Chapter IV: Darwin’s Theories of Artificial and of Natural Selection (1)

The Principle of Selection

Darwin’s theory of natural selection is preëminently a theory of adaptation. It appears, in fact, better suited to explain this phenomenon than that of the “origin of species.” Darwin prepared his reader for the ideas contained in the theory of natural selection by a brief consideration of the results of artificial selection; and since the key to the situation is, I believe, to be found in just this supposed resemblance, we cannot do better than examine the theories in the order followed by Darwin himself.

One of the means by which the artificial races of animals and plants have been formed by man is selection. The breeder picks out individuals having a certain peculiarity, and allows them to breed together. He hopes to find among the offspring, not only individuals like the parent forms, but also some that have the special peculiarity even more strongly developed. If such are found, they are isolated and allowed to breed, and in the next generation it is hoped to find one or more new individuals that show still more developed the special character that is sought. This process, repeated through a number of generations, is supposed to have led to the formation of many of our various forms of domesticated animals and plants.

This heaping up as a result of the union of similar individuals cannot for a moment be supposed to be the outcome of the addition of the two variations to each other. Such an idea is counter to all the most familiar facts of inheritance. For instance, when two similar forms unite, we do not find that the young show all the characters of the mother plus all those of the father, _i.e._ each peculiarity that is the same in both, increased twofold. On the contrary, the young are in the vast majority of cases not essentially different from either parent.

A more thorough examination of the facts shows that the problem is by no means so simple as the preceding general statement might lead one to suppose, for our experience shows that it is not always possible to increase all variations by selection, and, furthermore, there is very soon found a limit, even in favorable cases, to the extent to which the process can be carried. The most important point appears to be the nature of the variations themselves which may arise from different causes, and which have different values in relation to the possibility of their continuation.

We may begin, therefore, by following Darwin in his analysis of variation, as given in the opening chapter of the “Origin of Species.” He thinks that the great amount of variation shown by domesticated animals and plants is due, in the first place, to the new conditions of life to which they are exposed, and also to the lack of uniformity of these conditions. Darwin thinks, also, that there is some probability that this variability is due, in part, to an excess of food. “It seems clear that organic beings must be exposed during several generations to new conditions to cause any great amount of variation, and that when the organization has once begun to vary, it generally continues varying for many generations. No case is on record of a variable organism ceasing to vary under cultivation. Our oldest cultivated plants, such as wheat, still yield new varieties; our oldest domesticated animals are still capable of rapid improvement or modification.” In this statement of Darwin, full of significance, we must be careful to notice that he does not mean to imply, when he states that an organism that has once begun to vary continues to vary for many generations, that this continuous variation is always in the same direction, but only that new combinations, scattering in all directions, continue to appear.

The nature of the organism seemed to Darwin to be a more important factor in the origin of new variations than the external conditions, “for nearly similar variations sometimes arise under, as far as we can judge, dissimilar conditions; and, on the other hand, dissimilar variations arise under conditions which appear to be nearly uniform.” The following statement is important in connection with the origin of “definite” variations. “Each of the endless variations which we see in the plumage of our fowls must have had some efficient cause; and if the same causes were to act uniformly during a long series of generations on many individuals, all probably would be modified in the same direction.” Here we find an explicit statement in regard to the accumulation of variation in a given direction as the result of an external agent, but Darwin hastens to add: “Indefinite variability is a much more common result of changed conditions than definite variability, and has probably played a more important part in the formation of our domestic races. We see indefinite variability in the endless slight peculiarities which distinguish the individuals of the same species, and which cannot be accounted for by inheritance from either parent or from some more remote ancestor. Even strongly marked differences occasionally appear in the young of the same litter, and in seedlings from the same seed capsule. At long intervals of time, out of millions of individuals reared in the same country and fed on nearly the same food, deviations of structure so strongly pronounced as to deserve to be called monstrosities arise; but monstrosities cannot be separated by any distinct line from slighter variations.”

Another cause of variation, Darwin believes, is in the inherited effect of “habit and of the use and disuse of parts,” or what is generally known as the Lamarckian factor of heredity. Darwin believes that changes in the body of the parent, that are the result of the use or of the disuse of a part, may be transmitted to the descendants, and cites a number of cases which he credits to this process. As we shall deal more fully with this topic in another chapter, we may treat it here quite briefly. As an example of the inheritance of disuse, Darwin gives the following case: “I find in the domestic duck that the bones of the wing weigh less and the bones of the leg more in proportion to the whole skeleton than do the same bones in the wild duck, and this change may be safely attributed to the domestic duck flying much less and walking more than its wild parents.” The great and inherited development of the udders of cows and of goats in countries where they are habitually milked, in comparison with these organs in other countries, is given as another instance of the effect of use. “Not one of our domestic animals can be named that in some country has not drooping ears, and the view has been suggested that the drooping is due to the disuse of the muscles of the ears from the animals being seldom much alarmed.”

