Conclusions
In the light of the preceding discussion concerning the evidence in favor of the transmutation theory, we may now proceed to sum up our general conclusions, and at the same time discuss some further possibilities in regard to the descent theory.
The most widely accepted view in regard to the theory of organic evolution is that which looks upon the resemblances between the members of a group as due to their common descent from one original species that has broken up, as it were, into a number of new forms. Strictly applied, this means that all the vertebrates have come from one original species, all the mollusks from another, the echinoderms from a third, etc. Even farther back there may have been a common ancestral species for any two of the large groups, as, for example, the annelids and the mollusks; and if the relationship of all the many-celled forms be looked upon as probable, then they too have originated from one ancestral species.
Many zoologists appear to hesitate to apply strictly this fundamental idea contained in the transmutation theory, because, perhaps, they feel that it does not fit in with their general experience of living forms. Yet there can be no doubt that it is the primary conception of the transmutation theory. This is, however, not the whole question, for we must further consider the number of individuals of a species that are involved.
In some species there are smaller groups of individuals that are more like one another than like other individuals of the same species. Such groups are called varieties, and are often associated with certain localities, or with a special environment. In the latter case they are called local varieties. Some of these appear to breed true, not only when kept under the same conditions, but even when transferred to a new environment. Others change with the environment. It is not improbable that the varieties are of a different kind in these two cases, as shown by their different behavior when put under new and different surroundings. The variety that owes its peculiarities, not to the immediate environment, but to some internal condition independent of the surroundings, is recognized by some biologists as a smaller species. Such species appear to be commoner in plants than in animals, although it is possible that this only means that more cases have been found by the botanists, owing to the greater ease with which plants can be handled. These smaller species, in contradistinction to the ordinary Linnæan species, differ from the latter in the smaller amount of differences between the groups, and probably also in that they freely interbreed, and leave fertile descendants; but whether this is only on account of the smaller differences between them than between larger species, or because of some more fundamental difference in the kind of variation that gives rise to these two kinds of groups, we do not know.
These smaller species, or constant varieties, as we may call them, may be looked upon as incipient Linnæan species, which, by further variations of the same, or of other sorts, may end by giving rise to true species. A genus composed of several species might be formed in this way, and then, if each species again broke up into a number of new groups, each such group would now be recognized as a genus, and the group of genera would form a family, etc. The process continuing, a whole class, or order, or even phylum, might be the result of this process that began in a single species.
But we must look still farther, and inquire whether the start was made from a single individual, that began to vary, or from a number of individuals, or even from all the individuals, of a species. If we suppose the result to depend on some external cause that affects all the individuals of a species alike, then it might appear that the species, or at least as many individuals of a species as are affected, will give the starting-point for the new group. But if the new variation arises not directly as a response to some change in the surroundings, then it might appear in one or in a few individuals at a time. Let us consider what the results might be under these two heads.
If amongst the descendants of a single individual a new form or a number of new forms were to arise, then, if they represented only a variety, they would cross with the other forms like the parent species; and, under these conditions, it is generally assumed that the new variety would be swamped. If, however, the new forms have the value of new species, then, ex hypothese, they are no longer fertile with the original forms, and might perpetuate themselves by self-fertilization, as would be possible in some of the higher plants, and in those animals that are bisexual. But as a rule even bisexual forms are not self-fertilized, and, therefore, unless a number of offspring arose from the same form the chance of propagation would be small.
If, however, a number of new forms appeared at the same time and left a number of descendants, then the probability that the new group might perpetuate itself is greater, and the chance that such a group would arise is in proportion to the number of individuals that varied in the same direction simultaneously. In this case the new species has not come from a single individual or even from a pair of individuals, but from a number of individuals that have varied more or less in the same direction.
This point of view puts the descent theory in a somewhat unforeseen light, for we cannot assume in such a case that the similarities of the members of even the same species are due to direct descent from an original ancestor, because there are supposed to have been a number of ancestors that have all changed in the same direction. The question is further complicated by the fact that the new individuals begin to interbreed, so that their descendants come to have, after a time, the common blood, so to speak, of all the new forms. If with each union there is a blending of the substances of the individuals, there will result in the end a common substance representing the commingled racial germ-plasm.
A new starting-point is then reached, and new species may continue to be formed out of this homogeneous material. Thus, in a sense, we have reached a position which, although it appears at first quite different from the ordinary view, yet, after all, gives us the same standpoint as that assumed by the transmutation theory; for, while the latter assumes that the resemblances of the members of a group are due to descent from the same original form, and often by implication from a single individual, we have here reached the conclusion that it is only a common, commingled germ-plasm that is the common inheritance.
