318 2 i forean, ° 
nians. When the sexual forms appear the leaves have in many 
cases begun to dry up, or at least to harden so that nourishment 
may be more difficult to obtain. In the rose aphid the sexual 
forms are found on the lower, older leaves, while the partheno- 
genetic individuals are on the terminal young buds and younger 
leaves. If these are brought into the green-house they continue 
to produce parthenogenetically; the leaves of plants grown under 
such conditions remain soft, many young leaves are present 
and the terminal buds continue to grow. It seems very probable 
that temperature has no direct influence on the life-cycle in the 
aphids, and the facts here given suggest that the change may be 
connected with conditions of nutrition. 
The life-cycle of the rotifer, Hydatina senta, is similar in many 
ways to that of the aphids and phylloxerans. Here, too, all ferti-. 
lized eggs become females, and these females produce by partheno- 
genesis a succession of parthenogenetic individuals. Some of 
these may produce males, others sexual eggs, but the same female 
never produces more than one kind of egg. Maupas’s work on 
Hydatina senta seemed to show that a high temperature increases 
enormously the proportion of male-laying females (97 per cent) 
not by changing the character of the females so that a different 
kind of egg is produced, but by affecting the character of the laid 
egg, or its method of development, so that it becomes a male-lay- 
ing individual. Conversely Maupas believed that a low tempera- 
ture greatly increases the output of female-laying individuals (95 
per cent). Nussbaum, Punnett and Whitney, who have repeated 
the experiment, find no evidence that supports Maupas’s view. 
Whitney points out how Maupas’s facts may be correct, but his 
conclusions wrong. At a high temperature the male-laying females 
are not so much affected as to their output of eggs as are the female- 
laying individuals, hence the percentage of the male eggs will be 
proportionately increased. Furthermore while the average pro- 
portion of male-laying females is about 22 percent some individuals 
produce fifty per cent of these individuals, others only two to five 
per cent. ‘That these individuals are not fixed in respect to sex- 
production was also shown, for a low male-producing strain may 
later become a high one. It is difficult to understand how such 
