66 Bulletin Wisconsin Natural History Society. [Vol. 10, Nos. 1 & 2. 
caused by the decrease in temperature. In summer there is an 
abundance of food and the Daphnias then develop parthenogen- 
etically. When winter approaches, the food is reduced enor¬ 
mously, and the sexual process makes its appearance. 
Other workers, such as McClendon (To), Papanicolau (To), 
and Woltereck (Ti), on the other hand, are of the opinion that 
the environment does not directly account for the appearance of 
these different cycles of development, but that the sexual and par- 
thenogenetic phases are the results of internal factors within the 
organism. Environmental conditions may, however, act indirectly, 
and so interfere with the internal factors of the organism as to 
bring about a change in its life cycle. Woltereck (’ll), is the 
strongest adherent of this viewpoint. This investigator claims 
that the proportion of parthenogenetic males and parthenogenetic 
females produced in DapJmia may be Influenced by external con¬ 
ditions. He says that each egg has an inner mechanism of causes, 
or specific sex substances, which determines the sex. This 
mechanism of causes is not dependent on the equipment of the 
eggs with either plasma substances, or with heterochromosomes, 
but acts independently. Exterior conditions (such as temperature, 
food, chemicals, etc.), may sometimes, through their action on the 
inner mechanism of causes, have a distinct effect on the developed 
ovarial eggs, as well as on the eggs to be formed in the future. 
These specific sex substances (mechanism of causes) must be 
looked upon as ferment-like bodies, depending to a large extent 
upon external conditions for activation. The activation of one 
substance during the maturity of the egg accounts for the produc¬ 
tion of one sex, whereas the activation of another substance 
accounts for the other sex. 
In the face of all this evidence, Weismann dogmatically asserts 
that external factors have almost nothing to do with the appear¬ 
ance of the sexual cycle. According to him, this phase in the life 
history can only be accounted for on the grounds of internal 
factors of the organism, which are the outcome of natural selec¬ 
tion. Weismann says, “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 