It need scarcely be pointed out here, that, in the first case given, those ducks would have been most likely to remain in confinement that had less well-developed wings, and hence at the start artificial selection may have served to bring about the result. The great development of the udders of cows and of goats is obviously connected with the greater milk-giving qualities of these animals, which may have been selected for this purpose.

Another “law” of variation recognized by Darwin is what is called correlated variation. For example, it has been found that cats which are entirely white and have blue eyes are generally deaf, and this is stated to be confined to the males. The teeth of hairless dogs are imperfect; pigeons with feathered feet have skin between the outer toes, and those with short beaks have small feet, and _vice versa_.

Another source of variation is that of reversion, or the reappearance in the offspring of characters once possessed by the ancestors. Finally, Darwin thinks that a source of variation is to be found in modifications due to the influence of a previous union with another male, or, as it is generally called, telegony. As an example Darwin cites the famous case of Lord Morton’s mare. “A nearly purely bred Arabian chestnut mare bore a hybrid to a quagga. She subsequently produced two colts by a black Arabian horse. These colts were partially dun-colored and were striped on the legs more plainly than the real hybrid or even than the quagga.”[13] This case, however, is not above suspicion, since it is well known that stripes often appear on young horses, and the careful analysis made later by Ewart, as well as his other experiments on the possibility of the transmission of influences of this sort, puts the whole matter in a very dubious light.

These citations show that Darwin recognized quite a number of sources of variation, and, although he freely admits that “our ignorance of the laws of variation is profound,” yet some at least of these sources of variation are very questionable. Be this as it may, it is important to emphasize that Darwin recognized two main sources of variation,—one of which is the indefinite, or fluctuating, variability that appears constantly in domesticated animals and plants, and the other, definite variability, or a change in a definite direction, that can often be traced to the direct action of the environment on the parent or on its reproductive cells. It is the former, _i.e._ the fluctuating variability, that, according to Darwin, has been used by the breeder to produce most of our domestic races. In regard to the other source of variation, the definite kind, we must analyze the facts more closely.

Footnote 13:

“Animals and Plants under Domestication,” Chap. IX.

A definite change in the surroundings might bring about a definite change in the next generation, because the new condition acts either on the developing organism, or on the egg itself from which the individual develops. The distinction may be one of importance, for, if the new condition only effects the developing organism directly, then, when the influence is removed, there should be a return to the former condition; but if the egg itself is affected, so that it is fundamentally changed, then the effect might persist even if the animal were returned to its former environment. More important still is Darwin’s recognition of the cumulative effect in a given direction of external influences, for a new variation, that was slight at first, might, through prolonged action, continue to become more developed without any other processes affecting the organism.

From the Darwinian point of view, however, the all-important source for the origin of new forms is the fluctuating variation, which is made use of both in the process of artificial and of natural selection. We may now proceed to inquire how this is supposed to take place.

It has been stated that, by means of artificial selection, Darwin believes the breeder has produced the greater number of domesticated animals and plants. The most important question is what sort of variations he has made use of in order to produce his result. Has he made use of the fluctuating variations, or of the definite ones? It is difficult, if not impossible, to answer this question in most cases, because the breeder does not always distinguish between the two. There can be little question, however, that he may sometimes have made use of the definite kinds, whether these are the outcome of external or of internal influences. The question has been seriously raised only in recent years, and we are still uncertain how far we can accumulate and fix a variation that is of the fluctuating kind. In a few cases it has been found that the upper limit is soon reached, as shown by De Vries’s experiments with clover, and it is always possible that a definite variation of the right sort may arise at any stage of the process. If this should occur, then a new standard is introduced from which, as from a new base, variations fluctuating in the desired direction may be selected.

This question, before all others, ought to be settled before we begin to speculate further as to what selection is able to accomplish.

Darwin’s theory is often stated in such a general way that it would be applicable to either sort of variation; but if definite variation can go on accumulating without selection, then possibly we could account for evolution without supposing any other process to intervene. Under these circumstances all that could be claimed for selection would be the destruction of those variations incapable of living, or of competing with other forms. Hence the process of selection would have an entirely negative value.

