Life-cycle of the Lower Crustaceans 337 



seems to differ in different localities. In some local varieties 

 of this species the males are always present and abundant (as 

 in England, Odessa, Utah, and Cagliari), while in other races 

 the males are rare and not more than one or two males to a 

 thousand females are found. In the latter case all the summer 

 eggs develop by parthenogenesis. Whether the winter eggs 

 must be fertilized in these forms is not known. In the other 

 cases both the summer and Jhe winter eggs are fertilized, and 

 fail to develop if they are not, at least this has been definitely 

 shown to be the case at Cagliari by Artom. 



Most of the daphnias, or water fleas, show an alternation of 

 parthenogenetic and sexual forms. Many of the more familiar 

 species pass through the following cycle. From the winter egg 

 there emerges in the spring a female parthenogenetic individual. 

 Her offspring are also parthenogenetic, and throughout the 

 summer a series of these forms succeed one another in rapid 

 succession. In the autumn, males and sexual females appear, and 

 the latter, after fertilization, produce the winter eggs from which 

 the new brood of the following year emerges. 



This cycle differs somewhat in different species, and it is 

 known that sometimes males and sexual females are found dur- 

 ing the summer, and that in other species they appear under 

 special conditions. Kurz made some experiments which he 

 thought showed that the sexual forms appear when the water 

 becomes stale or begins to dry up. By hanging pieces of raw 

 cotton over the edges of the jar in which the daphnias lived, 

 the water was slowly drawn off, and after 14 days it was 

 reduced to about one sixth of its original volume. The experi- 

 ment was carried out in May, and many males and sexual 

 females appeared in the four different species present. In 

 another experiment he failed to obtain the same result, although 

 in foul water the sexual forms appeared. 



Weismann has made a most elaborate study of the life history 

 of the Daphnidae. He believed that his observations and ex- 

 periments show that external factors do not determine the ap- 

 pearance of the sexual generation, but that the change is due 



