The appearance of =sexual= reproduction in the history of the evolution of the organic world is an especially instructive example of the great importance of differentiation and variation as the most effective principle of evolution in general. The lowliest forms of life reproduce their kind in an extremely simple manner by a process of asexual cell division, which has not improperly been regarded as nothing more than a peculiar form of =growth=; and this simple process of cell division is retained as a mode of growth also in the higher organisms which reproduce their kind by sexual union. In some cases of simple cell division the secondary cell, the “daughter cell,” separates itself from the old cell, the “mother cell,” and forms a new complete individual; in other cases the cell division occurs as gemmiparous reproduction (budding or pululation), the daughter cell remaining united with the mother cell, so that a new organ is built up. Reproduction by cell division is found in many plants and lower animals side by side with sexual reproduction. This latter becomes the exclusive method of production in higher animals and in the human species, whose capacity for the procreation of new individuals by cell division, and for the replacement of lost organs by growth, has been lost. Thus, the progress and the gain which on the one hand are derived from the process of sexual reproduction, whose character we are about to investigate more closely, are balanced on the other hand by a loss. We shall often encounter this fact again in the history of the evolution of the sexual impulse, more especially in mankind and in relation to human love.