cHAP, 111.] CONDITIONS AFFECTING DISTRIBUTION. 37 
modifications of pre-existing Jand, than by the upheaval of 
entirely new continents in mid-ocean. These two principles 
will throw light upon two constantly recurring groups of facts 
in the distribution of animals,—the restriction of peculiar forms 
to areas not at present isolated—and on the other hand, the 
occurrence of allied forms in lands situated on opposite shores 
of the great oceans. 
Continental Areas.—Although the dry land of the earth’s 
surface is distributed with so much irregularity, that there is 
more than twice as much north of the equator as there is south 
of it, and about twice as much in the Asiatic as in the American 
hemisphere; and, what is still more extraordinary, that on a 
hemisphere of which a point in St. George’s Channel between 
England and Ireland is the centre, the land is nearly equal in 
extent to the water, while in the opposite hemisphere it is in 
the proportion of only one-eighth,—yet the whole of the land is 
almost continuous. It consists essentially of only three masses: 
the American, the Asia-African, and the Australian. The two 
former are only separated by thirty-six miles of shallow sea 
at Behring’s Straits, so that it is possible to go from Cape Horn 
to Singapore or the Cape of Good Hope without ever being 
out of sight of land; and owing to the intervention of the 
numerous islands of the Malay Archipelago the journey might 
be continued under the same conditions as far as Melbourne and 
Hobart Town. This curious fact, of the almost perfect continuity 
of all the great masses of land notwithstanding their extremely 
irregular shape and distribution, is no doubt dependent on the 
circumstances just alluded to; that the great depth of the oceans 
and the slowness of the process of upheaval, has almost always 
produced the new lands either close to, or actually connected 
with pre-existing lands; and this has necessarily led to a much 
greater uniformity in the distribution of organic forms, than 
would have prevailed had the continents been more completely 
isolated from each other. 
The isthmuses which connect Africa with Asia, and North 
with South America, are, however, so small and insignificant 
compared with the vast extent of the countries they unite that 
