CHAP, xv.] THE NEARCTIC REGION. 117 



separation was effected by an arm of the sea across what is 

 now Nicaragua, with perhaps another at Panama. This would 

 leave Mexico and Guatemala joined to North America, and 

 forming part of the Nearctic region, although no doubt contain- 

 ing many Neotropical forms, which they had received during 

 earlier continental periods ; and these countries might at other 

 times have been made insular by a strait at the isthmus of 

 Tehuantepec, and have then developed some peculiar species. 

 The latest climatal changes have tended to restrict these 

 Neotropical forms to those parts where the climate is really 

 tropical ; and thus Mexico has attained its present strongly 

 marked Neotropical character, although deficient in many of 

 the most important groups of that region. 



In view of these recent changes, it seems proper not to draw 

 any decided line between the Nearctic and Neotropical regions, 

 but rather to apply, in the case of each genus, a test which will 

 show whether it was probably derived at a comparatively recent 

 date from one region or the other. The test referred to, is the 

 existence of peculiar species of the genus, in what are un- 

 doubtedly portions of ancient North or South America. If, 

 for example, all the species of a genus occur in North America, 

 some, or even all, of them, migrating into the Neotropical region 

 in winter, while there are no peculiar Neotropical species, then 

 we must class that genus as strictly Nearctic; for if it were 

 Neotropical it would certainly have developed some peculiar 

 resident forms. Again, even if there should be one or two 

 resident species peculiar to that part of Central America north 

 of the ancient dividing strait, with an equal or greater number 

 of species ranging over a large part of Temperate North America, 

 the genus must still be considered Nearctic. Examples of the 

 former case, are Helminthophaga and Myiodioctes, belonging to 

 the Mniotiltidse, or wood- warblers, which range over all Tem- 

 perate North America to Canada, where all the species are found, 

 but in each case one of the species is found in South America, 

 probably as a winter migrant. Of the latter, are Ammodr&mus 

 and Junco (genera of finches), which range over the whole United 

 States, but each have one peculiar species in Guatemala. These 