The way in which domesticated animals and plants have originated is explained by Darwin in the following significant passage:—

“Let us now briefly consider the steps by which domestic races have been produced, either from one or from several allied species. Some effect may be attributed to the direct and definite action of the external conditions of life, and some to habit; but he would be a bold man who would account by such agencies for the differences between a dray- and race-horse, a greyhound and bloodhound, a carrier and tumbler pigeon. One of the most remarkable features in our domesticated races is that we see in them adaptation, not indeed to the animal’s or plant’s own good, but to man’s use or fancy. Some variations useful to him have probably arisen suddenly, or by one step; many botanists, for instance, believe that the fuller’s-teasel, with its hooks, which cannot be rivalled by any mechanical contrivance, is only a variety of the wild Dipsacus; and this amount of change may have suddenly arisen in a seedling. So it has probably been with the turnspit dog; and this is known to have been the case with the ancon sheep. But when we compare the dray-horse and race-horse, the dromedary and camel, the various breeds of sheep fitted either for cultivated land or mountain pasture, with the wool of one breed good for one purpose, and that of another breed for another purpose; when we compare the many breeds of dogs, each good for man in different ways; when we compare the game-cock, so pertinacious in battle, with other breeds so little quarrelsome, with ‘everlasting layers’ which never desire to sit, and with the bantam so small and elegant; when we compare the host of agricultural, culinary, orchard, and flower-garden races of plants, most useful to man at different seasons and for different purposes, or so beautiful in his eyes, we must, I think, look further than to mere variability. We cannot suppose that all the breeds were suddenly produced as perfect and as useful as we now see them; indeed, in many cases, we know that this has not been their history. The key is man’s power of accumulative selection: nature gives successive variations; man adds them up in certain directions useful to him. In this sense he may be said to have made for himself useful breeds.”

Darwin also gives the following striking examples, which make probable the view that domestic forms have really been made by man selecting those variations that are useful to him:—

“In regard to plants, there is another means of observing the accumulated effects of selection—namely, by comparing the diversity of flowers in the different varieties of the same species in the flower-garden; the diversity of leaves, pods, or tubers, or whatever part is valued, in the kitchen-garden, in comparison with the flowers of the same varieties; and the diversity of fruit of the same species in the orchard, in comparison with the leaves and flowers of the same set of varieties. See how different the leaves of the cabbage are, and how extremely alike the flowers; how unlike the flowers of the heartsease are, and how alike the leaves; how much the fruit of the different kinds of gooseberries differ in size, color, shape, and hairiness, and yet the flowers present very slight differences. It is not that the varieties which differ largely in some one point do not differ at all in other points; this is hardly ever,—I speak after careful observation,—perhaps never, the case. The law of correlated variation, the importance of which should never be overlooked, will insure some differences; but, as a general rule, it cannot be doubted that the continued selection of slight variations, either in the leaves, the flowers, or the fruit, will produce races differing from each other chiefly in these characters.”

Exception may perhaps be taken to the concluding sentence, for, interesting as the facts here recorded certainly are, it does not necessarily follow that all domestic products have arisen “by the continued selection of slight variations,” however probable the conclusion may appear. Darwin also believes that a process of “unconscious selection” has given even more important “results than methodical selection.” By unconscious selection is meant the outcome of “every one trying to possess and breed from best individual animals.” “Thus a man who intends keeping pointers naturally tries to get as good dogs as he can, and afterwards breeds from his own best dogs, but he has no wish, or expectation of permanently altering the breed. Nevertheless we may infer that this process, continued during centuries, would improve and modify any breed.... There is reason to believe that the King Charles spaniel has been unconsciously modified to a large extent since the time of that monarch.”

The enormous length of time required to produce new species by the selection of fluctuating variations is everywhere admitted by Darwin; nowhere perhaps more strikingly than in the following statement: “If it has taken centuries or thousands of years to improve or modify most of our plants up to their present standard of usefulness to man, we can understand how it is that neither Australia, the Cape of Good Hope, nor any other region inhabited by quite uncivilized man has afforded us a single plant worth culture. It is not that these countries, so rich in species, do not by a strange chance possess the aboriginal stocks of any useful plants, but that the native plants have not been improved by continued selection up to a standard of perfection comparable with that acquired by the plants in countries anciently civilized.”

In reply to this, it may be said that if the selection of fluctuating variations leads to an accumulation in the given direction, it is not apparent why it should take thousands of years to produce a new race, or require such a high degree of skill as Darwin supposes the breeder to possess.

The conditions favorable to artificial selection are, according to Darwin: 1. The possession of a large number of individuals, for in this way the chance of the desired variation appearing is increased. 2. Prevention of intercrossing, such as results when the land is enclosed, so that new forms may be kept apart. 3. Changed conditions, as introducing variability. 4. The intercrossing of aboriginally distinct species. 5. The intercrossing of new breeds, “but the importance of intercrossing has been much exaggerated.” 6. In plants propagation of bud variations by means of cuttings. The chapter concludes with the statement, “Over all these causes of Change, the accumulative action of Selection, whether applied methodically and quickly, or unconsciously and slowly, but more efficiently, seems to have been the predominant Power.”

