304 ANIMAL AGGREGATIONS 



Many workers have united in believing that the change from 

 parthenogenetic to sexual reproduction is due to internal factors, 

 perhaps to an innate sexual cycle. The problem has attracted much 

 attention because of the frequent coincidence between the appear- 

 ance of sexual forms, the production of the resistant sexual egg, and 

 the beginning of periods of special environmental stress. The whole 

 relationship appears to be highly adaptive in that the resistant egg 

 allows the species to survive periods of drought or of low tempera- 

 ture which would otherwise be fatal to it. The phenomenon is the 

 more impressive when we find that if a given pond contains more 

 than one cladoceran species, all show development of sexual repro- 

 duction at approximately the same time. 



In the laboratory, Banta has shown that one cladoceran, Moina 

 macrocopa, can be reared for at least 780 asexual generations with 

 complete vigor, and that another, Daphnia pulex, can be grown 

 similarly for at least 767 generations without the appearance of 

 sexual forms, providing cultural conditions remain favorable. Thus 

 it appears that the hypothesis of an innate sexual cycle will not hold 

 as a step toward a universal solution of the problem. 



Grosvenor and Smith (1913) suggested, as a result of their experi- 

 ence with another species of Moina, that the production of males is 

 initiated by the accumulation of waste products due to the crowded 

 condition of the mothers. Langhans (1909) had shown that accumu- 

 lations of excretory products reduce growth and reproduction and 

 hence presumably favor the production of sexual forms. Papani- 

 colau (19 10) confirmed an earlier report of Langhans that repeated 

 transfers of the Daphnia to fresh water postpones the appearance of 

 sexual forms indefinitely; but as a result of further work (1910a) he 

 concluded that crowding serves only to decrease numbers. Mc- 

 Clendon (19 10) also observed a hastening of sexual reproduction 

 which he interpreted as being due to excretions. Smith (191 5) 

 believed the effect of crowding is due to the excretions which ac- 

 cumulate, rather than to deficiency of food. Hartmann (19 19) con- 

 sidered the accumulation of excretions a probable cause of the out- 

 break of gamic reproduction. 



Banta and Brown, reporting in a series of papers beginning in 



