The geographical distribution of animals, with a study of the relations ...

chap, iv.] ZOOLOGICAL REGIONS. 79istics except the absence of some of the more highly specializedNeotropical groups. It is, however, a convenient division ascomprising the portion ofthe North American continent whichbelongs zoologically to South America.The fourth, or Antillean sub-region, consists of the WestIndia islands(except Trinidad and Tobago, which are detachedportions of the continent and must be grouped inthe first subregion);and these reproduce, in a much less marked degree,the phenomena presented by Madagascar.Terrestrial mammalsare almost entirely wanting, but the larger islands possess threegenera which are altogether peculiar to them. The birds areof South American forms, but comprise many peculiar genera.Terrestrial molluscs are more abundant and varied than inanypart of the globe of equal extent ; and if these alone wereconsidered, the Antilles would constitute an important Zoologicalregion.Nearctic Region.—This region comprises all temperate NorthAmerica and Greenland. The arctic lands and islands beyondthe limit of trees form a transitional territory to the Palsearcticregion, but even here there are some characteristic species.The southern limit between this region and the Neotropical islittle uncertain ; but it may be drawn at about the Rio Grandedel Norte on the east coast, and a little north of Mazatlan onthe west ; while on the central plateau it descends much farthersouth, and should perhaps include all the open highlands ofMexico and Guatemala.several characteristic Nearctic genera.This would coincide with the range ofDistinction of the Nearctic from the Palcearctic Region.—TheNearctic region possesses twelve peculiar families of vertebratesor one-tenth of its whole number. It has also twenty-fourpeculiar genera of mammalia and fifty-two of birds, in eachcase nearly one-third of all it possesses.This proportion is verynearly the same as in the Palsearctic region, while the number ofpeculiar families of vertebrata is very much greater.aIt has beenalready seen that both Mr. Blyth and Professor Huxley aredisposed to unite this region with the Palsearctic, while ProfessorNewton, in his article on birds in the new edition of the