Chapter IV: Darwin’s Theories of Artificial and of Natural Selection (2)

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.

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