120 DYNAMICAL GEOLOGY. 



pine-like trees and birches, corresponding to subarctic 

 and arctic zones ; 4. A circumpolar region of shrubs and 

 grasses ; and, 5. Perhaps a plantless or nearly plantless 

 region at the pole of cold. Here, again, No. 2 may be 

 subdivided into a warm-temperate region of evergreens, 

 and a cooZ-temperate region of deciduous trees. Here, 

 again, also the regions graduate insensibly into each 

 other. 



We have been speaking thus far of sea-level or near 

 sea-level. Of course, if a mountain in any latitude rises 

 to perpetual snow, we will have on its sides all the tem- 

 perature regions, except those south of it. Thus, in 

 ascending the Sierra Nevada we have a temperate region 

 (No. 2) at base, a subarctic region (No. 3) half-way up, 

 and a circumpolar region (No. 4) at the summits. 



Completer Definition of Regions. 1. All organic 

 forms will spread in all directions as far as physical con- 

 ditions and the struggle for life with other species will 

 allow. The area over which they thus spread may be 

 called their " range." Now, the range of one species may 

 be much greater than that of another, because more 

 hardy ; but the range of a species is always more restricted 

 than its genus, for when the species can go no farther, 

 another species of the same genus will continue the genus. 

 For the same reason the range of a family is greater than 

 that of its genera, etc. Thus, for example, in going up 

 the Sierra we find the range of pines extend from 2,000 

 to 10, 000 feet above sea-level, but the genus is represented 

 by a succession of species of much more restricted ranges. 

 We find, first, the nut-pine (Pinus tiabiniana), then the 

 yellow -pine (P. ponderosa), then the sugar-pine (P. Lam- 

 bertiana), then the tamarack-pine (P. contorta), and, 

 last, the mountain-pine (P. Jlexilis). Hereafter we shall 

 speak mostly of species. 



2. We have said that the several temperature regions 

 graduate insensibly into each other. We will now explain 



