342 DAVID DAY WHITNEY 
and B. In race D fifty-nine of the young females matured and 
the reproduction rate was twice that of race C and three times 
that of race A. 
When the two races A and B were in the 60th to 80th genera- 
tions their rates of reproduction were probably very much like 
that of race D although no exact data were taken in this period. 
However, from general observations made at this time it was 
clearly seen by the observer that each young female in both of 
these races under ordinary conditions matured and produced 
ten or more young daughter-females in forty-eight hours. It was 
customary in these early generations to take ten young daughter- 
females from a single mother with which to form the succeeding 
generation. Later as the generations increased this became im- 
possible and for the isolation of ten young daughter-females at 
the end of forty-eight hours two mothers were required and later 
still three mothers were required. At the same time it was 
noticeable that the females of race B in the same length of time 
were producing fewer daughter-females than the femalesof race A. 
During the summer of 1910 this was so apparent that difficulty 
was experienced in being able to isolate ten young daughter- 
females from both races which were of the same age and size at 
the end of forty-eight hours. In order to continue these two 
_ races in a parallel series of generations by isolations of young 
females of the same size from both races daughter-females of 
race A were isolated which were produced later in a family, from 
the tenth to the thirtieth, and the daughter-females from several 
mothers of race B were isolated which were the earliest ones pro- 
duced in each family. Thus by isolating the daughter-females . 
from near the middle of a family from race A and the ‘first born’ 
daughter-females from race B it was possible to keep the genera- 
tions of both races parallel. 
From table 1 and from these general observations it is readily 
seen that as the parthenogenetic race became older the rate of 
reproduction decreased very decidedly and also that the chances 
for each young female to grow to maturity were lessened. This 
decrease in the rate of reproduction may not necessarily be due 
to long continued parthenogenetic reproduction, but rather to 
