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, z.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 spherius 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 
