5] EVOLUTIONARY DEVELOPMENT AND PAST HISTORY 153 



when identified with plants of known climatic requirements, afford 

 a valuable indication of local conditions. And as a further instance 

 of the significance of climatic change, we shall see in the next 

 chapter how the great diversity of woody plants in North America 

 and eastern Asia is attributable to the fact that in these regions 

 such plants were able to migrate far south before the extending ice 

 and return north after its margin had receded, whereas in Europe 

 they were supposedly forced against the southern mountains or sea 

 and exterminated. For such reasons we must examine in the next 

 chapter the historical bases of modern geographical ranges before 

 we can deal in an understanding way with the distributions we 

 actually find. 



Further Consideration 



The facts and theories advanced in this chapter, as in the case of most 

 others, have usually been gleaned from various specialist and often highly 

 technical sources which the general reader will scarcely wish to consult 

 even if they are available to him. However, further details (and some- 

 times other opinions) may readily be obtained from one or more of the 

 following generalized or introductory books in English : 



Sir A. C. Seward. Plant Life Through the Ages (Cambridge University 



Press, Cambridge, Eng., pp. xxi -j- 601, 193 1). 

 C. A. Arnold. An Introduction to Paleobotany (McGraw-Hill, New 



York & London, pp. xi + 433. i947)- 

 H. N. Andrews. Ancient Plants and the World They Lived in (Comstock, 



Ithaca, N.Y., pp. ix + 279, 1947). 

 John Walton. An Introduction to the Study of Fossil Plants, second 



edition (Black, London, pp. x + 201, 1953). 



