ele Experimental Zoology 
subjected roses with aphids on them to the cold of an ice 
box without producing any effect. Moreover, the sexual form 
may appear in the autumn before the cold weather has come. 
Even under ordinary conditions all of the aphids do not become 
sexual forms and hundreds of them perish by the frost. I have 
kept a potted rose out of doors for three months after the sexual 
forms had appeared, and yet those individuals that remained 
alive after this time continued to reproduce parthenogenetically. 
Finally, the individuals that seem to be less affected are those 
that are found on the growing tips of the branches, where the 
leaves are still young and succulent. These observations sug- 
gest that the change is not due to the cold, but to some changes 
in the food plant that take place in the autumn. Whether the 
_result is due, as seems probable, to a lack of food, or to a reduc- 
tion in the amount of water, or to both combined, remains to be 
shown. 
A case apparently similar to that of the aphids is found in the 
Daphnians. These crustacea also produce parthenogenetically 
during the summer, and in the autumn the sexual forms appear. 
It has recently been shown by Issakowitsch that the change is 
probably due to a change in the food supply, and that the trans- 
formation can be quickly induced artificially by altering the 
external conditions. This and other cases will be given more 
fully in later chapters. 
In the life histories of the ciliate infusoria, periods of division 
are succeeded by periods of conjugation. Maupas thought that 
after a long succession of divisions this mode of propagation 
comes slowly to an end, and that unless conjugation occurs 
the individuals will die. Immediately after conjugation, when 
division is very active, there is little tendency to conjugate 
again; but the longer the time elapsing after this process, the 
more prone are the individuals to unite in pairs, especially those 
of a different parentage. It would seem that an internal factor 
is here involved; but there are some indications that external 
factors may also enter into the result. It has been shown by 
Calkins that when parameecium has undergone many divisions, 
