First of all, I do not see any greater difficulty in supposing that the “carriers of heredity” proceed centripetally from somatic-cells to germ-cells, than in supposing that they proceed centrifugally from the germ-cells to the somatic-cells which they are engaged in constructing. Nor do I see any more difficulty in imagining these “carriers of heredity” to be capable of constructing a new organism if they have first proceeded centripetally, and are thus severally representative of all parts of the parent organism _after its construction has been completed_, than I do if they have proceeded centrifugally, and are thus similarly representative of all parts of that organism _before its construction has been commenced_[74]. Similarly, it seems to me, whatever cogency there may be in Weismann’s objection to Darwin’s theory on the score that it must assume “an unknown controlling force in order to marshal the molecules,” is equally great as regards his own. True, Weismann has a lot to say about the control which nucleo-plasm can exercise on cell-formation, and germ-plasm on marshalling successive stages of ontogeny; but all that this amounts to is a re-statement of the facts. Such a controlling force must be equally assumed by both theories; but in each alike there is an absence of any ghost of an explanation. Again, whatever difficulty there may be in conceiving the transition of somatic substance, _mutatis mutandis_ there must be an equal difficulty in conceiving the transition of germinal substance into somatic substance. Indeed, as far as I can see, the difficulty is even greater in the latter case than it is in the former. For the very essence of Weismann’s view is that germ-plasm differs from all or any other “plasm” in origin or kind: germ-plasm, and germ-plasm alone, has been immortal, perpetually continuous, capable of indefinite self-multiplication, and so of differentiating itself into an endless number and variety of somatic tissues. But, according to Darwin’s view, there is not, and never has been, any such fundamental difference between the essential nature of somatic elements, and the essential nature of sexual elements. On the contrary, it is supposed that both formative and formed material are one in kind—that all the cellular tissues of a multicellular organism, like the single cell of a unicellular organism, are _per se_ endowed with the vital property of self-multiplication; and that whether this property finds its expression in normal growth, in abnormal increments of growth (such as tumours), in processes of repair, in the various forms of a-sexual reproduction, or in the more specialized form of sexual fertilization, there is everywhere an exhibition of one and the same capacity. Now, without going further than this contrast between the fundamental principles of the two theories, does it not become evident that the difficulty of conceiving a transition of A into A´ is at any rate no