240 1... Morgan 
4 
that either temperature or food is the sex-determining factor in 
this group. On this interpretation it might seem possible to 
regulate sex by controlling the conditions under which the aphids 
were kept. In the autumn of 1903 Miss. N. M. Stevens and I 
began our work on the group with these ideas in mind. Miss 
Stevens undertook the histological examination, while the experi- 
mental work was to be done conjointly. The former yielded the 
results that Miss Stevens published in 1905, but the experimental 
work gave only negative results. None of the external conditions 
to which we subjected the rose aphid produced the change from 
the parthenogenetic to the sexual forms. Later I tried a long 
series of other experiments in which twigs of the rose with aphids 
on them were kept in solutions of various salts, magnesium, cal- 
cium, potassium, lithium. The solutions, drawn up into the stem 
by the evaporation from the leaves should be imbibed by the 
aphids which procure their food by sucking the juices of the 
plant. These it was hoped might cause a change in the mode of 
reproduction. The results were negative. 
Observations made during the course of the experiments soon 
convinced us that temperature, at least, is not the cause of the 
change in the cycle in the rose aphid; for we found that the sexual 
forms may appear in the late summer before the cool weather has 
begun. Furthermore late in the autumn parthenogenetic individ- 
uals can always be found on the ends of the young twigs of the 
rose. In one case I kept a potted rose outside of a window and 
aphids remained on it until December, even after a freezing tem- 
perature outside. If brought into the green-house, these terminal 
aphids may continue to multiply throughout the winter without 
reproducing sexually. The facts show that temperature need take 
no direct part in the change in mode of reproduction; they also 
show that the conclusions that have been drawn from Kyber’s 
experiments are doubtful. The continued parthenogenesis of 
aphids brought into the greenhouse need not mean that the result 
is due to removal from the effects of cold, but that those individuals 
having escaped, so to speak, the influences, whether internal or 
external, that cause the cyclical change, continue to reproduce by 
parthenogenesis. Whether, having escaped at the critical period 
