:;S2 



THE AMERICAN NATURALIST 



[Vol. L 



ing of Europe, northern Africa and most of Asia. This mass 

 almost touches at its northeastern corner the somewhat smaller 

 mass of North America. The intervening space, Bering Strait, 

 is only about sixty-five miles wide, very shallow, and dotted with 

 islands. The other gap between these masses is somewhat wider, 

 but still not so great as the shortest distance between South 

 America and Africa. Thus the arctic region is almost encircled 

 by land and itself contains much land. The earth's surface is 

 really one huge northern land mass with three southward pro- 

 jections, namely America, Africa and the East Indian Islands, 

 including Australia. Antarctica is a small disconnected mass at 

 the other end. A species or group of species originating in, or 

 getting into, the far north could, as far as the present configura- 

 tion of the continents goes, populate the earth and have solid 

 ground under its feet most of the way. 



Probably one of the main reasons for the little consideration 

 which students of geographic distribution have given to this route 

 of dispersal is the present climatic conditions in the far north. 

 It seems to have been easier to imagine the ocean's bottom 

 heaved up between Africa and South America than to conceive 

 of a different climate in the northern regions. Yet we have defi- 

 nite and incontrovertible evidence of mild arctic climate in at 

 least several geologic periods, while the only moderately strong 

 evidence of a bridge across the Atlantic, for example, is the pres- 

 ence of related or identical forms on the opposite shores. 



Suppose two of us are known to have been together on Broad- 

 way, but now one of us is in eastern Connecticut and the other at 

 the eastern end of Long Island. One theory might be that we 

 traveled together to the eastern end of Connecticut and then, 

 while one of us stayed there, the other crossed the Sound in some 

 way or other, even though we could not swim that far, there was 

 no regular boat service, and the only evidence of a bridge having 

 been built and then destroyed is that one of us is in eastern Con- 

 necticut and the other in eastern Long Island. It would seem to 

 me more probable that we parted company in Manhattan and, 

 while one of us crossed by a known bridge to Long Island and 

 then had good going along the southern shore of the Sound, the 

 other crossed known bridges and travelled through Connecticut 

 along the northern shore of the Sound. This seems to be the way 

 the respective theories concerning the biogeographical relation- 

 ships of South America, Africa and Australia stand except that 

 we have some other facts. To continue the comparison : although 

 there may be no direct evidence as how one of us came to be in 



