CHAP. IV.] ZOOLOGICAL REGIONS. 79 
istics except the absence of some of the more highly specialized 
Neotropical groups. It is, however, a convenient division as 
comprising the portion of the North American continent which 
belongs zoologically to South America. 
The fourth, or Antillean sub-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 them. 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. 
Nearctic Region—This region comprises all temperate North 
America and Greenland. The arctic lands and islands beyond 
the limit of trees form a transitional territory to the Palearctic 
region, but even here there are some characteristic species. 
The southern limit between this region and the Neotropical is a 
little uncertain ; but it may be drawn at about the Rio 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 the open highlands of 
Mexico and Guatemala. This would coincide with the range of 
several characteristic Nearctic genera. 
Distinction of the Nearctic from the Palearctic 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 as in the Palearctic 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 Palearctic, while Pro- 
fessor Newton, in his article on birds in the new edition of the 
