REGIONAL DISTRIBUTION. 313 



has been divided into two smaller, north and south, districts — that bearing \rood, and that (the more 

 northern, called the Barren-Groimd region) too desolate and cold for wood to grow upon. South of 

 these northern districts the fauna indicates a tripartite longitudinal division ; and each of these stripes 

 is capable of further local subdi\-ision, according to its latitude and climate. We shoidd expect the 

 Rocky Mountain range to form the chief longitudinal line of separation, but, as already said, it 

 only separates species in a minor degree. The actual mountain barrier appears to be the 

 Cascade range on the west side of the Rocky Mountains, separating Oregon and California from 

 the rest of North America ; and the dividing limit between the two other regions seems to be 

 the Nebraska country, in which lie the Mauvaises Terres, and Nebraska and Niobrara beds, on 

 this side of the Rocky Mountains, — the line of separation, in fact, being marked by the site of 

 the ancient tertiary sea in which these beds had been deposited. 



These longitudinal sections reach as far south as Texas and New Mexico, when a new province 

 commences, which continues through Mexico and Central America to the Isthmus of Panama. 



As to the southern boimdary of the whole North American region I have already, when 

 speaking of the Vesjjer Mice, given my reasons for preferring Panama to a more northerly point. 

 Although South American sj)ecies extend to the north of this limit, few, if anj'^. North American 

 species pass to the south of it ; and this distribution concurs with the physical features of the 

 country in pointing out the narrowest and lowest neck of land as that most likely to have been 

 the place where a barrier between the two continents existed at some former period , which allowed 

 the shells of the Pacific to penetrate into the Gulf of Mexico. 



Dr. Sclater has carried the ornithic limit between the two regions up into the heart of Mexico, 

 but not without hesitation. Even more than in the Mammals, Central America is a sort of debateable 

 ground, in which the species of birds both from the south and the north meet and overlap each other. 



The same diiRculty occurs with regard to the West Indian Islands. Do they belong to the 

 north or south ? They have something of both in their character, besides a good deal that is 

 pecidiar to themselves. But, in the first place, a preponderance of essentially South-^\jnerican 

 forms occiu's in them, more especially the Phj-llostomatous Bats ; and in the next place, if we 

 look at Map 2, which shows the effect of a depression of land to the extent of 600 feet, (an 

 amount which must have been greatly exceeded before a mai'ine channel separating North from 

 South America coidd have been formed,) we see that although thej^ now lie so near North 

 America a great part of the southern extremity of that continent, viz. Florida, Georgia, Ala- 

 bama, South Carolina, &c., must then have been beneath the waves. This would place a much 

 greater distance between North America and these islands than there is now, while their pjresent 

 relations, so far as regards size and distance from South America, would remain comparatively 

 unchanged. The same is the case with Central America. It would still have stood then as 

 now ; and the configuration of the land and water in that region, imder such a depression, 

 gives a great temptation to suppose the connexion of the West Indian Islands to have been 

 with Central America on the one hand, and Venezuela on the other : but the distribution of 

 the mammals does not seem to sanction this, and I rather incline to think, that when North and 

 South America were disjoined it was by a strait at Panama, which tui-ncd up along the eastern 

 coast of Guatemala, and passed to the north of Cuba and Haiti, leaving them and the other West 

 Indian Islands connected with Venezuela on the east and south. 



The South-American half of the New World consists of the whole of South America, the West 

 Indian Islands, Tierra del Fuego, the Falkland Islands, and Galapagos. 



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