fine net a few times through the water, one can draw out a veritable animal soup. In our ponds and lakes these little Crustaceans form the fundamental food of numerous fishes. But notwithstanding the enormous havoc wrought among them by enemies, large numbers remain at the end of a favourable season, and these produce the lasting eggs, _after fertilization_. For shortly before the end of the season males appear among the progeny of the hitherto purely parthenogenetic females. Although each female will only produce a few of these 'lasting' eggs, which require fertilization and are richly supplied with yolk, the whole number in each colony is a very large one, because the number of individuals is very large; and it must be so, since the eggs, though secure against cold and desiccation, are very imperfectly protected against the numerous enemies which may do them injury. Of course the number of individuals which form a colony may vary greatly in the different species, and the same is true of the number of parthenogenetic generations which precede the bi-sexual generation. I have already shown in detail that this depends precisely on the average duration of the favourable conditions, so that, for instance, a species which lives in large lake-basins will produce many purely parthenogenetic generations before the bi-sexual one, which only appears towards autumn, while species which live in quickly-drying pools have only a few parthenogenetic generations, and the true puddle-dwellers give rise to males and sexual females along with the parthenogenetic females as early as the second generation. We thus find in the Daphnids an alternation, regulated and made normal by natural selection, of purely parthenogenetic with bi-sexual generations, and the result is that the uniformity of the germ-plasm, which is the necessary consequence of pure parthenogenesis, is interrupted after a longer or shorter series of generations by the occurrence of amphimixis. That the number of parthenogenetic generations may be so varied, though with a definite norm for each species, indicates again that amphimixis is not an absolute condition of the maintenance of life, not an indispensable rejuvenation, designed to counteract the exhaustion of vital force--whether this be meant in a transcendental sense or otherwise--but that it is an important advantage calculated to keep the species at its highest level, and that its influence appears whether it occurs in the species regularly, or frequently, or only rarely.