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-w r arblers, 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 Ammodramus 
and Junco (genera of finches), which range over the whole United 
States, but each have one peculiar species in Guatemala. These 
