During the millions of years that have rolled by since the first appearance of life on the earth, who shall count the number of types which have been exterminated without leaving the faintest trace of their having ever existed? The survivors which have contrived to maintain a place in the sun present an infinite range of variation in colour, size, habit, and structure, as well as in emotions. These varied aspects are all so many facets of the mysterious phenomenon we call Life: and they are so many witnesses of the versatility of Life. Not the least mysterious feature of this Life is its faculty of reproduction, which expresses itself in an infinite variety of ways, defying all but the crudest forms of analysis. The evolution of sex has exercised the speculative ingenuity of some of the acutest students of Nature from the earliest times, and we are still far from a satisfactory solution of the problems it presents. Hermaphroditism and Parthenogenesis are commonly regarded as degenerate forms of reproduction, but it would probably be more correct to see in them exceptional modes of adaptation enabling such individuals to occupy niches in the world untenable to creatures of more conservative habit. That the peculiar “strains” of animal life have turned into backwaters which offer no opportunity or possibility of further advancement seems clear enough, but they are nevertheless interesting and instructive. The parthenogenetic Crustacea and the Rotifers afford some good evidence of this adaptability—of the way in which creatures manage to cling to the skirts of life by reason of their power to survive the extremest tests of endurance. And this success has largely been due to some mysterious property of the germ-plasm enabling reproduction to take place through the female line alone, or in some cases with an occasional fillip from the intervention of males. Of the many marvellous things that could be related of these creatures but few instances can be cited here.