592 METASPEKMAE OF THE MINNESOTA VALLEY. 



lively 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 Appalachians; the Calif ornian 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 



