12 DISTRIBUTION OF ANIMALS. [part i. 



since they inhabit almost all the tropical regions but do not 

 range more than about 10° beyond the southern and 12° beyond 

 the northern tropic, while the great bulk of the species are 

 found only within an equatorial belt about 30° wide. But as 

 these animals are almost exclusively fruit-eaters, their distribu- 

 tion depends as much on vegetation as on temperature ; and this 

 is strikingly shown by the fact that the Semnopithecus schista- 

 ceus inhabits the Himalayan mountains to a height of 11,000 

 feet, where it has been seen leaping among fir-trees loaded with 

 snow-wreaths ! Some northern animals are bounded by the 

 isothermal of 32°. Such are the polar bear and the walrus, 

 which cannot live in a state of nature far beyond the limits of 

 the frozen ocean ; but as they live in confinement in temperate 

 countries, their range is probably limited by other conditions 

 than temperature. 



"We must not therefore be too hasty in concluding, that animals 

 which we now see confined to a very hot or a very cold climate 

 are incapable of living in any other. The tiger was once con- 

 sidered a purely tropical animal, but it inhabits permanently the 

 cold plains of Manchuria and the Amoor, a country of an almost 

 arctic winter climate. Few animals seem to us more truly in- 

 habitants of hot countries than the elephants and rhinoceroses ; 

 yet in Post-tertiary times they roamed over the whole of the 

 northern continents to within the arctic circle ; and we know that 

 the climate was then as cold as it is now, from their entire bodies 

 being preserved in ice. Some change must recently have 

 occurred either in the climate, soil, or vegetation of Northern 

 Asia which led to the extinction of these forerunners of existing 



o 



tropical species ; and we must always bear in mind that similar 

 changes may have acted upon other species which we now find 

 restricted within narrow limits, but which may once have roamed 

 over a wide and varied territory. 



Valleys and Rivers as Barriers to Mammals. — To animals which 

 thrive best in dry and hilly regions, a broad level and marshy 

 valley must often prove an effectual barrier. The difference of 

 vegetation and of insect life, together with an unhealthy atmos- 

 phere, no doubt often checks migration if it is attempted. Thus 