When we examine almost any group of living animals or plants, whether they are low or high in organization, we find that it is composed of a great many different species, and so far as geology gives any answer, we find that this must have been true in the past also. Why, then, do we suppose that all the members of the higher groups have come from a single original species or variety? Why may not all, or many, of the similar species of the lower group have changed into the species of the higher group,—species for species? If this happened, the resemblance of the new species of the group could be accounted for on the supposition that their ancestors were also like one another. The likeness would not be due, then, to a common descent, and it would be false to attempt to explain their likeness as due to a common inheritance. But before going farther, it may be well to inquire to what the resemblances of the individuals of the original species were due; for, if they have come from an older group that has given rise to divergent lines of descent, then we are only removing the explanation one step farther back. If this original group has come from numerous species of a still older group, and this, in turn, from an older one still, then we must go back to the first forms of life that appeared on the globe, and suppose that the individuals of these primitive forms are the originals of the species that we find living to-day. For instance, it is thinkable that each species of vertebrate arose from a single group of the earliest forms of life that appeared on the surface of the earth. If this were the case, there must have been as many different kinds of species of the original group as there are species alive at the present time, and throughout all the past. This view finds no support from our knowledge of fossil remains, and, although it may be admitted that this knowledge is very incomplete, yet, if the process of evolution had taken place as sketched out above, we should expect, at least, to have found some traces of it amongst fossil forms. Since this question is an historical one, we can, at best, only expect to decide which of all the possible suggestions is the more probable.
We conclude, then, that it is more probable that the vertebrates, the mollusks, the insects, the crustaceans, the annelids, the cœlenterates, and the sponges, etc., have come each from a single original species. Their resemblances are due to a common inheritance from a common ancestral species. Even if it be probable that at the time when the group of vertebrates arose from a single species, there were in existence other closely related species, yet we must suppose, if we adhere to our point of view, that these other related species have had nothing to do with the group of vertebrates, but that they have died out. Moreover, we must suppose that each order, each class of vertebrate, has come from a single original species; each family has had a similar origin, as well as each genus, but, of course, at different periods of time. Let us not shrink from carrying this principle to its most extreme point, for, unless the principle is absolutely true, then our much boasted explanation of the resemblances of forms in the same group will be thrown into hopeless confusion.
Let us ask another question in this connection. If a single species gave rise to a group of new species that represented the first vertebrates, they would have formed the first genus; and if the descendants of these diverged again so that new genera were formed, then a group which we should call a family would have been formed.
As the divergence went on, an order would be developed, and then a class, and then a phylum. The common characters possessed by the members of this phylum would have been present in the original species that began to diverge. Hence, we find the definition of the phylum containing only those points that are the features possessed by all of the descendants, and in the same way we should try to construct the definition of each of the subordinate groups. This is the ideal of the principle of classification based on the theory of descent with divergence. If we admit the possibility of the other view that I have mentioned above, or of any other of the numerous possibilities that will readily suggest themselves, then we must be prepared to give up some of the most attractive features of the explanation of resemblance as due to descent.
That all biologists believe strictly in divergent descent, to the exclusion of any other processes, is not the case. And, as I have said before, since we are dealing with an historical question, it would be very unwise, in our present ignorance on many points, to pretend that we have any direct proof of the explanation that we find generally given to account for the resemblances of the species of a group to each other. At most we can claim that it is the simplest point of view, and that most biologists believe it to be also the most probable. It has been suggested that, in some cases, the new forms that arise from two or more species run a parallel course. If the original forms from which they came were very much alike, it would soon be impossible to say what the parentage of a particular form was; that is, to which of the two original forms it belonged. It has also been suggested that even a convergence has at times taken place, so that the descendants of different species have become more alike than the original forms, at least in some one or more respects. This last limitation is the saving clause, for species differ in so many points that, even when they converge in a few, it is unlikely that they will do so in all, and, therefore, the deception may be discovered by the acute observer. One famous paleontologist has gone so far even as to suppose that a species may change its generic characters, so that it goes over bodily into a new genus without losing its specific characters. If such things do occur, then our classifications may well be the laughing-stock of Nature.
CHAPTER IV
DARWIN’S THEORIES OF ARTIFICIAL AND OF NATURAL SELECTION
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.
[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.