Variability, Darwin says, is governed by many unknown laws, and the final result is “infinitely complex.” If this is so, we may at least hesitate before we accept the statement that selection of fluctuating variations has been the only principle that has brought about these results. This is a most important point, for, as we shall see, the central question in the theory of natural selection has come to be whether by the accumulation of fluctuating variations a new species could ever be produced. If it be admitted that the evidence from artificial selection is far from convincing, in showing that selection of fluctuating variations could have been the main source, even in the formation of new races, we need not be prejudiced in favor of such a process, when we come to examine the formation of species in nature.

There are still other questions raised in this same chapter that demand serious consideration. Darwin writes as follows:—

“When we look to the hereditary varieties or races of our domestic animals and plants, and compare them with closely allied species, we generally perceive in each domestic race, as already remarked, less uniformity of character than in true species. Domestic races often have a somewhat monstrous character; by which I mean, that, although differing from each other, and from other species of the same genus, in several trifling respects, they often differ in an extreme degree in some one part, both when compared one with another, and more especially when compared with the species under nature to which they are nearest allied. With these exceptions (and with that of the perfect fertility of varieties when crossed,—a subject hereafter to be discussed), domestic races of the same species differ from each other in the same manner as do the closely allied species of the same genus in a state of nature, but the differences in most cases are less in degree. This must be admitted as true, for the domestic races of many animals and plants have been ranked by some competent judges as the descendants of aboriginally distinct species, and by other competent judges as mere varieties. If any well-marked distinction existed between a domestic race and a species, this source of doubt would not so perpetually recur.”

The point here raised in regard to the systematic value of the new forms is the question that first demands our attention. We must exclude all those cases in which several original species have been blended to make a new form, because the results are too complicated to make use of at present. The domesticated races of dogs appear to have had such a multiple origin, the origin of horses is in doubt; but the domesticated pigeons, ducks, rabbits, and fowls are supposed, by Darwin, to have come each from one original wild species. The great variety of the domestic pigeons gives perhaps the most striking illustration of changes that have taken place under domestication; and Darwin lays great stress on the evidence from this source.

It seems probable in this case, (1) that all the different races of pigeons have come from one original species; (2) that the structural differences are in some respects as great as those recognized by systematists as specifically distinct; (3) that the different races breed true to their kind; (4) that the result has been reached mainly by selecting and isolating variations that have appeared under domestication, and that probably some, at least, of these variations were fluctuating ones.

Does not this grant all that Darwin contends for? In one sense, yes; in another, no! The results appear to show that by artificial selection of some kind a group of new forms may be produced that in many respects resemble a natural family, or a genus; but if this is to be interpreted to mean that the result is the same as that by which natural groups have arisen, then I think that there are good reasons for dissenting from such a conclusion. Moreover, we must not grant too readily that the different races of pigeons have arisen by the selection of _fluctuating variations_ alone, for this is not established with any great degree of probability by the evidence.

In regard to the first point we find that one of the most striking differences between species in nature is their infertility, and the infertility of their offspring when intercrossed. This is a very general rule, so far as we know. In regard to the different races of domesticated forms, the most significant fact is that, no matter how different they may be, they are perfectly fertile _inter se_. In this respect, as well as in others, there are important differences between domesticated races and wild species. The further difference, that has been pointed out by a number of writers, should also not pass unnoticed, namely, that the domestic forms differ from each other in the extreme development of some one character, and not in a large number of less conspicuous characters, as is the case in wild species.

These considerations show that, interesting and suggestive as are the facts of artificial selection, they fail to demonstrate the main point for which they are used by Darwin. With the most rigorous attention to the process of artificial selection, new species comparable in all respects to wild ones have not been formed, even in those cases in which the variation has been carried farthest (where the history of the forms is most completely known).

There is another point on which emphasis should be laid. If by selecting the most extreme forms in each generation and breeding from them the standard can be raised, it might appear that we could go on indefinitely in the same direction, and produce, for instance, pigeons with legs five metres long, and with necks of corresponding length. But experience has shown that this cannot be done. As Darwin frequently remarks, the breeder is entirely helpless until the desired variation appears. It seems possible, by selecting the more extreme of the fluctuating variations in each generation, that a higher plane of variation is established, and even that more extreme forms are likely to arise for a few generations; but, even if this is the case, a limit is soon reached beyond which it is impossible to go.

