Chap, xxii.] jMICRATIONS OF DEEP-SEA ANIMALS. 507 



either side of that Ocean from one another, and were the con- 

 ditions 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 mi(^ht 

 be isolated from those of the Eastern Atlantic and be very 

 greatly different from them. 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 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 which 

 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 might have hoped to have met with animals of great 

 antiquity in the deepest holes, since these must possibly be 

 regarded as occupying the sites of very old depressions on the 

 earth's surface. 



Dr. Wallich, in his celebrated work, " The North Atlantic 

 Sea Bed," unfortunately never completed, though so full of 

 most important discussions of deep-sea phenomena, speaks 

 almost prophetically of the migrations of animals which " must 

 take place along the deep homothermal sea ; that great high- 

 way extending from Pole to Pole, which is for ever closed to 

 human gaze, but may nevertheless be penetrated by human 

 intelligence."* 



Marine animals may throughout all time have migrated in 

 the course of generations across the equator, from north to 

 south, by way of the deep sea, and on reaching temperate or 

 cold latitudes may have worked their way up into shallow 

 water and taken to coast life, and assumed forms more or less 

 like those of their ancestors who started on the journey. 



Regarded as a high-road for migration across the equator, 

 the deep sea may well be compared with the summits of those 

 mountain chains which, in a similar manner, have acted as 

 bridges across the tropics for the passage of non-tropical plants. 

 The"deep-sea animals themselves also, considered as a group, 

 may be well compared to Alpine floras, there being many 

 points of analogy between the two assemblages. 



As in the case of Alpine floras, plants which occur at sea- 



* G.C. Wallich, M.D., F.L.S., F.G.S., Surgeon-Major on the Retired 

 List, H.M. Indian Army, "The Atlantic Sea Bed," Pt. i, p. 105. London, 

 Van Voorst, 1862, 



