104 THE LIFE-STORY OF INSECTS [CH. 



evolved still more recently by slow stages, as the 

 continental climate has become more genial. In 

 tropical countries where instead of an alternation of 

 winter and summer, alternate dry and rainy seasons 

 prevail, somewhat similar seasonal dimorphism has 

 been observed among many butterflies. Not a few 

 forms of Precis, an African and Indian genus allied to 

 our Vanessa, that had long been considered distinct 

 species are now known, thanks to the researches of 

 G. A. K. Marshall (1898), to be alternating seasonal 

 forms of the same insect. The offspring when adult 

 does not closely resemble the parent ; its appearance 

 is modified by the climatic environment of the pupa. 

 The experiments of Weismann just sketched in out- 

 line show at least that the same principle holds for 

 our northern butterflies. 



We are thus led to see from the life-story of such 

 insects, that the course of the story is not rigidly 

 fixed ; the creature in its various stages is plastic, 

 open to influence from its surroundings, capable of 

 marked change in the course of generations. And 

 so the seasonal changes in the history of the indi- 

 vidual from egg to imago point us to changes in the 

 age-long history of the race. 



