Life-cycle of the Lower Crustaceans 339 



tion appears when food must be at its time of greatest abundance. 

 That Weismann is probably mistaken in this regard will be 

 shown presently. 



Weismann also rejects Kurz's idea that the drying up of the 

 water causes the sexual forms to appear, and bases his objection 

 most justly on the failure on Kurz's part to carry out control 

 experiments in which the water was not diminished. Weismann 

 points out that the sexual forms often appear in confinement 

 when the level of the water remains the same. Schmanke- 

 witsch's suggestion, that increasing the amount of salt contained 

 in the water causes the parthenogenetic forms to give rise to the 

 sexual generation, is also rejected by Weismann. Schmanke- 

 witsch pointed out that Moina rectirostris lives in small pools in 

 the spring and autumn ; it disappears in the summer when the 

 salt becomes too strong, but before this occurs the sexual genera- 

 tion appears. Weismann objects that while the facts may be 

 correct it does not follow that the salt is the cause of the change ; 

 for in other localities the same species of Moina lives in fresh 

 water and at all times of the year, and in every generation 

 except the first (that hatches from the winter eggs), brings 

 forth males and sexual females. 



Weismann carried out some experiments which show, he 

 thinks, that external conditions do not regulate the alternation 

 of generations. He found when two lots of daphnias are kept 

 in separate dishes under conditions as similar as possible that 

 one set may produce males and sexual females, while the other 

 set continues to reproduce parthenogenetically. It seems to 

 me hazardous to claim that the conditions are the same in 

 any two such dishes, for while light, temperature, and the amount 

 of water may be the same in both, the food supply may be very 

 different, depending in large part on the number and kind of 

 animals that are devouring the food. Experience will show that 

 it is well-nigh impossible to keep the relation between the plants 

 (that serve as food), the bacteria, and the animals in a constant 

 relation in any two dishes. Any initial difference at the beginning 

 (and such must nearly always exist) may lead to very different 



