IN BUTTERFLIES 249 



relaxation wliich such a life inevitably demands. 

 After a long absence in Europe, a keen observer 

 may even be directly conscious of this quickened 

 life. 



Now to what shall we ascribe such peculiarities 

 in animal life ? Naturally we look to climatic in- 

 fluences, and our attention is first attracted by the 

 well-known fact that if we compare two places in 

 Europe and America having the same mean an- 

 nual temperature, the extremes of variation will 

 prove much greater on this side of the Atlantic. 

 For example, while the mean annual temperature 

 of New York is about the same as that of Frank- 

 fort, the summer temperature of the former is that 

 of Rome and its winter that of St. Petersburg. 

 Moreover, the changes from summer to winter and 

 from winter to summer are more immediate in 

 America; or, in other words, the summers and 

 winters are longer by about three weeks. Such 

 long and hot summers are of course favorable to 

 the multiplication of broods in butterflies whose 

 history allows a repetition of the same cycle more 

 than once a year ; the length of the winter is of 

 slight consequence, as long as the insects can sur- 

 vive it ; and it can have no influence upon the 

 number of broods, unless there be species (of which 



