Since Darwin's epoch-making researches the scientific world has scarcely been more occupied by any subject than the inquiry into how the immense progress from the primitive cell up to the human organism may be satisfactorily explained. For a time the doctrine of heredity seemed to have furnished the correct answer, until Mr. Fr. Galton and Mr. August Weismann tried to explain the laws of development in a new and peculiar way. Darwin, as is well known, assumed that all the changes brought upon an organism from without are at once despatched to the germ, and through it transmitted to further generations. Some slight modification of this theory was adduced by Mr. W. K. Brooks, who attempted to prove that only those parts of the body that were not in sound condition despatched small particles (gemmules) to the germ-cells. Now these gemmules are but a mere hypothesis, and so the question again arises how it is that the germ-cell can assume all the hereditary tendencies of the whole organism. Here Mr. Weismann tries to show that “the germ-cells are not derived at all, as far as their essential and characteristic substance is concerned, from the body of the individual, but they are derived directly from the parent germ-cell”. Therefore all heredity is brought about by the peculiar substance of the “germ-plasm” alone, which is preserved unchanged—in spite of ordinary changes of the somatic cells—for the formation of the germ-cells of the following generation.