Even long before Darwin had published his Theory of Selection, some naturalists, and especially Goethe, had assumed the interaction of two distinct formative tendencies--a conservative or preserving, and a progressive or changing formative tendency--as the causes of the variety of organic forms. The former was called by Goethe the centripetal or specifying tendency, the latter the centrifugal tendency, or the tendency to metamorphosis (p. 89). These two tendencies completely correspond with the two processes of Inheritance and Adaptation. _Inheritance_ is the _centripetal_ or _internal formative tendency_ which strives to keep the organic form in its species, to form the descendants like the parents, and always to produce identical things from generation to generation. _Adaptation_, on the other hand, which counteracts inheritance, is the _centrifugal_ or _external formative tendency_, which constantly strives to change the organic forms through the influence of the varying agencies of the outer world, to create new forms out of those existing, and entirely to destroy the constancy or permanency of species. Accordingly as Inheritance or Adaptation predominates in the struggle, the specific form either remains constant or changes into a new species. The degree of constancy of form in the different species of animals and plants, which obtains at any moment, is simply the necessary result of the momentary predominance which either of these two formative powers (or physiological activities) has acquired over the other.
If we now return to the consideration of the process of selection or choice, the outlines of which we have already examined, we shall be in a position to see clearly and distinctly that both artificial and natural selection rest solely upon the interaction of these two formative tendencies. If we carefully watch the proceedings of an artificial selector--a farmer or a gardener--we find that only these two constructive forces are used by him for the production of new forms. The whole art of artificial selection rests solely upon a thoughtful and wise application of the laws of Inheritance and Adaptation, and upon their being applied and regulated in an artistic and systematic manner. Here the will of man constitutes the selecting force.
