I therefore believe that we must accept the hypothesis that, in indirect nuclear division, the formation of unequal halves may take place quite as readily as the formation of equal halves, and that the equality or inequality of the subsequently produced daughter-cells must depend upon that of the nuclei. Thus during ontogeny a gradual transformation of the nuclear substance takes place, necessarily imposed upon it, according to certain laws, by its own nature, and such transformation is accompanied by a gradual change in the character of the cell-bodies. It is true that we cannot gain any detailed knowledge of the nature of these changes in the nuclear substance, but we can very well arrive at certain general conclusions about them. If we may suppose, with Nägeli, that the molecular structure of the germ-idioplasm, or according to our terminology the germ-plasm, becomes more complicated according to the greater complexity of the organism developed from it, then the following conclusions will also be accepted,—that the molecular structure of the nuclear substance is simpler as the differences between the structures arising from it become less; that therefore the nuclear substance of the segmentation-cell of the earth-worm, which potentially contains the whole of the ectoderm, possesses a more complicated molecular structure than that of a single epidermic cell or nerve-cell. These conclusions will be admitted when it is remembered that every detail in the whole organism must be represented in the germ-plasm by its own special and peculiar arrangement of the groups of molecules (the micellae of Nägeli), and that the germ-plasm not only contains the whole of the quantitative and qualitative characters of the species, but also all individual variations as far as these are hereditary: for example the small depression in the centre of the chin noticed in some families. The physical causes of all apparently unimportant hereditary habits or structures, of hereditary talents, and other mental peculiarities, must all be contained in the minute quantity of germ-plasm which is possessed by the nucleus of a germ-cell;—not indeed as the preformed germs of structure (the gemmules of pangenesis), but as variations in its molecular constitution; if this be impossible, such characters could not be inherited. Nägeli has shown in his work, which is so rich in suggestive ideas, that even in so minute a space as the thousandth of a cubic millimetre, such an enormous number (400,000,000) of micellae may be present, that the most diverse and complicated arrangements become possible. It therefore follows that the molecular structure of the germ-plasm in the germ-cells of an individual must be distinguished from that of another individual by certain differences, although these may be but small; and it also follows that the germ-plasm of any species must differ from that of all other species.