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

chap, in.] CONDITIONS AFFECTING DISTRIBUTION. 37modifications of pre-existing land, than by the upheaval ofentirely new Continents in mid-ocean. These two principleswill throw light upon two constantly recurring groups of factsin the distribution of animals,—the restriction of peculiar formsto areas not at present isolated,—and on the other hand, theoccurrence of allied forms in lands situated on oppositeof the great oceans.shoresContinental Areas.—Although the dry land of the earth'ssurface is distributed with so much irregularity, that there ismore than twice as much north of the equator asof it,there is southand about twice as much in the Asiatic as in the Americanhemisphere; and, what is still more extraordinary, that on ahemisphere of which a point in St. George's Channel betweenEngland and Ireland is the centre, the land is nearly equal inextent to the water, while in the opposite hemisphere it is inthe proportion of only one-eighth,— yet the whole of the land isalmost continuous. It consists essentially of only three masses:the American, the Asia-African, and the Australian. The twoformer are only separated by thirty-six miles of shallow seaat Behring's Straits, so that it is possible to go from Cape Hornto Singapore or the Cape of Good Hope without ever beingout of sight of land ; and owing to the intervention of thenumerous islands of the Malay Archipelago the journey mightbe continued under the same conditions as far as Melbourne andHobart Town.This curious fact, of the almost perfect continuityof all the great masses of land notwithstanding theirextremelyirregular shape and distribution, is no doubt dependent on thecircumstances just alluded to ;that the great depth of the oceansand the slowness of the process of upheaval, has almost alwaysproduced the new lands either close to, or actually connectedwith pre-existing lands ;and this has necessarily led to a muchgreater uniformity in the distribution of organic forms, thanwould have prevailed had the continents been more completelyisolated from each other.The isthmuses which connect Africa with Asia,and Northwith South America, are, however, so small and insignificantcompared with the vast extent of the countries they unite that