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



different textures, wlien the deposit is once sufficiently firm to support 

 their weight. The distribution of the wandering species will therefore 

 depend, under the same conditions of temperature, water-movement, 

 etc., chiefiy upon the presence of a suitable food -supply and the 

 absence of enemies. 



In considering the geographical distribution of marine animals 

 Forbes introduced the idea of a specific centre, by which he meant 

 the locality, named also the metropolis of the species, where the 

 species flourished in large numbers, and he showed that as one passed 

 away from this locality the species became less and less abundant, 

 until it finally disappeared. In what follows I think I have been 

 able to show that the facts with regard to the local distribution of 

 species are of a very similar kind. Many instances will be given 

 of a species being specially abundant upon one particular patch of 

 ground, where the nature of the bottom-deposit or some other physical 

 condition is specially suited to its requirements, and in almost every 

 case of this kind it will be found that the species is taken in smaller 

 numbers on all the immediately surrounding grounds, even though the 

 general nature of the fauna and of the bottom-deposit of the latter 

 grounds may be very different. In this way the fauna of any particular 

 ground depends not only on the nature of the ground itself, and the 

 physical conditions to which it is subject, but also to a greater or less 

 extent upon the predominant fauna on neighbouring grounds, which 

 in its turn depends upon the physical conditions there existing. 



In the descriptions of the grounds, places where a species is especially 

 abundant will be referred to as centres of distribution of the species, 

 and such centres will be spoken of as influencing the fauna of sur- 

 rounding grounds. 



The numbers of the different grounds have been arranged according 

 to the texture and general character of the bottom-deposit. Grounds I. 

 to VIII. are all fine sand grounds, of which I., II., and III. show the 

 fine sand fauna in its most typical forms. Grounds IX. to XIII. are 

 coarse gravels mixed with sand or mud, of which IX. is the most 

 typical. Grounds XIV. and XV. are fine gravel, of which XIV. is 

 typical. Grounds XVI. and XVII. are clean broken-shell gravel, and 

 Ground XVIII. is stony. In order to understand the nature of the 

 fauna on many of the intermediate grounds, it is necessary that 

 acquaintance should be first made with that of the various typical 

 grounds. 



For a list of the individual hauls see Table I., p. 521. 



