164 THREE CRUISES OF THE " BLAKE." 



lids, etc., can hardly be found anywhere, — comprising genera 

 and species which do not seem to have an extreme bathymetrical 

 range, but are limited to a somewhat narrow continental belt, 

 as I have called it. 



We should remember that, while temperature and light are 

 important factors in the tropics, the temperature adapted for 

 deep-sea animals comes nearer to the surface than in the tem- 

 perate zone. It begins at from three hundred to four hundred 

 fathoms. In the temperate zone the same temperature is not 

 found till we reach a depth of six hundred fathoms. In the 

 polar regions we find the deep-sea forms cropping out far nearer 

 the surface than either in the tropics or in the temjDcrate zone. 

 We are not justified in disregarding the effect of temperature 

 because many species can withstand a considerable range, or be- 

 cause the same or a similar fauna occurs in distant localities 

 under different conditions of temperature. 



Other causes acting for a long period may have led to the 

 isolation of the fauna, as in the case of enclosed seas, like the 

 Mediterranean, the Red Sea, the Sulu Sea, the Caribbean, and 

 the Gulf of Mexico. Deep-sea forms may gradually have become 

 accustomed to varying temperatures and thus have acquired a 

 greater bathymetrical distribution. 



Since the temperature of the sea is nearly the same everywhere 

 in deep water, we have a uniform cosmopolitan fauna, of consid- 

 erable antiquity, at great depths, corresponding to that of high 

 isolated peaks or mountain chains. They have preserved down 

 to our own time the remnants of a former fauna, which may 

 once have been connected with a fauna at lower levels during 

 an epoch of ice or of lower temperature. 



Wallich speaks of the unchecked migrations along the homo- 

 thermal sea, and, as subsequent explorations show, the great 

 submarine folds, like the Dolphin and Challenger ridges, form 

 no barrier to migration. Except in the case of the Mediter- 

 ranean and some of the enclosed seas, they do not rise high 

 enough to reach the belts of higher temperature. In the Gulf 

 of Mexico and in the Caribbean (the American Mediterranean), 

 deep-sea forms are found in abundance, extending to upper lim- 

 its as high as any occurring in the great oceanic basins ; this is 



