It is true that it is only among the Vertebrates that we find 'the reproductive apparatus' so highly developed, but even among Molluscs and Arthropods 'sexual' reproduction, that is, reproduction associated with amphimixis, is the prevailing mode. In these, indeed, parthenogenesis does occasionally occur, that is to say, sexually differentiated female germ-cells are, by means of some slight variations in the maturation of the egg, rendered capable of developing without previous amphimixis, but this happens only in quite special cases as an adaptation to quite special circumstances, and can only be regarded as a temporary cessation of the association between reproduction and amphimixis. In some cases it is a moiety of the ova adapted for amphimixis which develop parthenogenetically, as it is the same sexually differentiated animals, true females, which produce both sorts, and this is often true to some extent when the differentiation in the direction of parthenogenesis has advanced further, and the ova have been separated into those requiring fertilization and those which are parthenogenetic (e.g. the winter and the summer eggs of the Daphnidæ). Parthenogenesis is not asexual but unisexual reproduction, a mode of multiplication which shows us that even in highly differentiated animals the apparently indissoluble association between reproduction and amphimixis can be dissolved if circumstances require it. But if amphimixis had to be retained in the higher animal forms--and we have seen reasons why this must be--it could only be effected by means of unicellular germs, for amphimixis is in essence a fusion of nuclei, and this is the reason why 'vegetative' reproduction, so-called, becomes less and less prominent in animals at least, and above the level of the Arthropods disappears almost entirely. Let us now return to the question we asked at the beginning--When and in what form was amphimixis first introduced into the world of organisms? The best way to answer this is by observation. We must turn to the lowest forms which now exhibit it, and see whether it occurs in them in a simpler form, so that we may draw conclusions as to its origin and its primitive significance, for it would be possible, _a priori_, that this was something different from what it is now in the relatively higher organisms, and that a change of function has gradually come about.