

BARRIERS AND MEANS OF DISPERSAL 63 



the pearl-gopher Citellus suslica from that of the common gopher 

 (C. citellus). 26 For lower animals, however, rivers often have more 

 importance as a connection than as a separation. There is no note- 

 worthy difference in the mollusk fauna of the north and south banks 

 of the Amazon, or of the Ganges. Floods cause shifting of the stream- 

 bed and create and destroy islands. Pieces of woodland may be pas- 

 sively transferred from one bank to the other, and with them their 

 fauna. The Hwang Ho, for example, has completely changed the lower 

 part of its course nine times in the past 2500 years. Floating trees 

 and roots afford a means of transport to many animals for an un- 

 planned crossing of the river. 27 Small rivers, and lakes, are of no 

 importance as barriers. 



Mountains and deserts may be placed with the oceans as barriers 

 of importance; in some respects they are even more effective, because 

 climatic changes are intimately associated with them. The sea, if the 

 body of water is not too broad, does not present a barrier to flying 

 animals; it equalizes temperature differences, and nowhere forms a 

 sharp climatic limit. In contrast, a mountain range of considerable 

 height not only forms a barrier to movement, by its steep slopes, but 

 also prevents many animals from reaching or crossing its passes on 

 account of the reduced temperature and lowered atmospheric pressure. 

 The faunae on the north and south slopes of the Himalayas, or on 

 the east and west sides of the American Cordillera, are entirely dis- 

 tinct. The fauna of Africa south of the Sahara exhibits a striking 

 uniformity, for no high mountain chains are present, comparable with 

 the Himalaya in Asia or the Cordillera in America. In consequence the 

 principal groups, and even a great number of species, have an un- 

 usually wide range both from east to west and from north to south. 

 This is equally true of reptiles, birds, mammals, insects, and land 

 isopods. 28, 29 



Mountains of even moderate height may form effective barriers 

 for certain species or even for whole faunae. The starling introduced 

 into New Caledonia in the 70's, has increased on the west coast to 

 such a degree as to be a pest; but it has not crossed the mountains 

 and is absent on the east coast. 30 If the mountain range in question is 

 older than an associated marine strait, its effectiveness as a barrier 

 will be apparent by comparison. Sumatra is traversed for its entire 

 length by a mountain range which separates a northeastern and a 

 southwestern coastal strip. The fauna of the former is in the main 

 like that of the Malay Peninsula, and to a lesser degree like that of 

 Borneo; the endemic species of Sumatra are found south of the moun- 

 tains, and the difference between these two parts of Sumatra exceeds 



