APRIL 57 



up thousands of lakes throughout northern 

 United States and Canada. With the slow 

 descent of the ice-cap the plants were forced 

 to move downward through Asia and 

 through North Ameriea. 



With the return of warmer seasons the 

 ice has retreated until now its southern 

 limit is in British America and lower Green- 

 land, and all these plants are slowly moving 

 back again. The ancestral arbutus broke 

 into two migrating parties, one of which 

 went down through Eastern Asia, the other 

 through Eastern North America. When 

 the time came for the return, these two sets 

 of arbutus had been separated just long 

 enough and had been under surroundings 

 just varied enough to have brought about a 

 little difference in their appearance and 

 habit, and yet their common origin is still 

 clearly traceable. It may seem strange to 

 many, to have the botanist speak of plants 

 migrating. Of course it is not to be under- 

 stood that any one plant uproots itself, 

 travels over the country, and replaces itself 

 again. But every plant tends to spread 



