DISPERSAL OF NORTH AMERICAN BIOTA. 



57 



first zone or wave (tundral type) have spread from the Ohio val- 

 ley to the Arctic sea, the members of the second wave have 

 moved from much the same southern limit to Hudson Bay, but 



FIG. I. Showing the present location of the five biotic types and the area in- 

 vaded by them in Postglacial times. The transverse lines indicates the southern 

 margin of the last (Wisconsin) ice sheet. 



the third wave, composed of the southeastern and southwestern 

 biota, has been relatively stationary. The first two waves entered 

 the territory they now occupy largely from the soiifh, although 



