338 Experimental Zoology 



solely to an internal factor. The latter, he believes, is the out- 

 come of natural selection which has acted in such a way that the 

 internal mechanism has been adjusted to meet the changes in the 

 environment, without, however, being affected directly by them. 

 In other words, natural selection has changed the life-cycle so that 

 it undergoes a parallel series of changes to the cyclical changes 

 in the environment, or as Weismann has put it, "The organism 

 is so constituted that it reproduces sexually at the proper time, 

 and it is to a certain degree a matter of no consequence what 

 external conditions exist at that time, so long as they are not of 

 a sort to very materially interfere with the process of assimila- 

 tion or to threaten the life of the individual." In other words, 

 natural selection has wound up the individual to go in the same 

 time as the environment. 



Weismann points out that the usual appearance of the sexual 

 forms in the autumn has suggested that the cold brings about 

 the result, but this is disproven by the fact that in the early spring 

 the water may be as cold as in the autumn, yet in the spring the 

 reproduction is only by parthenogenesis. He made an experi- 

 ment that confirmed this conclusion. During the summer he 

 kept pieces of ice in an aquarium in which a large number of 

 individuals lived, so that the temperature was reduced to from 

 5 to 10 C. The daphnias continued to reproduce partheno- 

 genetically, although more slowly. 



In another way, Weismann thinks, the insufficiency of the 

 temperate hypothesis is shown. Daphnias that live in swamps 

 (Polyphemus, for example) have a double sexual period : the 

 first at the beginning of July, when the water is at its warmest ; 

 and the second at the end of October, or the beginning of No- 

 vember, when the temperature is near the winter minimum. 



Weismann does not think that the condition of the food supply 

 can account for the result, because he believes that food is always 

 abundant. Most species of daphnias live on fine slime or par- 

 ticles of organic matter, sticking to the bottom or suspended in 

 the water. Weismann thinks there is a superfluity of this ma- 

 terial at all times. He states, moreover, that the sexual genera- 



