174 INTRODUCTION TO PLANT GKOGRAPHY [CHAP, 



{Fagus) and Hornbeam {Carpinus). In early historical times the 

 climate tended to be cool and moist, yielding 



(6) the ' Sub-Atlantic ' period which was characterized by con- 

 siderable bog formation. This appears to have given way during 

 the last millennium, and particularly in recent decades, to a warmer 

 and drier period. Meanwhile the forests have been gradually 

 destroyed by Man. 



So far as has been determined, and indeed as might be expected, 

 details of the above changes have varied considerably in different 

 areas, so that in some more, but in others fewer, phases have been 

 recognized. Nor, according to Dr. H. Godwin [in lift.), were the 

 periods as certain and defined as is commonly believed. In general, 

 the tendency has been to find less favourable temperatures to the 

 north, and especially in the Arctic, where at best conditions 

 approximating those of the present-day Subarctic have prevailed. 

 Moreover, difi^erent workers have placed diff"erent interpretations on 

 the ' sub-fossil ' and other evidence available, only the simplest 

 generalization being applicable to the majority of the drastically 

 aflfected parts of the world. However, there has clearly been a 

 climatic succession consisting of three main phases, viz. (i) increasing 

 temperature, (2) culmination of warmth-loving trees, and (3) decrease 

 of warmth-loving trees and appearance of those predominating today. 

 Thus in temperate America there are scarcely any profiles known 

 to yield such tundra plants as occurred in the Subarctic of Europe, 

 but the other periods are closely comparable with the European, 

 comprising a cool Pre-Boreal (Spruce — Fir), a warm and dry Boreal 

 (Pine), a warm and moist Atlantic (Oak — Hemlock — Beech, etc.), a 

 warm and dry Sub-Boreal (Oak — Hickory), and a cool and moist 

 Sub-Atlantic (Spruce — Hemlock, Oak — Beech, etc.). Age-checks by 

 such methods as ' carbon- 14 ' have recently indicated that these 

 climatic changes on the two sides of the Atlantic approximately 

 coincided in time. 



This hypothesis of climatic change suggests that, of the plants 

 which may be segregated into groups because of their preponderance 

 during one or another of these periods, the Atlantic and Sub- 

 Atlantic species favour regions having an oceanic climate and the 

 plants of the earliest or Subarctic period do not avoid coastal regions 

 nowadays, whereas the Boreal and Sub-l^oreal ones favour inland 

 areas of continental climate that tend to be drier and warmer though 

 often given to extremes. Sometimes the species of one category 

 will be so grouped together in an area as to suggest a particular 



