Each ontogeny (individual) begins in a minute germ cell, in which a small quantity of idioplasm is contained. In the cell divisions, by which the organism grows, the idioplasm divides into as many parts as there are single cells, while it continually increases in quantity in a corresponding degree. The ontogenetic increase of the idioplasm takes place by length growth of the strands--that is, by intercalation of micellae in each row of cells of the strands, which thereby grow in length without changing the configuration of the cross section.[C] Accordingly, each strand of idioplasm contains all the determinants that the particular individual has inherited in the germ cell, and each cell of the organism is idioplasmatically qualified to become the germ cell of a new individual. Whether this qualification may be realized depends upon the nature of the soma-plasm. In the lower plants this power belongs to each individual cell; in the higher plants many cells have lost it; in the animal kingdom it is possessed in general only by cells normally set apart as asexual or sexual reproductive cells. [C] Hence, according to Naegeli, every cell of the organism has idioplasm of identical structure. This at once suggests the objection, how can the idioplasm, for instance, of a pollen grain be the same as that of a leaf? Identical idioplasms should always produce identical structures. Naegeli attempts to explain this difficulty by attributing the different results to different "conditions of tension and movement," i.e., a dynamical difference between the idioplasms of the different parts of the organism. (_Abstammungslehre_, p. 53.) This idea of differences of structure being due to dynamic rather than to material causes plays a considerable part in Naegeli's theory, but is the point on which he speaks with least certainty--in fact with a noticeable hesitation. He does not clearly explain the phrase "conditions of tension and movement," nor does he give a convincing explanation of the known phenomena as results of the action of dynamic influence. Naegeli is not the only one who posits dynamic rather than material differences as to the basis of diversities of structure. More recently, Cope has built up a system of evolution founded largely on this idea.--_Trans._