SOCIAL TRANSITIONS 257 



nematode parasites are females; but if many eggs 

 are fed, the nematodes that hatch are almost all 

 males. The results are not to be ascribed to a differ- 

 ential death rate, for approximately 75 per cent of 

 the eggs develop in both cases. 



In Crepidula and Bonellia and nematodes, both 

 males and females are always present in a popula- 

 tion, though in differing ratios. In cladocerans, how- 

 ever, of which Daphnia is an example, the species 

 may be carried along for many generations by the 

 females alone. They produce eggs which do not re- 

 quire fertilization, but which develop directly into 

 females that again produce other females like them- 

 selves. In these cladocerans the race is usually made 

 up of females alone, but at times there is an out- 

 break of sexuality; males and sexual females appear 

 and the fertilized eggs which result from their union 

 are more resistant to adverse conditions than those 

 which are ordinarily produced and which require 

 no fertilization. These resistant eggs enable the spe- 

 cies to survive times of environmental stress, such as 

 winter's ice or the drying-up of the ponds in which 

 these small crustaceans live. 



In one species of Moina, (5) which has been 

 much studied by the biologists at Brown University, 

 crowding of the females is an effective method of 

 bringing on the outbreak of males and sexual fe- 



