DEEP-SEA FAUNA. 115 



oxygen, and of carbonate of lime, phosphate of lime, and other 

 materials necessary for their development, than upon any of the 

 conditions immediately connected with depth. 



"3. There is every reason to believe that the fauna of deep 

 water is confined principally to two belts, one at and near the sur- 

 face, and the other on and near the bottom ; leaving an intermediate 

 zone in which the larger animal forms, vertebrate and invertebrate, 

 are nearly or entirely absent. 



"4. Although all the principal invertebrate groups are repre- 

 sented in the abyssal fauna, the relative proportion in which they 

 occur is peculiar. Thus Mollusca in all their classes, brachyui'ous 

 Crustacea, and Annelida, are on the whole scarce ; while Echino- 

 dermata and Porifera greatly preponderate. 



" 5. Depths beyond five hundred fathoms are inhabited through- 

 out the world by a fauna which presents generally the same features 

 throughout ; deep-sea genera have usually a cosmopolitan extension, 

 while species are either universally distributed, or, if they differ 

 in remote localities, they are markedly representative; that is to 

 say, they bear to one another a close genetic relation. 



" 6. The abyssal fauna is certainly more nearly related than the 

 fauna of shallower water to the faunas of the Tertiary and Sec- 

 ondary periods, although this relation is not so close as we were 

 at first inclined to expect, and only a comparatively small num- 

 ber of types supposed to have become extinct have yet been dis- 

 covered. 



"7. The most characteristic abyssal forms, and those which are 

 most nearly related to extinct types, seem to occur in greatest 

 abundance and of largest size in the Southern Ocean ; and the gen- 

 eral character of the faunsB of the Atlantic and of the Pacific gives 

 the impression that the migration of species has taken place in a 

 northerly direction, that is to say, in a direction corresponding with 

 the movement of the cold undercurrent. 



"8. The general character of the abyssal fauna resembles most 

 that of the shallower water of high northern and southern latitudes, 

 no doubt because the conditions of temperature, on which the dis- 

 tribution of animals mainly depends, are nearly similar." ("The 

 Atlantic," II.) 



nature of the Deep-Sea Fauna.— Much diversity of opinion 

 exists among naturalists as to the nature of the deep-sea fauna. 



