114 



The St24,dy of Animal Life part iv 



to advantageous white. Not a few animals vary slightly 

 with the changing seasons. Thus many cases are known 

 where a butterfly produces in a year more than one brood, 



Fig. 67. — Seasonal dimorphism of PaJ>iUo ajax ; to the left the winter form 

 (variety Tc/ainonidcs), to the right the summer form (variety Marcellus). 

 (From Chambers's Encyclop. ; after Weisraann.) 



of which the winter forms are so different from those born 

 in summer that they have often been described as different 

 species. It is possible that this is a reminiscence of past 

 climatic changes, such as those of the Ice Ages, as the 



Fig. 63. — Seasonal changes of the bill in the puffin (Fratercuia arctica) ; to the 

 left the spring form, to the right the winter form, both adult males. (After 

 Bureau.) 



result of which a species became split up into two varieties. 

 Thus Araschnia levana and Araschnia prorsa are respect- 

 ively the winter and summer forms of one species. In the 



