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CHAP. XV.] THE NEAKCTIC REGION. 117
separation was effected by an arm of the sea across what is
now Nicaragua, with perhaps anoth# at Panama. This would
leave Mexico and Gimtemala 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 fl'om 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, o£ them, migrating into the Neotropical region
in winter, while there are no peculiar NeolToj>ical 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 Myioclioctes, belonging to
the Mniotiltidae, 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 Ammodramus
soldi JiLuco (genera of finches), which range over the whole United
States, but each have one peculiar species in Guatemala. These