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. 
Inother 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 asin 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 thé 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- 
