CHAP, v.] GENERAL CONCLUSIONS. 301 



sea-bottom by Mr. Murray ; and several questions of great inter- 

 est must be left open until tlieir investigations are completed. 



The lirst general survey of the deep-sea collections, under- 

 taken with a knowledge of the circumstances under which the 

 specimens were procured, justify us, I believe, in arriving at the 

 following general conclusions : 



1. Animal life is present on the bottom of the ocean at all 

 depths. 



2. Animal life is not nearly so abundant at extreme as it is 

 at more moderate depths ; but as well-developed members of 

 all the marine invertebrate classes occur at all depths, this ap- 

 pears to depend more upon certain causes affecting the compo- 

 sition of the bottom deposits, and of the bottom-water involving 

 the supply of 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 

 surface, and the other on and near the bottom ; leaving an in- 

 termediate zone in which the lai'ger animal forms, vertebrate 

 and invertebrate, are nearly or entirely absent. 



4. Although all the princiijal marine invertebrate groups are 

 represented in the abyssal fauna, the I'elative proportion in 

 which they occur is peculiar. Thus, Mollusca in all their class- 

 es, brachyourous Crustacea, and Annelida, are, on the whole, 

 scarce ; while Echinodermata and Porif era greatly preponder- 

 ate. 



6. Depths beyond 500 fathoms are inhabited throughout the 

 world by a fauna which presents generally the same features 

 throughout. Deep-sea genera have usually a cosmopolitan ex- 

 tension, while species are either universally distributed, or, if 

 they differ in remote localities, they are markedly representa- 

 tive ; that is to say, they bear to one another a close genetic 

 relation. 



