58 
ZOOLOGICAL GEOGRAPHY. 
[part III. 
American continent, and forming part of the Tertiary “ Nearctic 
region.” This is clearly indicated both by the many Nearctic 
forms which do not pass south of Nicaragua, of which the turkeys 
( Meleagris ) 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 continent. 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 Cam el id® ; 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 
