appear.”
If all this be granted, it is once more evident that the only variations that come under the action of selection are the limited number that are of vital importance to the organism. How little the theory of natural selection can be used to explain the origin of species will be apparent from the above quotation. This is, of course, not an argument against the theory itself, which would still be one of vast importance if it explained adaptive characters alone; but enough has been said, I think, to show that it is improbable that the origin of adaptive and non-adaptive characters are to be explained by entirely different principles.
In reply to a criticism of Mivart, Darwin makes the further admission as to the insufficiency of the theory of natural selection: “When discussing special cases, Mr. Mivart passes over the effects of the increased use and disuse of parts, which I have always maintained to be highly important, and have treated in my ‘Variation under Domestication’ at greater length than, as I believe, any other writer. He likewise often assumes that I attribute nothing to variation, independent of natural selection, whereas in the work just referred to I have collected a greater number of well-established cases than is to be found in any other work known to me.” If this is admitted, and if it can be shown that the evidence in favor of the inheritance of acquired characters is very doubtful at best, may we not conclude that Mivart’s criticisms have sometimes hit the mark?
The following objection appears to be a veritable stumbling-block to the theory. Flatfishes and soles lie on one side, and do not stand in a vertical position as do other fish. Some species lie on one side and some on the other, and some species contain both right-sided and left-sided individuals. In connection with this unusual habit we find a striking change in the structure. The eye that would be on the under side has shifted, so that it has come to lie on the upper side of the head, i.e. both eyes lie on the same side,—a condition found in no other vertebrate. As a result of the shifting of the eye, the bones of the skull have also become profoundly modified. The young fish that emerge from the egg swim at first upright, as do ordinary fish, and only after they have led a free existence for some time do they turn to one side and sink to the bottom. Unless the under eye moved to the upper side it would be of no use to the flatfish, and might even be a source of injury. Mivart points out that a sudden, spontaneous transformation in the position of eye is hardly conceivable, and to this Darwin, of course, assents. Mivart adds: “If the transit was gradual, then how such transit of one eye a minute fraction of the journey towards the other side of the head could benefit the individual is, indeed, far from clear. It seems even that such an incipient transformation must rather have been injurious.” Darwin’s reply is characteristic:—
“We thus see that the first stages of the transit of the eye from one side of the head to the other, which Mr. Mivart considers would be injurious, may be attributed to the habit, no doubt beneficial to the individual and to the species, of endeavoring to look upwards with both eyes, whilst resting on one side at the bottom. We may also attribute to the inherited effects of use the fact of the mouth in several kinds of flatfish being bent towards the lower surface, with the jaw-bones stronger and more effective on this, the eyeless side of the head, than on the other side, for the sake, as Dr. Traquair supposes, of feeding with ease on the ground. Disuse, on the other hand, will account for the less developed condition of the whole inferior half of the body, including the lateral fins; though Yarrell thinks that the reduced size of these fins is advantageous to the fish, as ‘there is so much less room for their action, than with the larger fins above.’ Perhaps the lesser number of teeth in the proportion of four to seven in the upper halves of the two jaws of the plaice, to twenty-five to thirty in the lower halves, may likewise be accounted for by disuse. From the colorless state of the ventral surface of most fishes and of many other animals, we may reasonably suppose that the absence of color in flatfish on the side, whether it be the right or left, which is undermost, is due to the exclusion of light.”
By falling back on the theory of inheritance of acquired characters Darwin tacitly admits the incompetence of natural selection to explain the evolution of the flatfish. If the latter theory prove incorrect, it must then be admitted that the evolution of the flatfishes cannot be accounted for by either of the two main theories on which Darwin relies.
Mivart further points out that the beginning stages of the mammary glands cannot be explained by Darwin’s theory. To which Darwin replies, that an American naturalist, Mr. Lockwood, believes from what he has seen of the development of the young of the pipe-fish (Hippocampus) that “they are nourished by a secretion from the cutaneous glands of the sac” in which the young are enclosed. This can scarcely be said to be a satisfactory reply; for, if it is true that this is the case for the pipe-fish,—and I cannot find on inquiry that this statement has been confirmed,—it is still rather speculative to suppose that the ancestral mammals nourished their young by secreting a fluid into the marsupial sac around the embryos.
