chap, in.] CONDITIONS AFFECTING DISTRIBUTION. 37 



modifications of pre-existing land, 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 



