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 hfe-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 



^ A few winged individuals appear in almost every generation, but these are 

 not the sexual forms as has been sometimes supposed. 



