FROM THE EDDYSTONE GROUNDS TO START POINT. 373 



remains clear and the light of the sun intense for long periods, than in 

 those where the opposite conditions prevail. The clearness of the 

 water, which depends upon the amount of solid matter in suspension, 

 will also be a factor of importance. 



In addition to tlie indirect effect of light upon the distribution of 

 animal life in the sea, owing to the dependence of the latter upon plant 

 life, there is also a direct effect, which, however, appears to be of 

 importance only within somewhat wide limits. The distribution of 

 those animals which seek their food chiefly by the aid of sight will 

 evidently be limited by the amount of light which penetrates, whilst 

 the safety of those which serve them as food will be correspondingly 

 increased as the amount of light diminishes. 



II. External Biological Conditions. 



The external biological conditions influencing the distribution of any 

 bottom-living organism (animal or plant), due to the existence at the 

 same time of other living organisms, are often of a complicated nature. 



1. One organism may exert an advantageous influence upon another. 



(ft) By serving as its food-supply ; 



(&) By serving as a fixed base to which it may attach itself ; 



(c) By serving as a movable base, and thus extending the area 



over which a fixed organism can collect its food-supply ; 



(d) By bringing supplies of food to the other organism as well 



as to itself, either by setting up a current, or in some 

 other way ; 

 (c) By affording the other organism means of protection or 

 concealment from its enemies, 



2. One organism may exert a disadvantageous influence upon another, 



(ft) By preying upon it ; 



(&) By fixing upon it in such a way as to destroy it ; 

 (c) As a competitor for a limited food-supply, or for a limited 

 amount of fixing space. 



3. The biological conditions by which the organisms on any particular 

 patch of ground are influenced depend not only upon the organisms 

 living on that ground itself, but also upon the nature and abundance 

 of the organisms living upon neighbouring grounds. 



The meaning of the latter statement can perhaps be best explained 

 by supposing that a particular patch of ground could be cleared of all 

 the organisms living upon it, and then allowed to remain until it had 

 acquired a new population. The nature of this new population would 

 depend upon the physical conditions of the ground, upon the nature 

 of the larval animals which were brought to it by the moving water 