The facts of observation show, that when a new variety appears its descendants are more likely, on the average, to produce proportionately more individuals that show the same variation, and some even that may go still farther in the same direction. If these latter are chosen to be the parents of the next generation, then once more the offspring may show the same advance; but little by little the advance slows down, until before very long it may cease altogether. Unless, then, a new kind of variation appears, or a new standard of variation develops of a different kind, the result of selection of fluctuating variations has reached its limit. Our experience seems, therefore, to teach us that selection of fluctuating variations leads us to only a certain point, and then stops in this direction. We get no evidence from the facts in favor of the view that the process, if carried on for a long time, could ever produce such great changes, or the kind of changes, as those seen in wild animals and plants.

Variation and Competition in Nature

Darwin rests his theory on the small individual variations which occur in nature, as the following quotation shows:—

“It may be doubted whether sudden and considerable deviations of structure such as we occasionally see in our domestic productions, more especially with plants, are ever permanently propagated in a state of nature. Almost every part of every organic being is so beautifully related to its complex conditions of life that it seems as improbable that any part should have been suddenly produced perfect, as that a complex machine should have been invented by man in a perfect state. Under domestication monstrosities sometimes occur which resemble normal structures in widely different animals. Thus pigs have occasionally been born with a sort of proboscis, and if any wild species of the same genus had naturally possessed a proboscis, it might have been argued that this had appeared as a monstrosity; but I have as yet failed to find, after diligent search, cases of monstrosities resembling normal structures in nearly allied forms, and these alone bear on the question. If monstrous forms of this kind ever do appear in a state of nature and are capable of reproduction (which is not always the case), as they occur rarely and singly, their preservation would depend on unusually favorable circumstances. They would, also, during the first and succeeding generations cross with the ordinary form, and thus their abnormal character would almost inevitably be lost.”

It is clear that Darwin does not think that the sudden and large variations that sometimes occur furnish the basis for natural selection, and the final statement in the last citation (which was added in later editions of the “Origin of Species”), to the effect that if such monstrous variations appeared as single or occasional variations they would be lost by intercrossing implies that, in general, single variations would likewise be lost unless they appeared in a sufficient number of individuals to maintain themselves against the swamping effects of intercrossing.

It is necessary to quote again, in order to show that, in some cases at least, Darwin believed selection plays little or no part in the origin and maintenance of certain peculiarities that are of no use to the species. “There is one point connected with individual differences, which is extremely perplexing: I refer to those genera which have been called protean or ‘polymorphic,’ in which the species present an inordinate amount of variation. With respect to many of these forms, hardly two naturalists agree, whether to rank them as species or as varieties. We may instance Rubus, Rosa, and Hieracium amongst plants, several genera of insects and of Brachiopod shells. In most polymorphic genera some of the species have fixed and definite characters. Genera which are polymorphic in one country seem to be, with a few exceptions, polymorphic in other countries, and likewise, judging from Brachiopod shells, at former periods of time. These facts are very perplexing, for they seem to show that this kind of variability is independent of the conditions of life. I am inclined to suspect that we see, at least in some of these polymorphic genera, variations which are of no service or disservice to the species, and which consequently have not been seized on by selection to act on and accumulate, in the same manner as man accumulates in any given direction individual differences in his domesticated productions. These individual differences generally affect what naturalists consider unimportant parts; but I could show by a long catalogue of facts, that parts which must be called important, whether viewed under a physiological or classificatory point of view, sometimes vary in the individuals of the same species. I am convinced that the most experienced naturalist would be surprised at the number of cases of variability, even in important parts of structure, which he could collect on good authority, as I have collected, during a course of years.”

After pointing out that naturalists have no definite standard to determine whether a group of individuals is a variety or a species, Darwin makes the highly important admissions contained in the following paragraph: “Hence, I look at individual differences, though of small interest to the systematist, as of the highest importance for us, as being the first steps toward such slight varieties as are barely thought worth recording in works on natural history. And I look at varieties which are in any degree more distinct and permanent, as steps toward more strongly marked and permanent varieties; and at the latter, as leading to subspecies, and then to species. The passage from one stage of difference to another may, in many cases, be the simple result of the nature of the organism and of the different physical conditions to which it has long been exposed; but with respect to the more important and adaptive characters, the passage from one stage of difference to another may be safely attributed to the cumulative action of natural selection, hereafter to be explained, and to the effects of the increased use or disuse of parts. A well-marked variety may therefore be called an incipient species; but whether this belief is justifiable must be judged by the weight of the various facts and considerations to be given throughout this work.”

