12 DISTRIBUTION OF ANIMALS. [PART I. 
PREC Laat ga) Es eA OEE Sore ST 
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 
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 
