The reason of this is not difficult to discover; it lies in the structure of multicellular animals, and in their differentiation according to the principle of division of labour, for since only particular cells are capable of reproduction, that is, of giving rise to the whole, it is in these necessarily that the process of amphimixis has to occur if its significance lies in its effects on the succeeding generations. It is true that in the lowest multicellular organisms, such as the species of _Volvox_, there are, in addition to the sex-cells, other reproductive cells quite similar to the ova, whose development into a new colony takes place without amphimixis, but the higher we ascend in the animal and plant series the rarer are these 'asexual' germ-cells or 'spores,' and in the highest animal types they are entirely absent and reproduction occurs only by means of the 'sex-cells.' I am inclined to look for the cause of this striking phenomenon mainly in the fact that, if amphimixis had to be retained, this was effected with increasingly great difficulty the more highly and complexly differentiated the organisms became, and that more complicated adaptations were therefore necessary in order that the union of the two germ-cells might be rendered possible at all. There is first of all the separation into two kinds of sex-cells, whose far-reaching differentiations and precise adaptations to the most minute conditions we have already discussed; then follow the innumerable adaptations to bring about the meeting of the sex-cells, the arrangements for copulation, and, finally, the instincts which draw the two sexes together, the means of attraction which are employed, whether decorative colours or attractive shapes, stimulating odours or musical notes, in short, all the diverse and intricate arrangements, which seem to be more subtly elaborated the higher the organism stands upon the ladder of life. When we call to mind that sexual differentiations finally go so far that they dominate the whole organism, alike in its external appearance and in its internal nature, its feelings, inclinations, instincts, its will and ability, as well as its structure down to the finest nerve-elements, we can understand that a mode of reproduction which demands such a composite disposition of details, involving a moulding of the whole organism, so to speak, from birth till death, must of necessity remain the only one, and that there was no room for the persistence of any essentially different mode of reproduction with quite different adaptations. Or, to speak metaphorically, the power of adaptation which is innate in the organism so exhausted itself in the establishment of this marvellous amphimixis adjustment that the possibility of any other was totally excluded.