584 A NATURALIST ON THE "CHALLENGER 



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animals in the deep sea. Time only is required for any deep-sea 

 animal to roam from any distant part of the earth to another. 



It is only in the strata of water, comparatively near the ocean 

 surface, that there is any great difference in range of temperature 

 in various latitudes. Up to a depth of 1,000 fathoms, even from 

 the greatest existing depths, the range amounts only to a few 

 degrees Fahrenheit ; and at 1,000 fathoms everywhere the water 

 is cold and dark, and the conditions of life practically the same 

 as those in the greatest depths ; even at a depth of 500 fathoms 

 the water is almost everywhere as cold as 40° F. The effects of 

 difference of pressure may be neglected, since, when encountered 

 gradually, they would be of no injury to migrating animals. 



Hence, even the ridges, which project up from the ocean 

 floor and separate areas of great depths from one another by 

 intervening expanses, over which the depth is only 1,000 

 fathoms or somewhat less, do not oppose any obstacle to the 

 migration of deep-sea animals. Such ridges will be seen, by 

 reference to the map at the commencement of this work, to exist 

 in the Atlantic and Pacific Oceans. 



In the Atlantic, a long sinuous ridge, with a depth of only 

 1,000 fathoms over it, separates the two deep troughs on either 

 side of the Atlantic from one another, and were the conditions 

 existing in 1,000 fathoms very different from those obtaining in 

 depths of 2,000 and 3,000 fathoms, it might well be conceived 

 that the Western Atlantic deep-sea animals might be isolated 

 from those of the Eastern Atlantic, and very greatly different 

 from them. As will be seen from the map, there is only one 

 narrow channel, lying just north of Tristan da Cunha, in the 

 South Atlantic, where a depth of 2,000 fathoms extends over 

 from one side of the Atlantic to another, and by which thus 

 migration in the supposed case would be possible. 



Similarly in the case of the Pacific, there is only a narrow 

 channel, situate between the Fiji Group and Tahiti, by winch the 

 deep waters of the Southern Pacific communicate directly with 

 those of the Northern. 



The deep-sea animals are however not restricted by these 

 ridges, and the shallows of 1,000 fathoms depth do not act as 

 barriers. Were there any marked isolation by great depth, we 



