58 ZOOLOGICAL GEOGRAPHY. [part hi. 



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 

 {Melcagris) 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 Camelidas ; 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 tlie two continents took place, the tropical 

 climate or the lower portions of Guatemala and Mexico would 

 invite rapid immigration from the south ; while some northern 

 forms would extend tlieir 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 