In this paragraph attention should be called especially, first, to the statement in respect to the origin of varieties, which are said to arise through individual differences. It is not clear whether these differences are supposed to have appeared first in one, or in a few individuals, or in large numbers at the same time. Again, especial note should be made of the striking admission, that the passage from one stage to another may, in many cases, be the simple result of the nature of the organism and of the physical conditions surrounding it; but with respect to the more important and adaptive differences, natural selection “may safely” be supposed to have intervened. Is it to be wondered at that Darwin’s critics have sometimes accused him of playing fast and loose with the origin of varieties? And since this question is fundamental for the theory of natural selection, it is much to be regretted that Darwin leaves the matter in such a hazy condition. It may be said that, at the time when he wrote, he made the best of the evidence in regard to the origin of varieties. Be this as it may, a theory standing on no better foundations than this is not likely to be found satisfactory at the present time.

We come now to the most important chapters, the third and the fourth, of the “Origin of Species,” dealing with “the struggle for existence,” “natural selection,” or the “survival of the fittest.” Behind these fatal phrases, which have become almost household words, lurk many dangers for the unwary.

“It has been seen in the last chapter that amongst organic beings in a state of nature there is some individual variability: indeed I am not aware that this has ever been disputed. It is immaterial for us whether a multitude of doubtful forms be called species or subspecies or varieties; what rank, for instance, the two or three hundred doubtful forms of British plants are entitled to hold, if the existence of any well-marked varieties be admitted. But the mere existence of individual variability and of some few well-marked varieties, though necessary as the foundation for the work, helps us but little in understanding how species arise in nature. How have all those exquisite adaptions of one part of the organization to another part, and to the conditions of life, and of one organic being to another being, been perfected? We see these beautiful coadaptions most plainly in the woodpecker and the mistletoe; and only a little less plainly in the humblest parasite which clings to the hairs of a quadruped or feathers of a bird; in the structure of the beetle which dives through the water; in the plumed seed which is wafted by the gentlest breeze; in short, we see beautiful adaptions everywhere and in every part of the organic world.

“Again, it may be asked, how is it that varieties, which I have called incipient species, become ultimately converted into good and distinct species, which in most cases obviously differ from each other far more than do the varieties of the same species? How do those groups of species, which constitute what are called distinct genera, and which differ from each other more than do the species of the same genus, arise? All these results, as we shall more fully see in the next chapter, follow from the struggle for life. Owing to this struggle, variations, however slight and from whatever cause proceeding, if they be in any degree profitable to the individuals of a species, in their infinitely complex relations to other organic beings and to their physical conditions of life, will tend to the preservation of such individuals, and will generally be inherited by the offspring. The offspring, also, will thus have a better chance of surviving, for, of the many individuals of any species which are periodically born, but a small number can survive. I have called this principle, by which each slight variation, if useful, is preserved, by the term Natural Selection, in order to mark its relation to man’s power of selection. But the expression often used by Mr. Herbert Spencer of the Survival of the Fittest is more accurate, and is sometimes equally convenient. We have seen that man by selection can certainly produce great results, and can adapt organic beings to his own uses, through the accumulation of slight but useful variations, given to him by the hand of Nature. But Natural Selection, as we shall hereafter see, is a power incessantly ready for action, and is as immeasurably superior to man’s feeble efforts, as the works of Nature are to those of Art.”

Darwin gives the following explicit statement of the way in which he intends the term “struggle for existence” to be understood: “I should premise that I use this term in a large and metaphorical sense, including dependence of one being on another, and including (which is more important) not only the life of the individual, but success in leaving progeny. Two canine animals, in time of dearth, may be truly said to struggle with each other which shall get food and live. But a plant on the edge of a desert is said to struggle for life against the drought, though more properly it should be said to be dependent on the moisture. A plant which actually produces a thousand seeds of which only one on an average comes to maturity may be more truly said to struggle with the plants of the same and other kinds which already clothe the ground. The mistletoe is dependent on the apple, and a few other trees, but can only in a far-fetched sense be said to struggle with these trees, for if too many of these parasites grow on the same tree, it languishes and dies. But several seedling mistletoes, growing close together on the same branch, may more truly be said to struggle with each other. As the mistletoe is disseminated by birds, its existence depends on them, and it may metaphorically be said to struggle with other fruit-bearing plants, in tempting the birds to devour and thus disseminate its seeds. In these several senses, which pass into each other, I use for convenience’ sake the general term ‘Struggle for Existence.’”

A number of writers have objected to the general and often vague way in which Darwin makes use of this phrase; but it does not seem to me that this is a serious objection, provided we are on our guard as to what the outcome will be in each case. In each instance we must consider the question on its own merits, and if it is found convenient to have a sufficiently general and non-committal term, such as the “struggle for existence,” to include all cases, I see no serious objection to the use of such an expression, although it is true the outcome has been that it has become a catchword, that is used too often by those who have no knowledge of its contents.

