1. The essential part of the reproductive plasm, termed idioplasm, since it divides and passes over from generation to generation, in higher as well as in lower organisms, has a continuous or "immortal" existence.[I] [I] Naegeli's idioplasm corresponds in many respects, though by no means in all, to Weismann's germ-plasm. Weismann's idea of continuity or "immortality," which has been so widely noticed, is set forth with equal clearness, though with less emphasis, by Naegeli. 2. During this continuous life the idioplasm goes through a development of its own, just as an individual organism goes through a certain cycle of development during its individual life. This development consists in a constantly increasing complexity of structure and differentiation of function. 3. This development is automatic, resulting from internal forces or movements, (_Vervollkommnungs-bewegungen_). 4. As a result of the increasing complexity of structure in the idioplasm the entire organism, which in each generation rearises therefrom, becomes, from generation to generation, more and more complex with greater and greater differentiation of function. Thus the progression of the idioplasm controls the phylogeny of the race. It marks out the course of evolution. 5. Since, according to Naegeli, new life with new idioplasms, may arise wherever and whenever the necessary conditions combine, the present organic world is not made up from branchings of a single original idioplasm, but each race or group may have its own specific idioplasm; and, since this has its own characteristic structure and its own specific internal perfecting forces, it passes through its own peculiar evolution, carrying with it its own depending race of organisms. The fact that animals and plants at the present time show such various degrees of organization is also accounted for on the last supposition, for those of lowlier organization are merely of more recent origin and have not progressed so far in idioplasmic development. This automatic perfecting principle has been the mark of much criticism. Some have confounded it with the mystical _nisus formativus_, or formative principle of preceding theorists. But, as Weismann remarks, Naegeli's phyletic force is conceived as a thoroughly scientific mechanical principle. Naegeli has simply made application in the organic world of the principle of entropy, as stated in the mechanical theory of heat. Naegeli himself also compares his internal perfecting principle to mechanical inertia. He says, "the force of evolution once started in a given direction, tends to continue in the same direction. This constitutes the law of inertia in the organic world." * * * * *