GEOGRAPHICAL DISTRIBUTION. 



All that we know of the Geographical Distribution of marine animals is, 

 with the exception of very contracted areas, limited to strictly littoral dis- 

 tribution. It is true that the Deep Sea Dredging Expeditions, since the 

 laying of the Atlantic cable, have given us considerable additional informa- 

 tion respecting the geographical distribution of marine life on the bottom of 

 the northern part of the Atlantic, but that is as yet only sketched out roughly, 

 though sufficiently to show us the probable mode of distribution of animals 

 at different depths according to the temperature, forming tolerably well- 

 defined belts, similar to the great belts of distribution of animals and plants 

 as we ascend in altitude or in latitude, though the latter does not form ap- 

 parently so important a part in the distribution of marine life as in the case 

 of land animals and plants. On land, when we have alpine and arctic as- 

 semblages of similar animals and plants occurring in far distant geographi- 

 cal points (latitudinally distant), the identity of forms extends to but few 

 types ; while, as far as we can judge from the little we know of the inhabi- 

 tants of the deeper parts of the Atlantic, the same species have a wide 

 geographical range far exceeding any similar distribution on land. While 

 on land we have a circumpolar arctic fauna extending with outliers at only 

 comparatively short distances on the highest parts of the adjoining temper- 

 ate tracts, we have probably an abyssal circumpolar marine fauna extending 

 far into the tropics, surrounded at its lowest base (the littoral edges) by 

 fauna? of the most, diversified character, — a feature entirely unknown in the 

 distribution of life upon the land. Of course we have the analogous feature 

 of the change of life as we proceed in altitude in a mountain-chain, but that 

 is a homologue of a similar change in marine life as we change the depth ; 

 and although we have something analogous to a well-characterized fauna 

 surrounded by widely different elements east and west and north and south 

 in the Rocky Mountains, yet we have nowhere such a diversified condition 

 of life as we find in the littoral elements surrounding what at present 

 we may call the Atlantic Realm, surrounded by the circumpolar littoral 