Darwin deals with instincts of animals in the same way as he deals with their structures. After pointing out that instincts are variable, and that the variations are hereditary, he proceeds to show how selection may act by picking out those individuals possessing the more favorable instincts. In other words, the theory of natural selection is applied to functions, as well as to structure. Darwin makes use here also of the Lamarckian factor of inheritance, and concludes that “in most cases habit and selection have probably both occurred.”
A few examples will sufficiently serve to illustrate Darwin’s meaning. The first case given is that of the cuckoo, which lays its eggs in the nests of other birds, where they are hatched and the young reared by their foster-parents. The starting-point for such a perversion of the ordinary habits of birds is to be found, he thinks, in the occasional deposition of eggs in the nests of other birds, which has at times been observed for a number of species. For instance, this has been seen in the American cuckoo, which ordinarily builds a nest of its own. It is recorded and believed to be true that the young English cuckoo, when only two or three days old, ejects from the nest the offspring of its foster-parents, and this “strange and odious instinct” is supposed by Darwin to have been acquired in order that the young cuckoo might get more food, and that the young bird has acquired during successive generations the strength and structure necessary for the work of ejection. This is of course largely speculative, and it is by no means obvious that it was a greater benefit to the cuckoo to have other birds rear its young than to do so itself. We can equally well imagine, since this is the turn the argument takes, that the occasional instinct to deposit eggs in the nests of other birds would be disadvantageous, and could not have been acquired by the selection of a fluctuating instinct of this sort. We have no right to assume, that because a new habit has been acquired, that it is a more advantageous one than the one that has been lost. All that we can legitimately infer is, that, although the normal instinct has been changed into another, the race has still been able to remain in existence. The same conclusion applies to the case of Molothrus bonariensis, cited by Darwin, and is here even more obvious:—
“Some species of Molothrus, a widely distinct genus of American birds, allied to our starlings, have parasitic habits like those of the cuckoo; and the species present an interesting gradation in the perfection of their instincts. The sexes of Molothrus badius are stated by an excellent observer, Mr. Hudson, sometimes to live promiscuously together in flocks, and sometimes to pair. They either build a nest of their own, or seize on one belonging to some other bird, occasionally throwing out the nestlings of the stranger. They either lay their eggs in the nest thus appropriated, or oddly enough build one for themselves on the top of it. They usually sit on their own eggs and rear their own young; but Mr. Hudson says it is probable that they are occasionally parasitic, for he has seen the young of this species following old birds of a distinct kind and clamoring to be fed by them. The parasitic habits of another species of Molothrus, the M. bonariensis, are much more highly developed than those of the last, but are still far from perfect. This bird, as far as is known, invariably lays its eggs in the nest of strangers; but it is remarkable that several together sometimes commence to build an irregular untidy nest of their own, placed in singularly ill-adapted situations, as on the leaves of a large thistle. They never, however, as far as Mr. Hudson has ascertained, complete a nest for themselves. They often lay so many eggs—from fifteen to twenty—in the same foster-nest, that few or none can possibly be hatched. They have, moreover, the extraordinary habit of pecking holes in the eggs, whether of their own species or of their foster-parents, which they find in the appropriated nests. They drop also many eggs on the bare ground, which are thus wasted.”
Can we possibly be expected to believe that it has been to the advantage of this species to give up its original regular method of incubating its own eggs, and acquire such a haphazard, new method? Does not the explanation prove too much, rather than give support to Darwin’s hypothesis? Is it not better to conclude, that despite the disadvantages entailed by a change in the original instincts, the species is still able to remain in existence?
Darwin points out, in the case of the slave-making ants, that the slave-making instinct may have arisen in the first instance by ants carrying pupæ, that they have captured, into their own nests. Later this habit might become fixed, and, finally, after passing through several stages of development, the ants might become absolutely dependent on their slaves. It is also supposed that those colonies in which this instinct was better developed would survive in competition with other colonies of the same species on account of the supposed advantage of owning slaves. In this way natural selection steps in and perfects the process.
