RELATIONSHIPS OP METASPERMAE. 593 



two continents. Second, it is important to observe the effect 

 of the profounder glaciation of the northern hemisphere 

 than of the southern. The largest continuous area of glaciation 

 is that of the North American continent. Here it comprises most 

 of the land east of long. 97 W. of Greenwich and north of lat. 

 42, although it extends south to 39. The next largest is the 

 area of western and central Europe where it comprises the 

 territory east of western Russia and north of Poland and 

 Germany. In the eastern hemisphere it extends south to 51 

 N. lat., or to a region of temperature approximately equal to 

 that of southern Illinois, in North America. Other drift-areas 

 in the northern hemisphere, such as those of the Alps, the 

 Pyrenees, the Carpathians, the Himalayas, the Cordilleran 

 range or the Tennessee mts. are more strictly local, but have 

 played their part in the commingling of plant forms. The 

 effect of the glaciation of the North American and western 

 European areas has been productive of a distribution of dis- 

 tinctively northern plants ("glacial plants") southward, as 

 one of the more simple results. More indirectly it has been 

 productive of diversity in the flora of the northern extra- 

 tropical regions by the forced origin of new forms during the 

 earlier southward movements and the succeeding northward 

 returns. As has been noticed by many writers this diversity 

 is greater in the western hemisphere than in the eastern, 

 evidently on account of the different continental positions of 

 the principal mountain ranges. In North America the Rocky, 

 Sierra, Coast and Appalachian systems all run from north 

 to south and present to north-bound or south-bound plants no 

 barrier, but rather an appreciable assistance by way of pro- 

 viding different altitudes at which acclimatisation might 

 progress most comfortably. In the old world, the Pyrenees, 

 Alps, Apennines, Carpathians, Caucasus and Himalaya moun- 

 tains maintain a generally east and west direction, and to plants 

 migrating southward before the glaciers would have presented 

 an impassable barrier. Decimation of old-world species 

 would thus result in the conditions of difference as seen to-day 

 between the old world and North America, where the migra- 

 tions were not opposed by the topography of the country. 

 In both the proximate and remote movements of plants under 

 the influence of widespread continental glaciation, the higher 

 mountain ranges, by presenting a wider range of temperature 

 in latitude, to be compared with the range of temperature 



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