INTRODUCTORY. [CHAP. 



especially in the southern half of that continent, they have a north- 

 and- south trend. Consequently, whereas in the former area the 

 mountain-ranges have acted as barriers to the dispersal of animal 

 life, there has been no such obstacle to diffusion in America, and 

 animals have been able to distribute themselves according to tem- 

 perature-conditions. It is in consequence of this physical feature 

 that a single species of cold-loving animal like the guanaco can 

 range at the present day from the equator to latitude 55 south, 

 whereas in the Old World the common ibex is now restricted to 

 the isolated mountain-ranges of Europe, and the Siberian ibex is 

 confined to the systems of the Himalaya and the Altai. Such 

 isolated stations could, of course, only have been reached during 

 a period when the general temperature of the northern hemisphere 

 was much colder than it is at present, and the animals were thus 

 enabled to cross the lowlands from one chain to the other ; and 

 that such a cold period formerly existed, there is abundant evidence 

 in the traces of an extensive glaciation found over a large portion 

 of Europe and Asia. Although, therefore, strictly speaking, tem- 

 perature has really had as much effect in the distribution of 

 animals in the Eastern Hemisphere as in the Western, yet, since 

 the glacial epoch, its influences have been considerably masked 

 by the trend of the chief mountain-ranges, and likewise by the 

 greater isolation of the different countries of the former area as 

 compared with the latter. 



These essential differences thus render it impossible to mark 

 out the Old World in the zones of animal distribution which have 

 been attempted for North America ; and they will likewise serve 

 largely to explain the divergence of views on this point which may 

 be noticed between the writings of Drs Wallace and Merriam. 



The latter writer 1 , whose conclusions are mainly based on the 

 evidence of North American animals and plants, is of opinion 

 that in the northern hemisphere " animals and plants are restricted 

 in southward distribution by the mean temperature of a brief 

 period covering the hottest part of the year"; and it is added that 

 in certain districts the mingling of essentially northern types with 

 those characteristic of a more southerly zone is due to the mean 

 temperature of the hottest part of the year being sufficiently low 

 1 Appendix, No. 20. 



