Page:Popular Science Monthly Volume 64.djvu/454

450 Indeed, it may be claimed that the kinetic theory of evolution affords the first concrete explanation of the workings of natural selection. Vital motion not only makes selective influence possible, but it meets the ancient and hitherto fatal objection to the theory of selection, since it shows how characters may originate and develop to the point of utility or harmfulness, where selection can take effect.

The hypothesis of selection as the active principle or causal agency of evolution became illogical and useless as soon as the inheritance of acquired characters was abandoned. The first idea without the second does not account for adaptations. The 'selection' of Nägeli, Weismann and other believers in a 'determining principle' or 'hereditary mechanism' of evolution is a very weak substitute for the Darwinian idea, able only to eliminate the hopelessly unfit, but quite without means of influencing the survivors. The recognition of a continuous and necessary vital motion permits us to understand that the rejection by the environment of a harmful variation encourages adaptation by accelerating the development of any more adaptive variation which may appear.

All organisms are subject to selective influence in the sense that variations are rejected with a promptness proportional to their harmfulness in the given environment, but generally this leaves a very wide latitude of possible changes in which selection does not interfere. The instances are relatively rare in which existence becomes acutely dependent upon the development of some one characteristic or quality, and such narrow selection does not strengthen the type, but insures and even hastens its extinction.

The traditional illustration of organic descent by a tree with ever-dividing branches is entirely misleading as a suggestion of the nature of evolutionary processes, because individuals do not follow each other in simple series. Successive generations are connected by endless intergraftings of the lines of descent. A species may be treated systematically or statistically as an aggregation of individuals, and may be described by an averaging of the characters of these; but from an evolutionary point of view it does not exist as a species because of the possession of a certain complex of characters, but because the component individuals breed together; through this alone is the integrity or coherence of the species maintained. For evolutionary purposes we may think of the same species existing thousands of years hence,