Life-cycle of the Lower Crustaceans 341 



such localities the winter eggs, more appropriately called the 

 resting eggs, must be often produced. Such, in fact, is the case, 

 and males and females may appear more than once a year. 

 For example, Moina rectirostris and M. paradoxa usually pro- 

 duce three to five successive parthenogenetic broods, but amongst 

 these, sexual forms are always to'be found, even at times in the 

 first parthenogenetic generation, but always in the second and 

 third. Again in Daphnia pulex, which lives in swamps, the 

 first generation consists of parthenogenetic individuals. Males 

 and females may appear in the second ; they are not infrequent 

 in the third, and almost always present in the fourth, fifth, and 

 sixth generations. 



There are a few species that seem to be acyclic, i.e. produce no 

 sexual forms at all, but throughout the winter and the summer 

 reproduce by parthenogenesis. Weismann believes that this is 

 a local condition, and that in other localities the same species 

 may have a sexual generation. Chydorus sphaerius produces 

 in Freiburg only parthenogenetic females, but in other places 

 sexual forms have been found. 



From the conditions under which these forms live Weismann 

 thinks that through natural selection the life-cycle has been made 

 what we find it to-day, so that each species has become adapted 

 to the special conditions under which it is found. It may be 

 pointed out that the results can just as readily be explained by 

 assuming that, the life-cycle being what we find it, certain species 

 can live only in certain localities and others in other localities. 

 How the species having these cycles originated, is a question 

 that we need not discuss at present. 



There are certain facts connected with the reproduction of 

 these crustaceans that are of general biological interest. The 

 same individual may alternately produce winter and partheno- 

 genetic eggs. If, for example, the winter egg is not fertilized, it 

 goes to pieces, and this "acts as a stimulus" to further partheno- 

 genetic reproduction. Four cells go to produce a partheno- 

 genetic egg, three of them serving as food for the fourth, but a 

 large number of cells contribute to the formation of the winter 



