CHAPTER XL 



DIMORPHISM AND POLYMORPHISM IN SPECIES. 



We have seen that the growing time of seeds, 

 and the time for putting forth leaves, is determined 

 by the annual changes of the seasons and by the 

 approach of the season of rapid increase of temper- 

 ature. We find an almost exactly similar series of 

 phenomena among animals, and especially is it the 

 case among insects. Insects, like plants, can flourish 

 only in warm weather, their lives being dependent, 

 in a large degree, both on the warmth and on the 

 plants. Each species of a temperate climate, there- 

 fore, is subjected by the regular return of the sea- 

 sons to a regularly recurring alternation of its 

 environment. Among many kinds of insects, only 

 the adult forms remain over from season to season, 

 passing the winter in a state of torpor. The young 

 are developed only in summer time, and, accordingly, 

 all generations are developed under the same condi- 

 tions. Consequently, all these generations, as we 

 would expect, are alike. On the other hand, there 



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