IV TEMPERATE FOREST REGIONS 97 



greater extent of northern land from which the existing 

 forest-trees originally came, and also a greater extent of 

 southern lowlands extending uninterruptedly into the 

 tropics, for them to retreat to during the period of cold. 

 All the conditions were here favourable, first for the 

 production and next for the preservation of a rich 

 flora. 



The poverty of Western America in deciduous trees 

 and its richness in conifers, Professor Asa Gray considers 

 to be a more difficult and at present an insoluble problem. 

 But here, too, a consideration of the physical character of 

 the country suggests an intelligible explanation. Conifers 

 are more especially mountain j^lants, while deciduous trees 

 abound most in the lowlands. Now in North-west 

 America there is a vast stretch of mountains from the 

 extreme north to the far south, and no extensive lowlands 

 — exactly the reverse of what obtains in Eastern America, 

 where the lowlands are vastly more extensive than the 

 mountains. Conifers, therefore, most likely always abounded 

 most on the western side of the continent, and during 

 their enforced southern migrations always found suitable 

 mountain habitats. The deciduous trees, on the other 

 hand (always, probably, few in number), were many of 

 them exterminated in their migrations first southward and 

 again northward, for want of suitable places of growth, or 

 were ovei'powered by the greater vigour of the competing 

 coniferous trees. 



Turning again to Eastern Asia wx find a combination of 

 both these conditions. Ample mountain ranges traverse 

 every part of it from the Arctic circle to the tropics, but 

 these are everywhere interrupted by great river- valleys 

 and extensive plateaus of moderate elevation, thus offering 

 equally favourable conditions for the preservation of both 

 kinds of trees ; and here we accordingly still find the richest 

 and most perfectly balanced woody vegetation of the north 

 temperate zone. 



The marvellous history that we have here sketched in 

 the merest outline, teaches us that our own country has 

 been denuded of its proper share of wild trees and shrubs 

 by a great natural catastrophe — the Glacial epoch — which 



VOL. II. H 



