140 ANIMAL LIFE AND SOCIAL GROWTH 



animals are most abundant in ponds which be- 

 come dry in summer. Their life history runs as 

 follows: A resistant winter egg hatches out in the 

 spring into a female which is capable of reproduc- 

 ing without being fertilized by a male. She is a 

 parthenogenetic female; her eggs will develop 

 without fertilization. The young produced are all 

 females and are parthenogenetic like their mother. 

 This may go on for many generations and then 

 an epidemic of sexuality appears, males and 

 sexual females occur. The latter produce eggs 

 which must be fertilized for development to take 

 place. These are the so-called resistant or winter 

 eggs, and after a period of quiescence which may 

 last over the period that the pond is dry in sum- 

 mer or during the period that it is frozen in winter, 

 this egg hatches into a parthenogenetic female 

 and the cycle continues. 



All the causes of the appearance of sexual forms 

 are not yet clear but recent work has shown that 

 the crowding of the parthenogenetic mothers 

 tends to produce a high percentage of males at 

 times when uncrowded sister cladocerans are 

 continuing to reproduce parthenogenetically. The 

 mechanism whereby crowding results in the pro- 

 duction of males is not known with certainty 

 but there is good evidence that the reduction of 

 available food will have this effect. The sug- 



