592 METASPERMAE OF THE MINNESOTA VALLEY. 
tively isolated island in this region and its distance from Nova 
Zembla on the one side and Greenland and Iceland on the other 
is slight compared with the distances between Kerguelen, the 
Cape, Terra del Fuego and New Zealand and Chatham Islands. 
The distances being less between the continental or island 
areas of the northern hemisphere than between such areas 
in the southern, we are prepared to expect smaller differences 
between regions on different meridians of the northern than in 
the case of regions similarly situated in the southern extratrop- 
ical regions. The facilities for migration and commingling are 
evidently much more favorable along parallels of latitude in 
the northern than in the southern hemisphere. It happens, 
then, that while in the southern hemisphere the Antarctic re- 
gion is the only one including land in both eastern and western 
hemispheres, in the northern hemisphere the next region south 
of the Arctic region is likewise common to both eastern and 
western hemispheres. This region is the Conifer region of En- 
gler and the Northern region of Drude. Further, in the north- 
ern hemisphere there is from Arctic circle to the equator a 
generally greater latitudinal mixing of plants than in the 
southern and this is apparent even when there is too little of 
it to permit grouping the regions affected under the same di- 
vision. For example, as pointed out by A. Gray and later by 
Miquel, the Japanese-Manchurian region presents striking re- 
semblances to that of the Appallachians; the Californian and 
Mediterranean-Oriental have much in common, and the Prairie 
province of North America is not unlike the Central-Asian 
steppes in its plant-population. Isolation of regions is there- 
fore characteristic rather of the Southern than of the North- 
ern realm and the difference in degree of isolation has had 
much to do with the differences which have arisen between the 
characteristic elements of the Northern and the Southern bo- 
tanical realms. 
Beside the geographical character of the northern hemi- 
sphere certain important geological characters have had an 
interesting effect upon the mixing of the plants in the Northern 
realm. First should be noted that the evidence, geological 
and biological, is in favor of supposing a closer union of 
Alaska with eastern Asia, in Tertiary times. The sharp dis- 
tinction between the plants of Greenland and the Scandinavian 
peninsula compared with the almost imperceptible differences 
between the floras of Alaska and Kamtschatka or Saghalin is 
interestingly explained by this ancient continuity between the 
