There have been few theories which have furnished a target for so much adverse criticism. It has been objected that natural selection cannot create but only destroy; that it cannot account for the beginnings of useful structures; that it cannot explain the development of organs of extreme complexity; that it does not explain how new variations are produced and hence compels us to seek elsewhere for the real causes of evolution in those agencies, whatever they may be, that give rise to variation. Much has been made of the difficulty that natural selection cannot cause the evolution of organs that involve simultaneous and appropriate modifications in a number of coöperating parts in order that each increment of improvement be of selective value. It has been urged that evolution has proceeded along definitely directed paths, and hence we must seek in some form of orthogenesis for the main cause of evolutionary progress. And then it has been claimed that natural selection is merely an unverified hypothesis, that, as Lord Salisbury has said, "No man has ever seen it at work."

These and other objections have afforded material for endless discussions. Accessory hypotheses such as panmixia, germinal selection, physiological selection, the intra-selection of Roux, and the theory of organic selection, have been advanced to meet various difficulties that confronted the theory of natural selection as formulated by Darwin, and these again have contributed further to swell the volume of controversial literature. It is not surprising, therefore, that symptoms of weariness developed in many quarters over the purely dialectical character of much of the writing on evolutionary theory. After the initial enthusiasm over a great conception had passed, students of biological science endeavored to penetrate more deeply into the workings of the forces which brought about the present order of the organic world, and it was found that the problem of the method of evolution was one with which little real headway had

been made. The problem seemed more amenable to the ingenuity of speculative thought than to actual investigation by observation or experiment.

One difficulty lay in bringing rival theories to the test of verification. We may study the method of evolution by observing the actual course of evolution in the organic world and then test our theories of the causes of the phenomena in the light of their conformity with the facts, or we may study the causes actually at work in modifying the characteristics of existing species. The first method involves the study of those fields of inquiry which reveal the actual course of evolutionary changes. But even if we had an exhaustive knowledge of these fields we should still be ignorant of the causes which lay back of the unfolding of life. Knowledge of the way in which evolution has proceeded would be of great service in the search for causes; but however well our theories might account for the course of evolutionary history, we could never be quite sure of their truth until in some way we had brought them to the test of verification.

While the progress of evolutionary theory between 1859 and 1900 failed to realize the possibly over-sanguine hopes of many biologists, it must not be inferred that no important advances had been made. The influence of geographical isolation in the formation of species had been brought out by A. R. Wallace, M. Wagner, L. Gulick, G. J. Romanes, and more recently by D. S. Jordan, C. H. Merriam, J. A. Allen, J. Grinnell, and other American workers on different groups of vertebrate animals. The isolationists had an adequate answer to the objection. based on the swamping effects of intercrossing, which Fleming Jenkin was the first to urge against Darwin's theory of the origin of species. No new variation, according to Jenkin, could establish itself, because it would breed with the parental type and produce an intermediate progeny which would gradually lose itself in the general average of the species. Moritz Wagner, in his well

known essay on "The Darwinian Theory and the Law of Migration," urged that the only way in which a variation may escape the swamping effect of intercrossing is to become isolated, by migration or otherwise, whereby it may develop in its own way, unchecked by interbreeding with the parent species. The large amount of careful work on geographical distribution which was stimulated by the problem of the origin of species has yielded abundant and conclusive evidence of the importance of isolation in the formation of distinct types. D. S. Jordan has formulated the principle that, "Given any species in any region, the nearest related species is not likely to be found in the same region nor in a remote region, but in a neighboring district separated from the first by a barrier of some sort." There is no doubt that this generalization, which J. A. Allen has designated as Jordan's Law, is quite generally exemplified in the distribution of vertebrate animals and in many species of invertebrates. It is fairly well borne out in the distribution of the races of man and in many species of higher plants. But it is no less evident that it meets with striking exceptions. In many cases closely allied forms inhabiting the same area do not interbreed. They may have breeding periods at different times, or live in different local habitats, or be in other ways sexually instead of geographically isolated, so that the broader principle of Moritz Wagner, "Ohne Isolierung keine Arten," may still be claimed to apply to them.

Just how isolation works to promote diversity has been variously interpreted. The controversy as to whether isolation per se is able to occasion divergence, independently of natural selection or the influence of environment, as maintained by Gulick in opposition to Wallace, we can now decide in favor of the former investigator. It is coming to be recognized more and more that the various marks which naturalists employ to distinguish species and varieties have no particular value

to their possessors. Much has been made of this fact by Eimer and others in attacks upon the theory of natural selection as an explanation of the origin of species. Still under the influence of teleological conceptions, Darwin and most of his contemporaries were prone to look upon everything in an organism as of some use, and, armed with a principle by which the origin of structures might be explained because of their use, they regarded specific characters as quite generally of some service to the organism. The selectionist had to recognize, however, a certain element of unfitness in every species. Darwin, who attempted to explain the origin of species by means of natural selection, believed that species were formed by the preservation of relatively minute favorable variations, and that therefore the characteristics by which one species differs from another represent the summation of a large number of successive improvements in the inherited endowments of the stock. He had read Paley's "Natural Theology" with great admiration and was strongly impressed with the wonderful adaptations of organic structure which are so clearly set forth in that able and celebrated work. Darwin was persuaded that he had to account for a condition of extreme perfection and nicety of adjustment in every organism. While abundantly conversant with the occurrence of sudden and well marked variations, he believed that such variations were of minor significance for evolution, because he deemed that, except in rare instances, they would be ill adapted and would tend to die out. As one is not likely to fit a peg neatly into a hole by going at it with an ax, so an organism is not apt to secure a close adaptation to its environment by a series of great and sudden mutations. Of course an organism must possess a considerable adaptiveness to its environment in order to survive. But if, as most biologists now hold, the margin of inutility in organic structures is wider than it was supposed to be by Darwin, it leaves a more open

field for other theories of the formation of species besides the one formulated in Darwin's celebrated work.

Since species-forming has been so frequently associated with isolation, it might be contended that it has taken place because of isolation. And undoubtedly much can be said for this viewpoint. Recent work in genetics has shown how a species containing a large amount of hereditary diversity may be broken up, through isolation, and quite independently of any selective elimination, into a number of different types. Such a segregation of types is simply an incidental product of inbreeding. Isolated groups tend to become homozygous or racially pure for different characters. And where any strongly heterozygous species is scattered into a number of restricted localities, it becomes inevitably broken up by inbreeding into distinct subdivisions.

The problem of the closeness of fit between the species and its environment has an important relation also to the theory of mutation. If specific characters are frequently of little or no biological importance, they may have come about full-fledged by a single variation. Whether species have been made by small steps or relatively large ones is, however, of subordinate importance for the larger problems of progressive evolution and the development of adaptations. Darwin attempted to explain the origin of species-which is essentially divergence and progressive adaptive development by the same method of survival of the fittest, with occasional aid from the transmission of acquired characters. Recent developments of evolutionary theory have tended to treat the formation of species as more or less independent of the progressive transformation of the world of life; and many who, like de Vries, oppose the view that species have arisen by means of natural selection, still hold that natural selection affords an adequate explanation of progressive adaptive development.