104 DOCTRINE OF EVOLUTION 



tells us that these islands were once the peaks of moun- 

 tain ranges rising from a Pacific continent which has 

 since subsided to such an extent that the mountain 

 tops have become separate islands. Thus the resem- 

 blances between Hawaiian and Polynesian snails, and 

 the closer similarities exhibited by the species of the 

 various groups of Polynesia, are intelligible as the marks 

 of a common ancestry in a widespread continental 

 stock, while the observed differences show the extent 

 of subsequent evolution along independent lines followed 

 out after the isolation of the now separated islands. 

 The principle may be worked out in even greater 

 detail, for ii appears that within the Umits of one group 

 diverse forms occupy different islands, evolved in 

 different ways in their own neighborhoods; while 

 in one and the same island, the populations of the 

 different valleys show marked effects of divergence in 

 later evolution, precisely as in the case of the classic 

 Achatinellidae of the Hawaiian Islands. 



The broad and consistent principle underlying these 

 and related facts is this : there is a general cor- 

 respondence between the differences displayed by the 

 organisms of two regions and the degree of isolation or 

 proximity of these two areas. Thus the disconnected 

 but neighboring areas of the Galapagos Islands and 

 South America support species that resemble each 

 other closely, for the reasons given before; long iso- 

 lated areas like Australia and its surroundings possess 

 peculiar creatures like the egg-laying mammals, and 

 all of the pouched animals or marsupials with only 

 one or two exceptions like our own x4merican opos- 

 sum, a correlation between a geological and geo- 



