ORGANIC E VOL UTION. 49 



into another genus, family, or order, we should 

 expect on this theory that the organic types 

 living on any given geographical area should be 

 found to resemble or to differ from organic types 

 living elsewhere, according as the area is con- 

 nected or disconnected with other geographical 

 areas. And this we find to be the case, as 

 abundant evidence proves. For, to quote from 

 Mr. Darwin, " barriers of any kind, or obstacles 

 to free migration, are related in a close and im- 

 portant manner to the differences between the 

 productions of various regions. We see this in 

 the great difference in nearly all the terrestrial 

 productions of the New and Old Worlds, ex- 

 cepting in the northern parts, where the land 

 .almost joins. . . . We see the same fact in the 

 great difference between the inhabitants of Aus- 

 tralia, Africa, and South America under the 

 same latitude, for these countries are almost as 

 much isolated from one another as possible. On 



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