-The Contemporary Review.

text:

De Rer. Nat., Book i. 54-62.

THE meaning of the term-now become a household word in science-"the origin of species," is often entirely misunderstood. It is very generally thought to mean the origin of life and of living things, and people are surprised and almost incredulous when told that Mr. Darwin himself, in the latest edition of his celebrated work, still refers that origin to divine agency. Such, however, is undoubtedly the case, as shown by the following passage which concludes the volume : There is grandeur in this view of life, with its several powers, having been originally breathed by the Creator into a few forms or into one; and that, while this planet has gone cycling on according to the fixed law of gravity, from so simple a beginning endless forms most beautiful and most wonderful have been, and are being, evolved."

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A species may be defined as a group of individuals of animals or plants which breed together freely and reproduce their like; whence it follows that all the individuals of a species, now living or which have lived, have descended from a few common ancestors, or perhaps from a single pair. Thus all horses, whether Shetland ponies, racers, or carthorses, form one species, because they freely breed together, and are known to have all descended from a common stock.

By the same test the common ass, the kiang, the quagga, and the zebra, are each shown to be distinct species; for though sometimes two of these species will breed together, they do not do so freely, they do not reproduce their like, but an intermediate form called a mule, and these mules are not capable of reproducing their kind, as are the offspring of any pairs of a single species. What Mr. Darwin did was to prove, by an overwhelming array of evidence and a connected chain of irresistible argument,

that, just as all horses and all asses have each descended from a few common ancestors, so have all asses, horses, quaggas, and zebras descended from a much more remote common ancestral form; and that the same thing has occurred with every group of allied species. This is the origin of species" by descent with modification, or, in other words, by evolution; while natural selection" was the term applied to the set of natural causes which formed the motive power and guiding principle by which the change from one species to another was brought about.

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In a very few years after the publication of this theory, it had literally extinguished among all thinking men the doctrine of special creation which had before largely prevailed; and some, who were its most violent opponents at the outset, now accept the fact of evolution as applied to almost every group of organized beings. At the present day there is perhaps no single naturalist of reputation who upholds that doctrine of the independent origin of each species of animal and plant, which was a very few years ago either tacitly accepted or openly maintained by the great majority of naturalists. Surely no such revolution in scientific thought was ever effected by one man in so short a period!

At first the opponents of Darwinism opposed evolution as well; but of late, years the opposition is directed wholly to the adequacy of the causes which Mr. Darwin maintains are sufficient to explain the origin of each species from some pre-existing species, and therefore the origin of all existing species from some one or more ancestral forms. It is maintained that there are other laws at work besides natural selection, and Mr. Darwin has himself admitted that there probably are such. Most of the opponents of Darwinism argue in favor of some guiding or organizing power, either internal or external, as absolutely necessary to the production of the kind and amount of variation necessary for the development of the various complex organs and special adaptations which characterize each important class of animals. Others go still further, and maintain that "natural selection" is powerless to produce new species in any case, its function being to keep those

which are produced in a state of health and perfection by weeding out all that are imperfect; or they argue that, so long as the "cause of variation" is unknown, the power that preserves those variations when they have arisen plays a very subordinate part. These last writers maintain that the causes, whatever they are, which produce certain variations in certain species at certain times, are the true and only causes of the origin of species.

Now all these objections, in so far as they refer to the origin of the different species of one genus from a common ancestral species, or even of all the species and genera of one family from some still more remote ancestor, may, I think, be shown to be invalid; because we have direct evidence, almost amounting to demonstration, that changes to this extent are producible by the known laws of variation and the admitted action of natural selection. But when we go farther back, and propose to account for the origin of distinct families, orders, and classes of animals by the same process, the evidence becomes far less clear and decisive. We find groups with organs of which no rudiment exists in other groups; we find classes differing radically in structure from other classes; and we have no direct evidence that changes of this nature are now in progress, as we have that the lesser changes resulting in new species and new genera are in progress.

Yet the evidence that these deeper and more important changes in the structure of organized beings have taken place by gradual steps through the ordinary processes of generation is overwhelming. The numerous intermediate links that have been discovered both among living and extinct animals, and especially the wonderful community perceptible in the embryological develop: ment of the most diverse living types, force upon us the conclusion that the entire animal and vegetable kingdoms owe the wonderfully diversified forms they now exhibit to one unbroken process of "descent with modification" from a few primeval types. It is indeed generally assumed that if we go so far, we must admit one original type of living organism; but this does not seem necessary. By means of whatever laws we

suppose living things first to have originated, why should not the primeval germs have appeared many times over, and in forms determined or modified by the infinitely varied chemical and physical conditions to be found in the crust of the earth? The identity of ultimate structure and wonderful similarities of development of all organisms may be due to the unity of the laws by which organic life was first produced; the diversity of the great types of animal and vegetable forms may be due to the operation of those laws at different places, acting on different combinations of elements, which are subject to unlike physical conditions.

