need for analysis of scientific conceptions. Under these conditions, while not departing in essentials from the position of Darwin, we are forced to bring forward isolation as one of the separate factors in the origin of species, and the factor on which the great and growing science of animal and plant geography mainly depends.

Nearly a decade after the publication of the Origin of Species, Dr. Moritz Wagner set forth the factor of isolation, and showed in convincing fashion its fundamental relation to the problem of the origin of species.

Wagner showed plainly that in the study of the evolution of any form we need to know where it lived, what it did, how it was bounded, and what was its relation to other forms, geographically as well as morphologically. "For me," he says, "it is the chorology of organisms, the study of all the important phenomena embraced in the geography of animals and plants, which is the surest guide to the knowledge of the real phases in the process of the formation of species."

The work of Wagner, a most necessary supplement to that of Darwin, has never received the attention it deserves. This is due in part to the fact that most of our investigators do not travel. They know little of animal or plant geography at first hand. They have had nothing to do with species as living, varying, reproducing, adapting, and spreading groups of organisms. Another reason lies in Wagner's own attitude of opposi

tion to Darwinism. He substituted separation, "raümliche Sonderung," for Natural Selection itself, and denied the potency of the latter factor. The two became in his philosophy competing, not coöperating, elements, and this threw on isolation the impossible task of accounting for all the phenomena of adaptation. We may not ascribe to Natural Selection the "Allmacht," or limitless power, which some Neo-Darwinians have ascribed to it, but on the other hand, those who reject it as a factor in organic evolution can give no rational explanation of the universality of adaptive organs and adaptive traits; no clue to the most universal characters of organic nature as it is.

Certain writers urge that neither selection nor isolation are factors in evolution, but rather elements in speciation or species-forming, a process defined as something distinct from evolution. Selection and isolation, as obstacles in the stream of life, help to split the on-moving group of organisms into different categories or species; but the impulse of the forward movement is internal, and the changes of evolution proper affect groups as a whole, and are not concerned with splitting them up into species.

This view may be questioned in two ways. It may be untrue as to fact, or it may be a matter of words only. As a matter of fact, we know nothing of evolution in vacuo, of progress in life without relation to environment. All forms of

life, we know, are split up into species, with adaptation to external conditions traceable in every structure. We know of no way in which organisms can become adapted to special conditions except by the progressive failures of those not adaptable. Hence we know of no organism which has escaped or can escape from the influence of selection. In like manner, as the world is covered with physical barriers, no organism can escape the form of evolutionary friction which prevents uniformity in breeding. There must be some degree of " räumliche Sonderung," even in a drop of water.

To admit these facts, and yet to say that selection and isolation are not factors in evolution, would appear to make the matter a mere question of words. If by evolution we mean the theoretical progress of life, in vacuo, the effects solely of forces intrinsic in organisms, then extrinsic forces or extrinsic obstacles are of course not factors in such evolution. If we mean by evolution, the actual life movements of actual organisms, on this actual earth, then forces and obstacles are alike factors in modifying change, and both speciation and adaptation as well necessary parts of the process.

We admit the primary necessity of variation and of heredity, but we can conceive of no case of actual animal or plant in the forming of which selection and isolation have not played each a large and persistent part. Among the factors

everywhere and inevitably connected with the course of descent of any species, variation, heredity, selection, and isolation must appear; the first two innate, part of the definition of organic life, the last two extrinsic, arising from the necessities of environment, and not one of these can find leverage without the presence of each of the others. Isolation as the factor longest overlooked, though to the field naturalist the most conspicuous of the four, must be advanced to the post of honor beside the others, not instead of any of them.

THE CELL IN RELATION TO HERED

ITY AND EVOLUTION

BY

EDMUND B. WILSON

I TRUST that my colleagues in this symposium will not suspect me of any intention disrespectful to them if I speak of my own small contribution to it as the voice of one crying in the wilderness. I do not mean to imply by the Scriptural phrase that the cytologist has to announce the coming of a new gospel of heredity or of evolution. He is, to say the least, as much in need of light as are others. I wish only to suggest the somewhat isolated position of the subject assigned to me, dealing, as it mainly must, with matters with which Darwin's own work was not very directly concerned, and which in their detailed aspects belong mainly to the post-Darwinian period. With the notable exception of the provisional hypothesis of pangenesis Darwin made no systematic attempt to correlate his own conclusions with those towards which cell-research was already tending in his day; and pangenesis was rather a speculative construction than an induction from known cytological facts. Nevertheless my intrusion into this circle may perhaps be justified on two grounds. One is the keen interest in the inter