When Darwin first turned the search-light of his genius upon the world of Nature, and under its illumination men were compelled to replace their static views of organic creation by a dynamic representation that made the history of life a connected and, in great part, progressive process from the beginning, attention was mainly concentrated on the fitness of the organism to its environment. The fact of such fitness had long been obvious in differing degrees, but the problem of its causation as a factor in survival was then for the first time philosophically treated in the doctrine of Natural Selection. Nevertheless, it must be admitted that through all the earlier discussions that ranged round these topics the point of view was more or less one-sided. The fitness of the organism to its environment was stressed and stressed again; the question of the fitness of the environment to the organism was seldom raised, or even realized. In some cases, along with views advancedly transmutational, a conception of the environment was maintained that was almost static. The organism, isolated from its environment, was ransacked for its history in the laboratory or made the subject of experiment in order to elucidate its behavior. The conception of the organism and its environment as vitally and reciprocally connected, as a single system undergoing change, had not yet been reached.