394 THE STUDY OF ANIMAL LIFE CHAP. 



In many cases the heat or cold is influential simply 

 as what we may call a trigger-puller. It induces certain 

 predispositions to express themselves, as is well illus- 

 trated by seasonal changes. The variable hare (Lepus 

 variabilis), the stoat (Putorius ennineus), the ptarmigan 

 (Lagopus mutus), and some other animals turn white 

 every year when the winter approaches, and what the 

 cold does is to set a-going a periodic change which has 

 been engrained in the constitution in the course of ages. 

 We may think of the environment in such cases as a 

 liberating stimulus. Many cases are known where a 



FIG. 122. SEASONAL DIMORPHISM OF Papilio ajax ; TO THE LEFT THE 

 WINTER FORM (VARIETY telamonides), TO THE RIGHT THE SUMMER 

 FORM (VARIETY marcellus). 



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



butterfly produces in a year more than one brood, 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 result of which a species became split up into 

 two varieties. Thus Araschnia levana and Araschnia 

 prorsa are respectively the winter and summer forms of 

 one species. In the glacial epoch there was perhaps only 

 A. levana, the winter form ; the change of climate has 

 perhaps evolved the summer variety A. prorsa. Both 

 Weismann and Edwards have succeeded, by artificial 



