158 



INTRODUCTION TO PLANT GEOGRAPHY 



northern Asia and smaller ones in northern continental North 

 America, as well as the northernmost insular tracts, were free from 

 major ice-sheets throughout the Pleistocene, owing for example to 

 their ' continental ' climate and especially low precipitation, it is 

 believed that ice at one or more stages of the Pleistocene engulfed 



Fig. 43. — Past and present distributions of Redwoods. Abo\e, known localities 

 of fossil Redwoods (after Chaney) ; below, modern (relic) areas of Coastal Redwood 

 {Sequoia sempervirens) on left and of Sierra Redwood or Big-tree {Sequoiodendron 

 iii<4onteum) on right. (Modified from Cain : Foundations of Plant Geography, copy- 

 right 1944, Harper & Brothers.) 



much of northern North America {see F^ig. 44) and virtually the 

 whole of western and central h^urope as far south as the Thames 

 Valley in the west and the Carpathians in the east. Nor could such 

 extensive glaciation of northern territories obtain without being 

 reflected in a lowering of temperatures to the south and even in 

 the tropics, where some mountains became glaciated, and where 



