DISPERSAL AND MIGRATIONS 151 



bird colonists that the avifauna of the two 

 islands differs more than does that of Japan 

 from Great Britain. As Wallace writes : " The 

 birds of the one are extremely unlike those of 

 the other, the difference being such as to strike 

 even the most ordinary observer. Bali has red 

 and green Woodpeckers, Barbets, Weaver-birds, 

 and black-and-white Magpie-Robins, none of 

 which are found in Lombok, where, however, 

 we find screaming Cockatoos and Friar Birds, 

 and the strange mound-building Megapodes, 

 which are all equally unknown in Bali. Many 

 of the Kingfishers, Crow-shrikes, and other birds, 

 though of the same general form, are of very 

 distinct species ; and though a considerable 

 number of birds are the same in both islands 

 the difference is none the less remarkable, as 

 proving that mere distance is one of the least 

 important of the causes which have determined 

 the likeness or unlikeness in the animals of 

 different countries." The world contains many 

 other similar contrasts. 



But seas are not the only barriers to the 

 colonising movements or dispersal of birds. 

 Mountain ranges in not a few parts of the world 

 have proved effectual obstacles to the range ex- 

 pansion of birds. In a similar manner deserts 



