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58 ZOOLOGICAL GEOGRAPHY. [part hi.
American continent, and forming part of the Tertiary " Xearctic
region." This is clearly indicated both by the many Nearctic
forms which do not pass south of Nicaragua, of which the turkeys
(Mekagris) are a striking example, and by the comparative
poverty of this area in typical Neotropical groups. During the
Miocene period there was not that marked diversity of climate
between North and South America that now prevails ; for when
a luxuriant vegetation covered what are now the shores of the
Arctic Ocean, the country south of the great lakes must have been
almost or quite tropical. At an early Tertiary period, the zoological
differences of the Nearctic and Neotropical regions were probably
more radical than they are now. South America being a huge
island, or group of islands—a kind of Australia of the New
World, chiefly inhabited by the imperfectly organized Edentata
while North America abounded in Ungulata and Carnivora, and
perhaps formed a part of the great Old World contineDt. There
were also one or more very ancient unions (in Eocene or Miocene
times) of the two continents, admitting of the entrance of the
ancestral types of Quadrumana into South America, and, somewhat
later, of the Camelidae ; while the isthmus south of Nicaragua
was at one time united to the southern continent, at another made
insular by subsidence near Panama, and thus obtained that rich
variety of Neotropical types that still characterises it. When
the final union of the two continents took place, the tropical
climate of the lower portions of Guatemala and Mexico would
invite rapid immigration from the south ; while some northern
forms would extend their range into and beyond the newly
elevated territory. The Mexican sub-region has therefore a
composite character, and we must not endeavour too rigidly to
determine its northern limits, nor claim as exclusively Neotro-
pical, forms which are perhaps comparatively recent immigrants
and it would perhaps be a more accurate representation of the
facts, if we were to consider all the highlands of Mexico and
Guatemala above the limits of the tropical forests, as still
belonging to the Nearctic region, of which the whole country so
recently formed a part.
The long-continued separation of North and South America