As I have already said in an earlier lecture, Darwin adhered to Lamarck's assumption of the transmission of functional adaptations, and perhaps the easiest way to make clear the theoretical difficulties which stand in the way of such an assumption is to show how Darwin sought to present this principle as theoretically conceivable and possible. Darwin was the first to think out a theory of heredity which was worthy of the name of theory, for it was not merely an idea hastily suggested, but an attempt, though only in outline, at elaborating a definite hypothesis. His theory of 'Pangenesis' assumes that cells give rise to special gemmules which are infinitesimally minute, and of which each cell brings forth countless hosts in the course of its existence. Each of these gemmules can give rise to a cell similar to the one in which it was itself produced, but it cannot do this at all times, but only under definite circumstances, namely, when it reaches 'those cells which precede in order of development' those that it has to give rise to. Darwin calls this the 'elective affinity' of each gemmule for this particular kind of cell. Thus, from the beginning of development there arises in every cell a host of gemmules, each of which virtually represents a specific cell. These gemmules, however, do not remain where they originated, but migrate from their place of origin into the blood-stream, and are carried by it in myriads to all parts of the body. Thus they reach also the ovaries and testes and the germ-cells lying within these, penetrate into them, and there accumulate, so that the germ-cells, in the course of life, come to contain gemmules from all the kinds of cells which have appeared in the organism, and, at the same time, all the variations which any part may have undergone, whether due to external or internal influences, or through use and disuse. In this manner Darwin sought to attribute to the germ-cells the power of giving rise, in the course of their development, to the same variations as the individual had acquired during its lifetime in consequence of external conditions or functional influences. I abstain from analysing the assumptions here made; their improbability and their contradictions to established facts are so great that it is not necessary to emphasize them; the theory shows plainly that it is necessary to have recourse to very improbable assumptions, if an attempt is to be made to find a theoretical basis for the transmission of acquired (somatogenic) characters. Even when Darwin formulated his theory of Pangenesis his assumptions were hardly reconcileable with what was known of cell-multiplication; now they are above all irreconcileable with the fact that the germ-substance never arises anew, but is always derived from the preceding generation--that is, with the continuity of the germ-plasm.