314 



The Study of A nimal 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 Papilio ajax ; to tlie left the winter form 

 (variety TeiaTnonides), to the right the summer form (variety Marcellus). 

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



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. 68. — Seasonal changes of the bill in the puffin {Fratercula tirciicd) ; 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 ^rorsa are respect- 

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



