Page:Popular Science Monthly Volume 9.djvu/439

Rh the course of their development, run through a series of changes of the same order as those which are postulated by the evolution-theory for life in time.

Again, the facts of geographical distribution, as now known, are absolutely incompatible with the hypothesis that existing animals and plants have migrated from a common centre, whether Mount Ararat or any other; and, by demonstrating the similarity of the existing fauna and flora of any locality to that which inhabited the same area in the immediately precedent epoch, have furnished a strong argument in favor of the modifiability of species. Thus, it is not too much to say that the facts of biology known at the present day are all consistent with and in favor of the view of species entertained by Lamarck, while they are unfavorable to, if not incompatible with, those advocated by Cuvier; and that, even if no suggestion has been offered, or could be offered, as to the causes which have led to the gradual evolution of species, the hypothesis that they have arisen by such a process of evolution would be the only one which would have any scientific foundation.

The great service which has been rendered to science by Mr. Darwin, in the "Origin of Species" is that, in the first place, he has marshaled the ascertained facts of biology in such a manner as to render this conclusion irresistible; and, secondly, that he has proved the following proposition: Given, the existence of living matter endowed with variability, the interaction of variation with the conditions of existence must tend to give rise to a differentialism of the living matter into forms having the same morphological relations as are exhibited by the varieties and species which actually exist in Nature.

What is needed for the completion of the theory of the origin of species is, first, definite proof that selective breeding is competent to convert permanent races into physiologically distinct species; and, secondly, the elucidation of the nature of variability. It is conceivable that both the tendency to vary and the directions in which that tendency takes effect are determined by the molecular constitution of a living body, in which case the operation of the changes of external conditions will be indirect, and, so to speak, permissive. It is conceivable, on the other hand, that the tendency to vary is both originated and directed by the influence of external conditions, while it is also conceivable that both variation and the direction which variation takes are partly determined by intrinsic and partly by extrinsic conditions.—The American Cyclopœdia.