It is far from proven, or even made probable, that a species of ant that becomes gradually dependent on its slaves is more likely to survive than other colonies that are not so dependent. All we can be certain of is that with slaves they have still been able to maintain their own. Moreover, we must not forget that it is not enough to show that a particular habit might be useful to a species, but it should also be shown that it is of sufficient importance, at every stage of its evolution, to give a decisive advantage in the “struggle for existence.” For unless a life and death struggle takes place between the different colonies, natural selection is powerless to bring about its supposed results. And who will be bold enough to affirm that the presence of slaves in a nest will give victory to that colony in competition with its neighbors? Has the history of mankind taught us that the slave-making countries have exterminated the countries without slaves? Is the question so simple as this? May not the degeneration of the masters more than compensate for the acquirement of slaves, and may not the loss of life in obtaining slaves more than counterbalance the advantage of the slaves after they are captured? In the face of these possibilities it is not surprising to find that Darwin, when summing up the chapter, makes the following admission: “I do not pretend that the facts in this chapter strengthen in any degree my theory; but none of the cases of difficulty, to the best of my judgment, annihilate it.” Darwin, with his usual frankness, adds:—
“No doubt many instincts of very difficult explanation could be opposed to the theory of natural selection,—cases, in which we cannot see how an instinct could have originated; cases, in which no intermediate gradations are known to exist; cases of instincts of such trifling importance, that they could hardly have been acted on by natural selection; cases of instincts almost identically the same in animals so remote in the scale of nature, that we cannot account for their similarity by inheritance from a common progenitor, and consequently must believe that they were independently acquired through natural selection. I will not here enter on these several cases, but will confine myself to one special difficulty, which at first appeared to me insuperable, and actually fatal to the whole theory. I allude to the neuters or sterile females in insect communities; for these neuters often differ widely in instinct and in structure from both the males and fertile females, and yet, from being sterile, they cannot propagate their kind.
“The subject well deserves to be discussed at great length, but I will here take only a single case, that of working or sterile ants. How the workers have been rendered sterile is a difficulty; but not much greater than that of any other striking modification of structure; for it can be shown that some insects and other articulate animals in a state of nature occasionally become sterile; and if such insects had been social, and it had been profitable to the community that a number should have been annually born capable of work, but incapable of procreation, I can see no especial difficulty in this having been effected through natural selection. But I must pass over this preliminary difficulty. The great difficulty lies in the working ants differing widely from both the males and the fertile females in structure, as in the shape of the thorax, and in being destitute of wings and sometimes of eyes, and in instinct. As far as instinct alone is concerned, the wonderful difference in this respect between the workers and the perfect females, would have been better exemplified by the hive-bee. If a working ant or other neuter insect had been an ordinary animal, I should have unhesitatingly assumed that all its characters had been slowly acquired through natural selection; namely, by individuals having been born with slight profitable modifications, which were inherited by the offspring; and that these again varied and again were selected, and so onwards. But with the working ant we have an insect differing greatly from its parents, yet absolutely sterile; so that it could never have transmitted successively acquired modifications of structure or instinct to its progeny. It may well be asked, how is it possible to reconcile this case with the theory of natural selection?”
Darwin’s answer is that the differences of structure are correlated with certain ages and with the two sexes, but this is obviously only shifting the difficulty, not meeting it. He concludes, “I can see no great difficulty in any character becoming correlated with the sterile condition of certain members of the insect communities, the difficulty lies in understanding how such correlated modifications of structure could have been slowly accumulated by natural selection.” “This difficulty, though appearing insuperable, is lessened, or, as I believe, disappears, when it is remembered that selection may be applied to the family, as well as to the individual, and may thus give the desired end.”
