)i4 



The Study of Animal Life part iv 



to advantiiyeous white. Not a few animals vary slightly 

 with the changiny seasons. Thus many cases are known 

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



Fig. 67. — Se.isoiial dimorpliism of Pa/'ilip njax ; to the left the winter rorm 

 (variety J\/ajiwrn'(lcs), to the right the siimnier fui-tn (variety I\Iarccthcs). 

 (From Chamberb's Encyctop. ; after Weisniann.) 



of which the winter forms arc so different from those born 

 in sunnncr that ihcy have often been described as diftcrent 

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

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



Flc. 68.— Seasonal chances nftlie bill in the ptiflln {FraUirii/a anli'ca) ■ to the 

 left the spring form, to the right the winter form, both adult males. ' (.\fter 

 Eureau.) 



result of which a species became split u]) into two varieties. 

 Thus Arasclinia Icvana and Arasclinid prorsa are respect- 

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



