830 Proccedivgs of the British Association. 



animals of North America and those of Europe, as regarded their 

 generic distinctions, connected with the dissimilarity of their species, 

 rendered them well adapted to inquiries connected with their re- 

 spective geographic distribution. Hitherto the trivial names bestow- 

 ed by the colonists upon many of those of North America, had tend- 

 ed to mislead naturalists. The observations in the present report 

 would principally refer to the western parts of North America, in- 

 cluding New Mexico, the Peninsula of Florida and California, down 

 to the well-defined limits of the South American zoological province. 

 Dr Richardson then proceeded to describe the physical structure of 

 this country, of which the Rocky Mountains formed a most remark- 

 able feature. The altitude of many of their peaks rose above the 

 limits of perpetual snow, and their sides were flanked by zones of 

 different temperature, affording passages for animals from the Arctic 

 circle to the table lands of Mexico, without any great alteration of 

 climate throughout the whole extent. The temperate zones of both 

 hemispheres might, in this way, be connected, were it not that the 

 Cordilleras were greatly depressed at the Isthmus of Panama, and 

 that a plain extended from sea to sea a little further to the south. 

 As yet we possess no information of the elevation of the backs of 

 these mountains, independent of the heights of some of the peaks, and 

 the elevation of the base of the range is equally unknown. The 

 depths of some of the transverse valleys are considerable, and the^e 

 afford passages for the migration of animals. Most of the princi- 

 pal rivers flowing to the east cut across the chain, and one actually 

 rises to the west of the crests of the range. On the Atlantic side 

 are prairies, composing plains gently inclining to the east, and there 

 is an extent of land which may be likened to a long valley, which 

 stretches from the Arctic sea to Mexico, without any transverse 

 ridges dividing it, but merely affording three distinct water-sheds. 

 The greatest width of the plain is about 15° of longitude, in the 

 40° to 50° of north latitude. This configuration gives great facility 

 for the range of herbivorous quadrupeds from north to south, and 

 for the migration of low-flying birds ; whilst the Mackenzie fur- 

 nishes a channel by which the anadromous fish of the Arctic Sea 

 can penetrate 10° or 11° of latitude to the southward, and the 

 Mississippi enables those of the Gulf of Mexico to ascend far to the 

 north. The most remarkable chain east of the Mississippi, is that of 

 the Alleghanies, which is .about 100 miles broad, rises from a base 

 between 1000 and 1200 feet, and attains an elevation from 2000 to 

 3000 feet above the sea. The strip of land between them and the 



