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 twodishes. Any initial difference at the beginning 
(and such must nearly always exist) may lead to very different 
