134 ZOOLOGICAL GEOGRAPHY. 



rows. The northern mass of land causes its isothermal lines to bend south- 

 wards; and its winter temperature especially, is far lower than at corres- 

 ponding latitudes in Europe. This diminishes the available area for sup- 

 porting animal life; the amount and character of which must be, to a great 

 extent, determined by the nature of the least favorable part of the year. 

 Again, owing to the position of its mountain ranges and the direction of 

 prevalent winds, a large extent of its interior, east of the Eocky Mountains, 

 is bare and arid, and often almost desert; while the most favored districts 

 — those east of the Mississippi and west of the Sierra Nevada, bear but a 

 small proportion to its whole area. Again, we know that at a very recent 

 period geologically, it was subject to a very severe glacial epoch, which 

 wrapped a full half of it in a mantle of ice, and exterminated a large num- 

 ber of animals which previously inhabited it. Taking all this into account, 

 we need not be surprised to find theNearctic region somewhat less rich and 

 varied in its forms of lite than the Palsearctic or the Australian regions, with 

 which alone it can fairly be compared. The wonder rather is that it should 

 be so little inferior to them in this respect, and that it should possess such 

 a variety of groups, and such a multitude of forms, in every class of ani- 

 mals. * * ^ * 



In our chapter on extinct animals, we have shown that there is good 

 reason for believing that the existing union of JSorth and South America 

 is quite a recent occurrence; and that the separation was effected by an arm 

 of the sea across what is now Nicaragua, with perhaps another at Panama. 

 This would leave Mexico and Guatemala joined to North America, and 

 forming part of the Nearctic region, although no doubt containing many 

 Neotropical forms, which they had received during earlier continental pe- 

 riods; and these countries might at other times have been' made insular by 

 a strait at the isthmus of Tehuantepec, and have then developed some pe- 

 culiar species. The latest climatal ehanges have tended to restrict these 

 Neotropical forms to those parts where the climate is really tropical; and 

 thus Mexico has attained its present strongly marked Neotropical charac- 

 ter, although deficient in many of the most important groups of that region. 



In view of these recent changes, it seems proper not to draw any de- 

 cided line between the Nearctic and Neotropical regions, but rather to 

 apply, in the case of each genus, a test which will show whether it was 

 probably derived at a comparatively recent date from one region or the 

 other. * * * * - 



The Central or Mocky Mountain sub - region is, for the greater part of 

 its extent, from 2,000 to 5,000 feet above the sea, and is excessively arid; 

 and, except in the immediate vicinity of streams and on some of the higher 

 slopes of the mountains, is almost wholly treeless. Its zoology is therefore 

 peculiar. Many of the most characteristic genera and families of the East- 

 ern States are absent; while a number of curious desert and alpine forms 

 give it a character of its own, and render it very interesting to the nat- 

 uralist. * * ^ * 



