340 Experimental Zoology 
results. Weismann cites the following experiments in support 
of his statement. Six young daphnias, born parthenogenetically 
from the same mother, were isolated each in a separate glass. 
Fourteen days later No. 1 had produced three parthenogenetic 
young; No. 2, eight of the same; No. 3, six; No. 4, five; 
No. 5, seven sexual females and three males; No. 6, five 
parthenogenetic females. 
In another experiment a male and a female were placed in the 
same small glass on March 5. They belonged to the fourth 
generation from the winter egg. They were not observed to 
pair. Three weeks later 8 parthenogenetic females were pres- 
ent. After another 3 weeks 50 young were counted, of which 
many were males. Ten days later there were 18 sexual females 
and 5 males. A week later numerous young were found, and 9 
days later still, when they were counted, there were found 3 
females with winter eggs, 20 parthenogenetic females, 1 male, 
and 79 young. In this experiment the sexual forms did not ap- 
pear for some time; but in a parallel experiment beginning with 
25 males and 8 females (March 24) the sexual forms appeared 
at once. It is difficult to understand why Weismann should con- 
clude from these experiments that the life-cycle is independent 
of the environment. It is more probable that there was present 
some unknown condition that caused the difference in the results. 
One such condition may have been the crowding with the conse- 
quent decrease in the available food. 
Weismann has laid a good deal of emphasis on the differences 
in the life history of species of Daphnide that live under different 
conditions. Thus monocyclic species in which the sexual genera- 
tion appears only once in the course of the year are found in 
large bodies of water that never dry up, —in lakes, ponds, 
swamps, and in the ocean. Sida crystallina is a species of this 
kind. Throughout the summer only parthenogenetic indi- 
viduals appear, and in the autumn males and sexual females. 
Polycyclic species inhabit puddles and small swamps that are 
often dried up. The winter eggs can withstand the drying, but 
not the parthenogenetic adults; therefore in order to exist in 
