3G8 ON THE FAUNA AND BOTTOM-DEPOSITS NEAR THE 30-FM. LINE 



adapted. So much attention has in the past been devoted to the rarer 

 species occurring in any district that the exact conditions under which 

 the common and prevailing forms exist have often received but little 

 attention. Yet the fact that a species is rare in any locality no doubt 

 in most cases means that it is only imperfectly adapted to the con- 

 ditions there prevailing, and that if the locality were cut off from 

 communication with the real centre of distribution the species would 

 soon cease to exist in it. Illustrations of this could be almost in- 

 definitely multiplied. One case in point is referred to in some detail 

 on p. 473. 



It is clear, therefore, that much yet remains to be done all round our 

 coasts in the way of dredging work. A number of detailed surveys in 

 the different districts and in water of different depths is what is really 

 required. Continuous areas should where possible be examined, 

 samples of the bottom - deposits obtained, and all the physical con- 

 ditions observed. The nature of the fauna should in each case be 

 described in such a way that the common and characteristic species 

 are clearly indicated and given due prominence, and the inter-relations 

 of the different species pointed out. Special attention should be 

 directed to all spots where any one species is exceptionally plentiful, 

 in order to ascertain as. exactly as possible the conditions under which 

 it is best able to flourish. 



Section II. 



THE CAUSES WHICH INFLUENCE THE DISTEIBUTION 

 OF LIFE ON THE SEA-BOTTOM. 



In this section I propose to give a brief summary of some of the more 

 important causes influencing the distribution of the animals and plants 

 which live on the sea-bottom, as it will be necessary to constantly bear 

 these causes in mind in considering the nature of the fauna on the 

 various grounds to be described. This summary is based largely on 

 the writings of Forbes, Gwyn Jeffreys, Wyville Thompson, Norman, 

 Berthold, Herdman, MTntosh, Petersen, and Walther. 



The definite distribution of animals and plants in the sea, as on the 

 land, is due to causes some of which are obscure, whilst others, and 

 perhaps the most important ones, are sufficiently obvious. They 

 may, as a matter of convenience, be divided into two classes, the 

 physical causes and the biological causes. The physical causes are 

 the result of those physical conditions which immediately affect the life 

 of the individual members of a species. The biological causes include, 

 in addition to such as are due to the structure and activity of the 