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The point here insisted upon is, that the origin of all organisms, living and extinct, by descent with modification," is not necessarily the same thing, and is not included in, "the origin of species by means of natural selection." The latter we not only know has occurred, but we can follow the process step by step by means of known facts and known laws; the former, we are almost equally certain, has occurred, but we cannot trace its steps, and there may have been facts and laws involved of which we have no certain knowledge. The terms laws of growth," "laws of development," "laws of inheritance," laws of inheritance," "laws of variation," laws of correlation," direct action of the environment, laws of habit and instinct," with some others, are used to express the action of causes of which we are almost wholly ignorant, as we are of the nature of life itself. Now Mr. Darwin has himself admitted that there are these unknown causes at work, and that "natural selection is the most important but not the exclusive means of modification." There may be some question as to the term "most important," if, as is not improbable, the most radical differences in animals and their most important organs could not have been produced by it alone in the same way as the specific modifications of a genus or family may be produced. This, however, is a fair matter for discussion and research, and will probably continue to be so for many generations; and even if it should be ever proved that higher laws than "natural selection" have brought about the more fundamental divergences of

the animal and vegetable kingdoms, this will not be held to detract in any way from the greatness and the value of Mr. Darwin's work, any more than it will be held to detract from the greatness of Newton, if it should some day be demonstrated that the law of gravitation as expressed by him is not absolutely true, but that (as some physicists now suppose) it should be found to be subject to a higher law for remote stellar distances.

No thoughtful person can contemplate without amazement the phenomena presented by the development of animals. We see the most diverse forms-a mollusk, a frog, and a mammal-arising from apparently identical primitive cells, and progressing for a time by very similar initial changes, but thereafter each pursuing its highly complex and often circuitous course of development, with unerring certainty, by means of laws and forces of which we are totally ignorant. It is surely a not improbable supposition that the unknown power which determines and regulates this marvellous process may also determine the initiation of those more important changes of structure and those developments of new parts and organs which characterize the successive stages of the evolution of animal forms. In so far as Mr. Darwin denies the necessity of any such power, and maintains that the origin of all the diverse forms and types and all the complex structures of the organic world are due to identically the same laws and processes as are adequate to produce the different species of Rubus or of Canis, from some ancestral bramble or dog respectively, his opponents have undoubtedly a case well worthy of being argued out in the courts of science. They should, however, remember that no final judgment has been given or can be given while the evidence on both sides is not only circumstantial but imperfect and contradictory; and it would be well not to declare too confidently that Mr. Darwin's theory has hopelessly broken down, since a majority both of naturalists and geologists, whose evidence as experts will undoubtedly have great weight with the educated public, are at present altogether in his favor.

Leaving this great case to be discussed

and argued in weighty volumes by specialists in science, I here propose to deal briefly with that much smaller but still important question, of the origin of the species of a genus or of a family-that is, of groups of organisms differing, as the wolf, dog, and fox among animals, or as the numerous species of oaks or of primulas among plants; and I hope to be able to show that in these cases there is hardly any room for doubt as to the mode in which the change from species to species has been effected.

We have to inquire, then, how it is that new species arise, supposing the world to have been then very much as it is now; and what becomes of them after they have arisen. In the first place we must remember that new species can only be formed when and where there is room for them. If a continent is well stocked with animals and plants, there is a balance between the different species, those best adapted to the varied existing conditions maintaining themselves in the largest numbers, while others, being only adapted to special conditions that occur in limited areas, are far less numerous; the former are common and widespread, the latter rare or local species. If the set of organisms in any country has existed for a sufficient time to have been subjected to all the varying conditions which occur during considerable cycles of climatal and other changes, the balance will have become well established, and so long as no change takes place in the conditions no new species will arise.

But now let us suppose some change to begin, either of climate or geography. The land may sink or it may be elevated, in the former case diminishing in area and perhaps becoming divided by an arm of the sea, in the latter case increasing in area and perhaps becoming united. with extensive lands formerly separated from it; or the climate may become moister or drier, hotter or colder, more extreme or more equable, and any one of these changes or any combination of them would, it is easy to see, produce a special effect on the forms of life. The vegetation would in almost any case become changed, and this would affect both the insects and the higher animals in a variety of ways. New enemies or new competitors might be admitted, and

these would certainly cause the extermination of some of the rarer species, and perhaps greatly reduce the numbers of those which had been most numerous. Others might, from the same general causes, obtain fresh supplies of food, or have opened to them fresh areas over which to spread themselves.

