The physiological fact which is most material for a correct appreciation of individual psychogeny is the _continuity_ of the _psyche_ through the rise and fall of generations. A new individual comes into existence at the moment of conception; yet it is not an independent entity, either in respect of its mental or its bodily features, but merely the product of the blending of the two parental factors, the maternal egg-cell and paternal sperm-cell. The cell-souls of these two sexual cells combine in the act of conception for the formation of a new cell-soul, just as truly as the two cell-nuclei, which are the material vehicles of this psychic potential energy, unite to form a new nucleus. As we now see that the individuals of one and the same species--even sisters born of the same parents--always show certain differences, however slight, we must assume that these variations were already present in the chemical plasmatic constitution of the generative cells themselves.[17] These facts alone would suffice to explain the infinite variety of individual features, of soul and of bodily form, that we find in the organic world. As an extreme, but one-sided, consequence of them, there is the theory of Weismann, which considers the _amphimixis_, or the blending of the germ-plasm in sexual generation, to be the universal and the sole cause of individual variability. This exclusive theory, which is connected with his theory of the continuity of the germ-plasm, is, in my opinion, an exaggeration. I am convinced, on the contrary, that the great laws of _progressive heredity_ and of the correlative _functional adaptation_ apply to the soul as well as to the body. The new characteristics which the individual has acquired during life may react to some extent on the molecular texture of the germ-plasm in the egg-cell and sperm-cell, and may thus be transferred to the next generation by heredity in certain conditions (naturally, only in the form of latent energy).