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 temperate 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 1s 