These are the first and most obvious effects of such changes, but there are others still more important, and not less certain to be produced. We have supposed each of the species which inhabited the country to be well adapted to the conditions of its existence, to be able to obtain food for itself and young, to protect itself against all kinds of enemies, and to be able to resist the ordinary inclemencies of the seasons, and to do all this in competition with the numerous other species by which it was surrounded. But now all these conditions and surroundings are undergoing change, and, in order to become equally well adapted to the new conditions, some of the species will require to undergo a corresponding change, either in structure, habits, color, or some other characters. New enemies may necessitate greater swiftness, or greater cunning, or less conspicuous colors; less abundant food may necessitate some modification in structure better adapted to secure it, or the means of ranging over a wider area to search for it; while a severer climate may necessitate a thicker covering, or more nourishing food, or new kinds of shelter. To bring about these changes, "variation" and the "struggle for existence" come into play. Each year the old and less adapted forms die out, while those variations which are more in harmony with the new conditions constantly survive; and this process, continued for many hundreds or thousands of successive generations, at length results in the production of one or more new species.

We now come to the difficulty which has been repeatedly put forward, and which seems very great to all who have not studied groups of species as they occur in nature, and which is expressed in the question, "How comes it that variations of the right kind and sufficient in amount have always occurred just when they were wanted, so as to form the endless series of new species that

have arisen?" and it is more especially to answer this question that the present paper has been written.

Few persons consider how largely and universally all animals are varying; yet it is certain that if we could examine all the individuals of any common species we should find considerable differences, not only in size and color but in the form and proportions of all the parts and organs of the body. In our domesticated animals we know this to be the case, and it is by means of the continued selection of such slight varieties to breed from that all our extremely varied domestic animals and cultivated plants have been produced. Think of the difference in every limb, in every bone and muscle, and probably in every part, internal and external, of the whole body, between a greyhound and a bulldog! Yet, if we had the whole series of ancestors of these two breeds before us, we should find them gradually converge till they reached the same original type, while between no two successive generations would there be any greater difference than now sometimes occurs in the same litter. It is often thought, however, that wild animals do not vary sufficiently to enable any such change as this to be brought about in the same limited time; and though naturalists are well aware that there is little, if any, difference in this respect between wild and domesticated species, it is only recently that they have been able to adduce positive proof that this is the

case.

We owe this proof to an American naturalist, Mr. J. A. Allen,* who has made an elaborate series of observations and measurements of the mammals, and more especially of the birds, of the United States; and he finds a wonderful and altogether unsuspected amount of variation between individuals of the same species even when inhabiting the same locality. They differ in the general tint, and in the distribution of the colors and markings; in general

*"On the Mammals and Winter Birds of East Florida; with an Examination of certain assumed Specific Characters in Birds, and a Sketch of the Bird Fauna of 'Eastern North America.' By J. A. Allen. (Bulletin of the Museum of Comparative Zoology at Harvard College, Cambridge, Mass., vol. ii. No. 3.

NEW SERIES.-VOL. XXXI., No. 3

size, and in proportions; in the length of the head, feet, wings, and tail; in the length of particular feathers, thus altering the shape of the wing or tail; in the length of the tarsi and of the separate toes; and in the length, width, thickness, and curvature of the bill. These variations are by no means small in amount or requiring very accurate measurements for their detection, since they often reach one seventh, one sixth, or sometimes even one fourth, of the entire average dimensions. Thus, in twelve species of small birds, all taken in the same locality, the variation in twenty-five or thirty specimens of the same sex and age was, in the length of the folded wing, from 14.5 to 21 per cent, and in the length of the tail from 14 to 23.4 per cent. If we take individual cases, we find equally striking facts. Wilson's Thrush (Turdus fuscescens) was found to vary in length of wing from 3.58 to 4.15 inches, and in the tail from 3.55 to 4 inches. In the Bluebird (Sialia sialis) the middle toe varied from 0.77 to 0.91 inch, and the hind toe from 0.58 to 0.72 inch; while the bill varied from 0.45 to 0.56 inch in length, and from 0.30 to 0.38 inch in width. In the Yellow-crowned Warbler (Dendræca coronata) the quills vary in proportionate length, so that the first, the second, the third, or the fourth, is sometimes the longest; and a similar variation of the wing, involving a change of proportion between two or more of the feathers, is recorded in eleven species of birds. Color and marking vary to a still greater extent. The dark streaks on the under parts of the American Song-sparrow (Melospiza melodia) are sometimes reduced to narrow lines, while in other specimens they are so enlarged as to cover the greater part of the breast and sides of the body,. sometimes uniting on the middle of the breast into a nearly continuous patch. In the small spotted Wood-thrushes (of the sub-genus Hylocichla) not only does. the general tint of different parts vary greatly, but this is accompanied by great variation in the markings, some speci-mens being very pale with indistinct narrow lines on the breast, while others. have dark, plumage and dark, broad, triangular markings. It must be re

membered that all these differences are

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