Were it not that each animal and plant gives birth, on an average, to more than two offspring, the species would soon become exterminated by accidents, etc. We find in some of the lower animals, and in some of the higher plants, that thousands and even millions of eggs are produced by a single individual in the course of its life. A single nematode may lay sixty million eggs, and a tapeworm one thousand million. A starfish may produce about thirty-nine million eggs, a salmon may contain fifteen thousand, and a large shad as many as one hundred thousand. The queen of a termite nest is said to lay eighty thousand eggs a day.

In the higher vertebrates the number of young is considerably less, but since the young stages are passed within the body of the parent, proportionately more of them reach maturity, so that even in man the population may be doubled in twenty-five years, and in the elephant, slowest breeder of all animals, Darwin has calculated that, if it begins breeding when about thirty years old and goes on until ninety years, bringing forth six young in the interval, after 750 years there will be nearly nineteen million elephants alive which have descended from the first pair.

Obviously, then, if all the descendants of all the individuals of a species were to remain alive, the world would be over-crowded in a very short time, and the want of room would in itself lead to the destruction of countless individuals, if for no other reason than lack of food. We can easily carry out on a small scale an experiment that shows how the overstocking, resulting from favorable conditions, comes about, and how it checks itself. If we make a meat broth suitable for the life of a particular bacterium, and sow in the broth a very few individuals, we find in the course of several days the fluid swarming with the descendants of the original individuals. Thus it has been shown that, if we start with a few hundred bacteria, there will be five thousand after twenty-four hours, and twenty thousand, forty-eight hours later; and after four days they are beyond calculation.

Cohn found that a single bacterium produces two individuals in one hour, and four in two hours, and if they continue to multiply at this rate there will be produced at the end of three days 4,772 billions of descendants. If these are reduced to weight, they would weigh seventy-five hundred tons. Thus when the conditions are favorable, bacteria are able to increase at such an enormous rate that they could cover the surface of the earth in a very few days. The reason that they do not go on increasing at this rate is that they soon exhaust the food supply, and the rate of increase slows down, and will finally cease altogether. If the bacteria were dependent on a continuous supply of food, they would perish after the supply had been exhausted, so that the rapid rate of multiplication would serve only to bring the career of the organism to an untimely end. If the weaker individuals were to die first, the products of their disintegration might serve to nourish the stronger individuals; hunger coming on again, the next weakest might die; and the same process continuing, we might imagine that the bacteria were finally reduced to a single one which would then die in turn for lack of food. Like a starving shipload of men, reduced by hunger to cannibalism, the life of some and finally of the last individual might be prolonged in the hope of rescue, but if this did not arrive, the last and perhaps the strongest individual would perish. But this is not what we find occurring in these lower organisms, for, as a rule, they gradually cease to increase when the food supply becomes lessened, and their activities slow down. Finally, when the food is gone, they pass into a resting stage, in which condition they can remain dormant for a long time, even for years. If they should again find themselves in favorable surroundings, they become active, and begin once more their round of multiplication. We cannot follow the individuals in such a culture of bacteria, but there is nothing to be seen that suggests a struggle for existence, if this idea conveys the impression of the destruction of certain individuals by competition with others. In fact, the results are in some respects exactly the reverse. Millions of individuals are present at the time when the food supply becomes exhausted, and they all pass into a protected resting stage.

The enormous rate of increase in this case finds its counterpart in higher animals when the food supply, or the absence of enemies, allows a species to multiply at its maximum rate of increase. The introduction of rabbits into Australia was followed by an enormous increase in a few years, and the introduction of the English sparrow into the United States has had a similar result. But in no country can such a process continue beyond a certain point, because, in the first place, the scarcity of food will begin to keep the birth-rate down, and in the second place, the increase in numbers may lead to an increase in the number of its enemies, or even induce other forms to feed on it. Crowding will also give an opportunity for the spread of disease, which again may check the increase. Sooner or later a sort of ever shifting balance will be reached for each species, and after this, if the conditions remain the same, the number of individuals will keep approximately constant.

