IN BUTTERFLIES 245 



intervenes. Who solves this problem will win de- 

 served renown. 



What may be the exact climatic features which 

 determine the number of generations of a butterfly 

 has not yet been studied ; but there are some curi- 

 ous difficulties in the way of understanding them. 

 The Painted Lady (Vanessa cardui), for instance, 

 is double brooded in New England, both in the 

 districts where the contrasts of heat and cold, 

 moisture and drought, are excessive, — that is, 

 where the climate has those peculiarities which are 

 termed " continental ; " and also on islands such 

 as Nantucket in southern New England, where a 

 much greater evenness prevails and the climate 

 partakes of an '*• insular " character ; yet in the 

 valleys of Switzerland, where perhaps of all places 

 in Europe the climate presents the greatest and 

 most sudden inequalities, and therefore is most 

 similar to that of New England, and certainly more 

 '* continental " than that of Nantucket, this butter- 

 fly is single brooded. We have exceedingly few 

 identical butterflies in Europe and the United 

 States, and this apparently is the only one of them 

 that differs in its broods in the two countries ; but 

 there are several of our butterflies which are repre- 

 sented by very closely allied species in Europe, 



