154 SEASONAL CHANGES AND HISTORIES. 



But the most striking example of all will be 

 found in the species of the genus Iphiclides. The 

 European I. Podalirius is confined to the Medi- 

 terranean region, while our Zebra Swallow-tail 

 belongs to the southern half of the United States ; 

 the regions are therefore fairly comparable ; yet 

 we find no mention of more than two broods of I. 

 Podalirius, while Mr. Edwards has shown that, 

 even as far north as the Appalachian valley of 

 West Virginia, I. Ajax has four and sometimes 

 five generations during the year. 



These cases might perhaps be multiplied, but 

 further positive evidence is not at hand ; it should 

 be remarked, however, that there is no reversal 

 of the rule : among all the butterflies properly 

 comparable on the two continents, there is no 

 single instance where the European butterfly has 

 more broods than the American. 



This result of a comparison of the annual his 

 tories of similar European and American butter- 

 flies furnishes but another instance of that in- 

 tensity which seems to characterize all life in 

 America. The expenditure of nervous and vital 

 energy, against which physicians vainly inveigh, 

 which superannuates our merchants, lawyers, 

 clergymen, and other professional men, is not in- 

 duced by the simple passion for gain, place, 

 power or knowledge, but by an uncontrollable 

 restlessness, a constant dissatisfaction with pre- 



