If the above considerations are well founded it seems permissible to accept the general view according to which sexual reproduction is derived from conjugation. It is necessary to suppose that, in a colony of Protozoa in the course of becoming a Metazoon, the capacity of reproduction by spores became localized in certain definite cells, and although the formation of spores from these cells may have been possible without previous conjugation, yet that conjugation gradually became established as the rule. The differentiation of primitively similar conjugating cells into male and female cells was probably a very early occurrence, since indications of an analogous differentiation, as has already been mentioned, are found in certain existing Protozoa (Monads, Vorticella, etc.). I have attempted to shew in the second chapter that the breaking up of the cell into spores without previous conjugation is perhaps provided against in the extrusion of the so-called 'directive body'. With the differentiation of special germinal cells, to take the place of the whole individual in the act of conjugation, the possibility of each act of conjugation resulting in the production of only a single organism became introduced. Germinal cells can be indefinitely produced, and the reproductive capacity of a single individual is therefore unlimited; while if two whole individuals conjugated and only produced _one_ from the process, the result would be a diminution instead of an increase in the race[6]. [6] In the vegetable kingdom there are numerous types of Thallophytes, which throw a considerable amount of light on the relation between sexual reproduction and conjugation. Subjoined are a few of the more striking cases. In Pandorina at the time of sexual reproduction the cells which constitute a colony divide each into sixteen, and the products of their division are set free. Pairs of them then conjugate and permanently fuse. After a resting stage the protoplasm is set free from its envelope after division into two or four parts. Each of these then divides into sixteen coherent cells and constitutes a new Pandorina colony. In OEdogonium the fertilization is effected by a spermatozoon fusing with an oosphere (ovum). The fertilized oosphere (oospore) then undergoes segmentation like the ovum of an animal; but the segments, instead of uniting to form a single organism, separate from each other, and each of them gives rise to a fresh individual (swarm-spore) which grows into a perfect OEdogonium. In Coleochæte the impregnation and segmentation take place nearly as in OEdogonium, but the segments remain united together, acquire definite cell walls, and form a single embryo. There is in fact in Coleochæte a true sexual reproduction of the ordinary type. (_Vide_ S. H. Vines "On alternation of generation in the Thallophytes." _Journal of Botany_, Nov., 1879.)