Life-cycle of the Lower Crustaceans 257 
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 the 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 Daphnide. 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 
Zz 
