RELATIONSHIPS OF METASPERMAE. 593 
two continents. Second, it is important to observe the effect 
of the profounder glaciation of the northern hemisphere 
than ofthe 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 89°. 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 plantforms. 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 Appallachian 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 
—38 