Darwin admits that the “causes which check the natural tendency of each species to increase are most obscure.” “We know not exactly what the checks are even in a single instance.” This admission may well put us on our guard against a too ready acceptation of a theory in which the whole issue turns on just this very point, namely, the nature of the checks to increase. Darwin gives the following general cases to show what some of the checks to increase are. He states that eggs and very young animals and seeds suffer more than the adults; that “the amount of food for each species of course gives the extreme limit to which each can increase; but very frequently it is not the obtaining food, but the serving as prey to other animals which determines the average numbers of a species. Thus, there seems to be little doubt that the stock of partridges, grouse, and hares on any large estate depends largely on the destruction of the vermin.” “On the other hand, in some cases, as with the elephant, none are destroyed by beasts of prey; for even the tiger in India most rarely dares to attack a young elephant protected by its dam.” “Climate plays an important part in determining the average number of a species, and periodical seasons of extreme cold or drought seem to be the most effective of all checks.” “The action of climate seems at first sight to be quite independent of the struggle for existence; but in so far as climate acts in reducing food, it brings on the most severe struggle between the individuals, whether of the same, or of distinct species which subsist on the same kind of food.”

We need not follow Darwin through his account of how complex are the relations of all animals and plants to one another in the struggle for existence, for, if true, it only goes to show more plainly how impossible it is to establish any safe scientific hypothesis, where the conditions are so complex and so impossible to estimate. To show that the young Scotch fir in an enclosed pasture is kept down by the browsing of the cattle, and in other parts of the world, Paraguay for instance, the number of cattle is determined by insects, and that the increase of these flies is _probably_ habitually checked by other insects, leads to a bewilderingly complex set of conditions. We cannot do better than to quote Darwin’s conclusion: “Hence, if certain insectivorous birds were to decrease in Paraguay, the parasitic insects would probably increase; and this would lessen the number of the navel-frequenting flies—then cattle and horses would become feral, and this would certainly greatly alter (as indeed I have observed in parts of South America) the vegetation: this again would largely affect the insects; and this, as we have just seen in Staffordshire, the insectivorous birds, and so onwards in ever increasing circles of complexity. Not that under nature the relations will ever be as simple as this. Battle within battle must be continually recurring with varying success; and yet in the long run the forces are so nicely balanced, that the face of nature remains for long periods of time uniform, though assuredly the merest trifle would give the victory to one organic being over another. Nevertheless, so profound is our ignorance, and so high our presumption, that we marvel when we hear of the extinction of an organic being; and as we do not see the cause, we invoke cataclysms to desolate the world, or invent laws on the duration of the forms of life!”

The effect of the struggle for existence in determining _the distribution of species_ is well illustrated in the following cases:—

“As the species of the same genus usually have, though by no means invariably, much similarity in habits and constitution, and always in structure, the struggle will generally be more severe between them, if they come into competition with each other, than between the species of distinct genera. We see this in the recent extension over parts of the United States of one species of swallow having caused the decrease of another species. The recent increase of the missel-thrush in parts of Scotland has caused the decrease of the song-thrush. How frequently we hear of one species of rat taking the place of another species under the most different climates! In Russia the small Asiatic cockroach has everywhere driven before it its great congener. In Australia the imported hive-bee is rapidly exterminating the small, stingless native bee. One species of charlock has been known to supplant another species; and so in other cases. We can dimly see why the competition should be most severe between allied forms, which fill nearly the same place in the economy of nature; but probably in no one case could we precisely say why one species has been victorious over another in the great battle of life.”

All this goes to show, if it really shows anything at all, that the distribution of a species is determined, in part, by its relation to other animals and plants—a truism that is recognized by every naturalist. The statement has no necessary bearing on the origin of new species through competition, as the incautious reader might infer. Not that I mean in any way to imply that Darwin intended to produce this effect on the reader; but Darwin is not always careful to discriminate as to the full bearing of the interesting illustrations with which his book so richly abounds.

At the end of his treatment of the subject, Darwin emphasizes once more how little we know about the subject of the struggle for existence.

“It is good thus to try in imagination to give to any one species an advantage over another. Probably in no single instance should we know what to do. This ought to convince us of our ignorance on the mutual relations of all organic beings; a conviction as necessary, as it is difficult, to acquire. All that we can do, is to keep steadily in mind that each organic being is striving to increase in a geometrical ratio; that each at some period of its life, during some season of the year, during each generation or at intervals, has to struggle for life and to suffer great destruction. When we reflect on this struggle, we may console ourselves with the full belief, that the war of nature is not incessant, that no fear is felt, that death is generally prompt, and that the vigorous, the healthy, and the happy survive and multiply.”

The kindliness of heart that prompted the concluding sentence may arouse our admiration for the humanity of the writer, but need not, therefore, dull our criticism of his theory. For whether no fear is felt, and whether death is prompt or slow, has no bearing on the question at issue—except as it prepares the gentle reader to accept the dreadful calamity of nature, pictured in this battle for existence, and make more contented with their lot “the vigorous, the healthy, and the happy.”

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.”