FROM THE EDDYSTONE GROUNDS TO START POINT. 369 



organism itself, all those influences to which a species is subjected in 

 consequence of the existence at the same time of other living organisms. 

 Such influences may be spoken of as the result of the external biological 

 conditions to which the species is subjected. In ultimate analysis, 

 however, the external biological conditions affecting the distribution 

 of a species in any locality are themselves largely, if not entirely, 

 due to physical conditions acting in the same or in neighbouring 

 localities. 



Since the assemblage of animals and plants forming the fauna and 

 flora of any particular area is merely a collection of a number of 

 individual species, the causes determining its nature will be the same 

 as those which determine the distribution of species. In the present 

 paper we are more particularly concerned with the distribution of a 

 portion of the bottom fauna and flora of the sea, of those organisms 

 which live in intimate relation with the bottom-deposit and are captured 

 by the use of the naturalist's dredge or of the trawl. 



I. Physical Conditions. 



Those physical conditions, the variations of which influence the life 

 of bottom-living species, are capable of definite statement, and for the 

 most part of accurate measurement. They are — 



1. The constitution of the sea- water. 



2. The nature of the bottom-deposit. 



3. The movements of the water, due to 



(a) wave action, 

 (&) currents, 

 (c) tides. 



4. The temperature of the sea-water. 



5. The pressure, varying with the depth of water, to which the 

 organisms are exposed. 



6. The amount of light which penetrates to the sea-bottom. 



1. The Constitution of the Sea-water. Variations in the constitution 

 of the sea- water of sufficient importance to have any marked effect 

 upon the bottom-living animals and plants are only met with in the 

 neighbourhood of the land. These variations are (1) the addition of 

 fresh-water, indicated by a lowering of density, which is the result 

 of the land-rainfall ; (2) the presence of soluble substances derived 

 from the drainage of the land ; (3) the presence of solid substances in 

 suspension either brought down directly by the land-water, or stirred 

 up by the action of waves on the coast and in shallow parts of the sea, 

 and (4) in all probability the amount of oxygen and carbonic acid 

 dissolved in the sea-water. 



