CHAP, v.] GENERAL CONCLUSIONS. 281 



The Distribution and Nature of the Deep-sea Fauna. — The 

 most prominent and remarkable biological result of the recent 

 investigations is the final establishment of the fact that the dis- 

 tribution of living beings has no depth-limit ; but that animals 

 of all the marine invertebrate classes, and probably fishes also, 

 exist over the whole of the floor of the ocean ; and some of the 

 most interesting of the problems which are now before us have 

 reference to the nature and distribution of the deep-sea fauna, 

 and to its relations with the fauna of shallower water, and with 

 the fauntB of past periods in the earth's history. This is, how- 

 ever, precisely the class of questions which w^e are as yet least 

 prepared to enter into, for every thing depends upon the care- 

 ful study and the critical determination of the animal forms 

 which have been procured ; and this task, which will occupy 

 many specialists for several years, has been only just com- 

 menced. 



My present impression is that although life is thus univers- 

 ally extended, the number of species and of individuals dimin- 

 ishes after a certain depth is reached, and that at the same time 

 their size usually decreases. This latter observation is not, 

 however, true for all groups ; a peculiar family of the Holo- 

 thuridea, very widely distributed in deep water, maintain the 

 full dimensions of the largest of their class, and even exhibit 

 some forms of unusual size. Of the value of our present im- 

 pressions on such questions I am by no means sure. Using 

 all precautions, and with ample power and the most complete 

 appliances, it is extremely difiicult to work either with the 

 dredge or with the trawl at depths approaching or exceeding 

 3000 fathoms. A single dredging operation in such depths 

 takes a long time ; the dredge is put over at day-break, and it 

 is usually dark before it is recovered, so that the number of 

 such ojierations must be comparatively small. It is necessary 

 to take every precaution to keep the ship as nearly as possible 

 in the same place ; and as this can never be done absolutely, it 



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