IXBEEEDING, PARTHENOGENESIS, ASEXUAL REPRODUCTION 243 



Thus we see that in the animal kingdom hermaphroditism is 

 always associated with cross-fertilization in some way or other, even 

 though the latter may occur rarely, being usually periodically 

 interpolated, and thus bringing new ids into the germ-plasm which is 

 rapidly becoming monotonous or uniform. Adaptations quite analogous 

 to these are found in relation to parthenogenesis, and it will repay us 

 to give a brief summary of these. 



Parthenogenesis effects a very considerable increase in the 

 fertility of a species, and in this increase the reason for its introduction 

 among natural phenomena obviously lies. By the occurrence of 

 parthenogenesis, the number of ova produced by a particular colony 

 of animals may be doubled, because each individual is a female, and as 

 the multiplication increases in geometrical ratio a few parthenogenetic 

 generations result in a number of descendants enormously in excess of 

 those produced by bi-sexual reproduction. We can therefore under- 

 stand why parthenogenesis should obtain among animals whose 

 ■conditions of life are favourable only for a short time, and are then 

 uncertain and dangerous for a long period. This is the case with the 

 water-fleas, the Daphnids (see Figs. 57 and 58), whose habitats 

 — pools, ponds, and marshes — often dry up altogether in summer, or 

 freeze in winter, so that it becomes almost if not quite impossible for 

 the colonies to go on living, and the preservation of the species can 

 only be secured by the production of hard-shelled 'lasting' eggs, 

 which sink to the bottom, dry up in the mud, or become frozen, or at 

 least remain latent in a sort of slumber. As soon as the favourable 

 conditions reappear, young animals which emerge from the eggs 

 are all females and reproduce parthenogenetically, so that after a few 

 days there is a numerous progeny swimming freely about, which in 

 their turn are all females, and reproduce after the same manner. In 

 many Daphnids this goes on for a series of generations, and there 

 thus arises an enormous number of animals, which may fill a marsh 

 so densely that, by drawing a 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 ; 



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