BIOGEOGRAPHY 



nerific zone. But at the edge of the eontinental shelf, the ocean floor drops 

 off rapidlv to great depths. In this great expanse of open sea, there are 

 several depth zones. The surface waters, to a depth of 100 fathoms, com- 

 prise the pelagic zone, which is inhabited by widely ranging fishes. The 

 water here is subject to wave action, and is well oxygenated and lighted. 

 The deeper water, down to 1000 fathoms, comprises the hathyal zone. 

 Here the water is always quiet and poorlv lighted. It grows progressively 

 colder with increasing depth, and is scantily inhabited. Below this is the 

 abyssal zone, into which the sunlight never penetrates. Here the water is 

 always cold and quiet. The living forms to be found there are profoundly 

 modified for life in the abyss. 



The littoral and neritic zones are by far the most richly inhabited. The 

 deep ocean basins form a barrier to the dispersal of these inhabitants of 

 the continental shelves, with the result that different marine floras and 

 faunas may be quite as isolated from one another as are those of the dif- 

 ferent continents. Thus Darwin pointed out that the organisms of the east 

 and west coasts of the Americas are quite different, because they are sepa- 

 rated by a great land mass. Yet about 30 per cent of the fishes on opposite 

 coasts of Panama are identical. Correlated with this is the known geologi- 

 cal fact that the Isthmus of Panama was sul^merged during much of the 

 Tertiary Period. But westward of the continental shelf of the west coast 

 there lies a great expanse of open sea until the islands of the Orient and 

 the south Pacific are reached. The organisms found here are utterlv dif- 

 ferent from those of the American continental shelf, because the open sea 

 has been a formidable barrier to the littoral and neritic flora and fauna. 

 But from the islands of the Orient to Africa, a much greater expanse, there 

 is an almost continuous chain of islands or of continental coast, and the 

 flora and fauna are rather uniform throughout this great region. 



DARWIN'S EXPLANATION 



It appeared to Darwin that these and other puzzling problems of the 

 distribution of plants and animals could be understood if it be assumed 

 that all organisms of a particular group (species or higher group) had 

 migrated from a common place of origin, with subsequent modification. 

 On this basis, one would expect the floras and faunas of those areas which 

 have been isolated from one another longest (the biogeographical re- 

 gions) would be most sharply differentiated. The inhabitants of different 

 parts of the same region ( for example, the mountains and plains of South 

 America ) should resemble one another more closely than the inhabitants 

 of similar parts of different regions ( for example, the mountains of South 

 America and of Africa). That is, when the mountains in any region are 

 elevated during geological ages, the new mountains will be colonized by 

 the inhabitants of the surrounding lowlands. Some of these will be alto- 

 gether unfit for the mountain environment; others will be adaptable to 

 the lower but not to the higher altitudes; while a few can invade the 

 highest ranges; and so the proportions of the various organisms will be 

 different from those which characterize the surrounding lowlands. As a 



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