Darwin did not fail to see that there is a further difficulty even greater than the one just mentioned. He says: “But we have not as yet touched on the acme of the difficulty; namely, the fact that the neuters of several ants differ, not only from the fertile females and males, but from each other, sometimes to an almost incredible degree, and are thus divided into two or even three castes. The castes, moreover, do not commonly graduate into each other, but are perfectly well defined; being as distinct from each other as are any two species of the same genus, or rather as any two genera of the same family. Thus in Eciton, there are working and soldier neuters, with jaws and instincts extraordinarily different: in Cryptocerus, the workers of one caste alone carry a wonderful sort of shield on their heads, the use of which is quite unknown: in the Mexican Myrmecocystus, the workers of one caste never leave the nest; they are fed by the workers of another caste, and they have an enormously developed abdomen which secretes a sort of honey, supplying the place of that excreted by the aphides, or the domestic cattle as they may be called, which our European ants guard and imprison.”
“It will indeed be thought that I have an overweening confidence in the principle of natural selection, when I do not admit that such wonderful and well-established facts at once annihilate the theory. In the simpler case of neuter insects all of one caste, which, as I believe, have been rendered different from the fertile males and females through natural selection, we may conclude from the analogy of ordinary variations, that the successive, slight, profitable modifications did not first arise in all the neuters in the same nest, but in some few alone; and that by the survival of the communities with females which produced most neuters having the advantageous modification, all the neuters ultimately came to be thus characterized. According to this view we ought occasionally to find in the same nest neuter insects, presenting gradations of structure; and this we do find, even not rarely, considering how few neuter insects out of Europe have been carefully examined.”
From this the conclusion is reached:—
“With these facts before me, I believe that natural selection, by acting on the fertile ants or parents, could form a species which should regularly produce neuters, all of large size with one form of jaw, or all of small size with widely different jaws; or lastly, and this is the greatest difficulty, one set of workers of one size and structure, and simultaneously another set of workers of a different size and structure;— a graduated series having first been formed, as in the case of the driver ant, and then the extreme forms having been produced in greater and greater numbers, through the survival of the parents which generated them, until none with an intermediate structure were produced.
“I have now explained how, as I believe, the wonderful fact of two distinctly defined castes of sterile workers existing in the same nest, both widely different from each other and from their parents, has originated. We can see how useful their production may have been to a social community of ants, on the same principle that the division of labor is useful to civilized man. Ants, however, work by inherited instincts and by inherited organs or tools, whilst man works by acquired knowledge and manufactured instruments. But I must confess, that, with all my faith in natural selection, I should never have anticipated that this principle could have been efficient in so high a degree, had not the case of these neuter insects led me to this conclusion. I have, therefore, discussed this case, at some little but wholly insufficient length, in order to show the power of natural selection, and likewise because this is by far the most serious special difficulty which my theory has encountered. The case, also, is very interesting, as it proves that with animals, as with plants, any amount of modification may be effected by the accumulation of numerous, slight, spontaneous variations, which are in any way profitable, without exercise or habit having been brought into play. For peculiar habits confined to the workers or sterile females, however long they might be followed, could not possibly affect the males and fertile females, which alone leave descendants. I am surprised that no one has hitherto advanced this demonstrative case of neuter insects, against the well-known doctrine of inherited habit, as advanced by Lamarck.”
We may dissent at once from Darwin’s statement which, he thinks, “proves that any amount of modification may be affected by the accumulation of numerous slight variations which are in any way profitable without exercise or habit having been brought into play”; we may dissent if for no other reason than that this begs the whole point at issue, and is not proven. It does not follow because in some colonies all intermediate stages of neuters exist, that in other colonies, where no such intermediate stages are present, these have been slowly weeded out by natural selection, causing to disappear all colonies slightly below the mark. It is this that begs the question. Because we can imagine that intermediate stages between the different castes may have been present, it neither follows that such fluctuating variations have been the basis for the evolution of the more sharply defined types, nor that the imagined advantage of such a change would have led through competition to the extermination of the other colonies. However much we may admire the skill with which Darwin tried to meet this difficulty, let us not put down the results to the good of the theory, but rather repeat once more Darwin’s own words at the end of this chapter, to the effect that the facts do not strengthen the theory.