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

-CHAPTEK III.DISTRIBUTION AS AFFECTED BY THE CONDITIONS AND CHANGES OFTHE EARTH'SSURFACE.The distribution of animals over the earth's surface, is evidentlydependent in great measure upon those grand and importantcharacteristics of our globe, the study of which is termed physicalgeography. The proportion of land and water ; the outlines anddistribution of continents; the depth of seas and oceans; theposition of islands ;the height, direction, and continuity of mountainchains ;the position and extent of deserts, lakes, and forests ;the direction and velocity of ocean currents, as well as of prevalentwinds and hurricanes ; and lastly, the distribution of heat andcold, of rain, snow, and ice, both in their means and in theirextremes, have all to be considered when we endeavour toaccount for the often unequal and unsymmetrical manner inwhich animals are dispersed over the globe. But even thisknowledge is insufficient unless we inquire further as to theevidence of permanence possessed by each of thesefeatures, inorder that we may give due weight to the various causes thathave led to the existing facts of animal distribution.Land and Water.—The well-known fact that nearly threefourths of the surface of the earth isoccupied by water, and buta little more than one-fourth by land, is important as indicatingthe vast extent of ocean by which many of the continents andislands are separated from each other. But there is another fact