SPECIAL ADVANTAGES OF N. AMEEICA 151 



arctic section suffers from the opposite defect. 

 Separated by imperfect barriers from the Oriental 

 Eegion, its butterfly fauna is complicated by 

 much invasion of specially protected species from 

 the tropics, and the examples of Mimicry are too 

 numerous and too little known. North America 

 occupies a position conveniently intermediate 

 between the two sections of the Palaearctic por- 

 tion of the circumpolar land-belt. It has been 

 invaded by models from the eastern tropics of 

 the Old World and also probably from the tropics 

 of the New ; but the species are few and their 

 effects upon the indigenous butterflies sharp and 

 distinct. The Mimicry itself affords striking and 

 remarkable evidence of the lines of migration 

 followed by some of the intruding models. The 

 ancestral forms from which the mimics were 

 derived, have nearly always persisted, and enable 

 us to unravel the history of the change, with 

 exceptional clearness. The examples bear in a 

 most interesting manner upon the two great 

 hypotheses associated respectively with the names 

 of H. W. Bates and Fritz Muller. Although the 

 butterfly fauna is as well known as that of any 

 part of the world, the mimetic resemblances 

 supply material for a large amount of much- 

 needed original investigation, inviting the atten- 

 tion of American naturalists in almost every 

 locality. 



