In our chapter on extinct animals, we have shown, that there is good reason for believing that the existing union of North and South America is a quite recent occurrence; and that the {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 containing 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 undoubtedly 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 Mniotiltidæ, or wood-warblers, which range over _all_ Temperate 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 {118}may be claimed as exclusively Nearctic genera, on the ground that Guatemala was recently Nearctic; and is now really a transition territory, of which the lowlands have been invaded and taken exclusive possession of by a Neotropical fauna, while the highlands are still (in part at least) occupied by Nearctic forms.