I.] BARRIERS TO DISPERSAL. 15 



owing to the fact that in districts where vegetation is luxuriant, 

 huge natural rafts are formed by the trunks of trees intermingled 

 with vegetable matter, upon which numbers of animals may be 

 borne down stream, and thus transferred from one bank to the 

 other. Nevertheless, in treeless districts, or near their mouths, 

 large rivers afford absolutely impassable barriers to the movements 

 of mammals. In South America, for instance, even such an 

 aquatic creature as the carpincho, or capibara (ffydrochcerus) has 

 been unable to cross the Plata river from Uruguay into the 

 Argentine, while, conversely the viscacha (Lagostomus) of Argentina 

 is prevented by the same river from reaching Uruguay. 



Deserts are, perhaps, even more impracticable than rivers ; the 

 Sahara which was long supposed to have been the site of an 

 ancient sea, although it has really been a desert since very remote 

 ages having apparently formed a barrier preventing the fusion of 

 the mammals of North Africa with those to the south of that tract 

 since at least the Pliocene epoch. It must not, however, be sup- 

 posed that what are desert-tracts at the present day have always 

 been such, the existence of a fossil chimpanzee in North-Western 

 India during the Pliocene period indicating that the open sandy 

 plains of the Punjab were at that time covered with dense tropical 

 forests, and probably that the same was the case with parts of 

 Syria and Arabia. 



Before taking leave of seas and deserts, it should be mentioned 

 that in the polar regions ice may act in lieu of a land-connection 

 to enable mammals to pass from one country to another. On this 

 subject Dr Heilprin writes that " the reindeer is stated to cross 

 the Bering Strait by way of the Aleutian Islands and the Frozen 

 Sea, and in a somewhat similar manner the musk-ox finds its way 

 to Melville Island; it is, however, somewhat singular that the last- 

 named animal, despite its long ice-journeys, never manages to 

 reach either the continent of Asia or Greenland." 



High mountain ranges form an effectual barrier to the migration 

 of mammals, not only on account of the physical difficulties of 

 crossing them, but likewise by the lowness of the temperature at 

 great altitudes, coupled with the absence of proper food, being 

 fatal to the existence of many. As already stated, however, 

 mountain-ranges are much more far-reaching in their effects on such 



