CHAP. IV.] ZOOLOGICAL REGIONS. 79 



istics except the abseuce of some of the more highly specialized 

 Neotropical groups. It is, however, a convenient division as 

 comprising the portion of the ISTorth American continent which 

 belongs zoologically to South America. 



The fourth, or Antillean suh-region, consists of the West 

 India islands (except Trinidad and Tobago, which are detached 

 portions of the continent and must be grouped in the first sub- 

 region) ; and these reproduce, in a much less marked degree, 

 the phenomena presented by ^Madagascar. Terrestrial mammals 

 are almost entirely wanting, but the larger islands possess three 

 genera which are altogether peculiar to tliem. The birds are 

 of South American forms, but comprise many peculiar genera. 

 Terrestrial molluscs are more abundant and varied than in any 

 part of the globe of equal extent; and if these alone were 

 considered, the Antilles would constitute an important Zoological 

 region. 



Nearctlc Bcgion. — Tliis region comprises all temperate North 

 America and Greenland. The arctic lands and islands beyond 

 the limit of trees form a transitional territory to the Paltearctic 

 region, but even here there are some characteristic species. 

 The southern limit between tliis region and the Neotropical is a 

 little uncertain ; but it may be drawn at about the Eio Grande 

 del Norte on the east coast, and a little north of Mazatlan on 

 the west ; while on the central plateau it descends much farther 

 south, and should perhaps include all tlie open liighlands of 

 Mexico and Guatemala. This would coincide with the range of 

 several characteristic Nearctic genera. 



Distinction of the Kcardic from the Pakeardic Region. — The 

 Nearctic region possesses twelve peculiar families of vertebrates 

 or one-tenth of its whole number. It has also twenty-four 

 peculiar genera of mammalia and fifty-two of birds, in each 

 case nearly one-third of all it possesses. This proportion is very 

 nearly the same ?s in the Palaearctic region, while the number of 

 peculiar families of vertebrata is very much greater. It has been 

 already seen that both Mr. Blyth and Professor Huxley are 

 disposed to unite this region with the Palsearctic, while Pro- 

 fessor Newton, in his article on birds in the new edition of the 



