Involuntarily the question arises—how is it that such an acute thinker fails to perceive this contradiction? But the answer is not far to seek, and Nägeli himself indicates it when he adds these words to the sentence quoted above: ‘It follows therefore that if a cell is detached as a germ-cell in any stage of ontogenetic development, and from any part of the organism, such a cell will contain all the hereditary tendencies of the parent individual.’ In other words, if we are restricted to different ‘conditions of tension and movement’ as an explanation, it seems to follow as a matter of course that the idioplasm can re-assume its original condition, and therefore that the idioplasm of any cell in the body can again become the idioplasm of the germ-cell; for this to take place it is only necessary that the greater tension should become the less, or _vice versa_. But if we admit a real change in constitution, then the backward development of the idioplasm of the cells of the body into germ-cells appears to be very far from a matter of course, and he who assumes it must bring forward weighty reasons. Nägeli does not produce such reasons, but considers the metamorphosis of the idioplasm in ontogeny as mere differences in the ‘conditions of tension and movement.’ This phrase covers the weak part of his theory; and I look upon it as a valuable proof that Nägeli has also felt that the phenomena of heredity can only find their explanation in the hypothesis of the continuity of the germ-plasm; for his phrase is only capable of obscuring the question as to how the idioplasm of the cells of the body can be re-transformed into the idioplasm of germ-cells. I am of the opinion that the idioplasm cannot be re-transformed, and I have defended this opinion for some years past[118], although I have hitherto laid especial stress on the positive aspect of the question, viz. on the continuity of the germ-plasm. I have attempted to prove that the germ-cells of an organism derive their essential nature from the fact that the germ-plasm of each generation is carried over into that which succeeds it; and I have tried to show that during the development of an egg into an animal, a part of the germ-substance—although only a minute part—passes over unchanged into the organism which is undergoing development, and that this part represents the basis from which future germ-cells arise. In this way it is to a certain extent possible to conceive how it is that the complex molecular structure of the germ-plasm can be retained unchanged, even in its most minute details, through a long series of generations.