186 DEVELOPMENT AND HEREDITY. 



be applied to it. Among a large number of speci- 

 mens of this winter generation which were devel- 

 oped under high temperature, there were a few — 

 generally imperfectly developed — individuals of the 

 summer form, showing that heat is not altogether 

 without effect upon this generation. 



When we seek for an explanation of this phenom- 

 enon of a periodic change of form in insects which 

 spring successively from the same lineal stock, we 

 can compare it to the periodic change in the root- 

 pressure of plants, which we have discussed in 

 Chapter IV. In both cases, when undisturbed, 

 the periods of change are coincident with periods 

 of profound changes in the environment, — in the 

 one case the periods of day and night, and in the 

 other, periods of winter and summer. VVe have seen 

 that light caused the increase of root-pressure, and 

 we have seen that cold caused the development of 

 the winter form of the butterfly. We may therefore 

 safely conclude that as day and night cause the 

 periodic changes of root-pressure, so summer and 

 winter cause the periodic change of form of the 

 butterfly. In the one case the daily alternation of 

 stimuli have effected co-ordinations in the living 

 matter of the plant, which cause a periodicity in its 

 activity ; in the other case, the annual alternation of 

 ■stimuli have effected co-ordinations in the living 



