Page:Popular Science Monthly Volume 64.djvu/453

Rh physical barriers in the work of subdividing species, this would mean that evolution sometimes results in segregation, not that segregation results in evolution.

Evolution is a process of change in species; it is the journey of which individual variations are steps. Evolution changes the characters of species, but it does not originate species.

Natural selection may assist geographical and other influences tending to the division of species, but it is not on that account a cause of evolution; it represents the determining aspect of the environment—the factors which influence the direction of the vital motion, but not those which induce the motion. Natural selection may explain differences between two species, but not the becoming different. It is an external incident or influence and not an active principle or agency of organic evolution. Adaptation is possible because there is a vital motion which can be deflected, not because the environment changes the characters of species. The river of evolution flows through the land of environment; the conformation of the valley determines the course of the stream, but the water descends by its own gravity.

In the course of its progress the species explores all the adjacent territory and follows the line of least resistance to the variations it is able to put forth. Changes are necessary to maintain the vitality of the species and also to keep it abreast of its environmental opportunities, and if no adaptive movement can be made it is still unable to remain stationary, but continues to change in characters indifferent to the environment, or even actually detrimental.

The species encounters obstacles and subdivides because it is in motion; the division takes place when variations can no longer spread freely among the individuals of the species, not because the environment introduces new characters.

That species occupy definite regions of distribution has been taken by some to mean that the individuals are similar because they are molded by similar influences, but that this inference is wrong is shown both by the wide diversity of conditions under which some species exist, and by the even wider diversity of form and structure often found among the members of the same species in the same environment. Similarity of conditions may permit plants and animals of different origins to develop similar variations, and to share, finally, the same adaptive characters, but identical conditions do not put an end to individual variations or to evolutionary progress.

By denying that selection has any power to initiate or actuate developmental changes it is not intended to imply that it has not profoundly influenced the course of evolution in many organic groups.