The geographical distribution of animals, with a study of the relations ...

;12 DISTRIBUTION OF ANIMALS. [part i.since they inhabit almost all the tropical regions but do notrange more than about 10° beyond the southern and 12° beyondthe northern tropic, while the great bulk of the species arefound only within an equatorial belt about 30° wide. But asthese animals are almost exclusively fruit-eaters, their distributiondepends as much on vegetation as on temperature ;and thisis. strikingly shown by the fact that the Semnopitheeus schistaceusinhabits the Himalayan mountains to a height of 11,000feet, where it has been seen leaping among fir-trees loaded withsnow-wreaths ! Some northern animals are bounded by theisothermal of 32°. Such are the polar bear and the walrus,which cannot live in a state of nature far beyond the limits ofthe frozen ocean ;but as they live in confinement in temperatecountries, their range is probably limited by other conditionsthan temperature.We must not therefore be too hasty in concluding, that animalswhich we now see confined to a very hot or a very cold climateare incapable of living in any other. The tiger was once considereda purely tropical animal, but it inhabits permanently thecold plains of Manchuria and the Amoor, a country of an almostarctic winter climate. Few animals seem to us more truly inhabitantsof hot countries than the elephants and rhinocerosesyet in Post-tertiary times they roamed over the whole of thenorthern continents to within the arctic circle ;and we know thatthe climate was then as cold as it is now, from their entire bodiesbeing preserved in ice. Some change must recently haveoccurred either in the climate, soil, or vegetation of NorthernAsia which led to the extinction of these forerunners of existingtropical species ;and we must always bear in mind that similarchanges may have acted upon other species which we now findrestricted within narrow limits, but which may once have roamedover a wide and varied territory.Valleys and Rivers as Barriers to Mammals.—To animals whichthrive best in dry and hilly regions, a broad level and marshyvalley must often prove an effectual barrier. The difference ofvegetation and of insect life,together with an unhealthy atmosphere,no doubt often checks migration if it is attempted. Thus