Changes in the Life-cycle 311 
the winter the hydra produces buds that pinch off and become 
new individuals instead of remaining a part of the parent to form 
a colony. The size of hydra remains more or less constant. In 
the spring, sperm and eggs are produced. If the hydras are 
brought into a warm place in the autumn or in the winter, they 
will begin to produce sperm and eggs in a few weeks. This fact 
seems to indicate that the change produces the result; but whether 
this change is the effect of the cold, or of the warmth after the 
cold, or of both combined, or of some food relation, has not yet 
‘been made out. 
Passing now to an examination of those cases where the 
changes in the life-cycle are known to be definitely connected with 
changes in the environment, I shall mention first the well-known 
case of the rose aphids or plant lice. These insects produce 
generation after generation of wingless,’ parthenogenetic indi- 
viduals throughout the summer. In the autumn a generation 
of winged males and wingless females appears. These pair 
and afterward the females deposit each a few “winter eggs” 
on the food plant. From these eggs the young parthenogenetic 
females hatch in the following spring and start the summer 
broods. 
It has been shown for the rose aphid that if the parthenogenetic 
summer forms are brought into the greenhouse and put on roses, 
they will continue to produce parthenogenetic young, and the 
sexual forms never appear. Bonnet saw nine generations of 
parthenogenetic forms; Duval counted eleven in the course of 
seven months; Kyber kept aphids in a hothouse for four years 
and observed only parthenogenetic reproduction. It is evident, 
therefore, that if the external conditions are favorable, the non- 
sexual mode of reproduction may continue forever, as far as we 
can see. What, then, are the external conditions that determine 
the production of the sexual forms? The change occurs in the 
autumn when the cold weather begins to set in, and it may 
appear that the cold is the cause of the change. I have 
1A few winged individuals appear in almost every generation, but these are 
not the sexual forms as has been sometimes supposed. 
