372 ON THE FAUNA AND BOTTOM-DEPOSITS NEAR THE 30-F.M. LINE 



4. Temperature of the Sea-water. There seems to be some difference 

 of opinion amongst recent writers as to the extent to which temperature 

 is a determining cause in the distribution of marine species. 



The following general principles appear to be beyond dispute. 



Change of temperature, having regard both to the amount of the 

 change and to the rapidity with which the change takes place, is of 

 greater importance in its influence upon marine life than the actual 

 degree of temperature, though at the same time the latter must not be 

 considered to be without effect. 



For each species there is a maximum rate of change of temperature 

 which the species is able to endure without injury, and this rate of 

 change differs greatly for different species. 



Each species is specially adapted to flourish within a particular range 

 of temperature, which differs both in actual degree and in extent of 

 range for different species. 



It also seems probable that, provided the change takes place 

 sufficiently slowly, the particular range of temperature to which a 

 species is adapted may alter, and a race of the species may arise 

 adapted to a temperature range different to that of the species in 

 general. Whether or not selection plays a part in the formation of 

 such races it is impossible in the present state of knowledge to say. 



5. Pressure. The range of many species in depth is very great, so 

 that it is evident — and the fact has been sufficiently recognised since 

 the time of Forbes and Gwyn Jeffreys — that pressure in itself has 

 little influence on the distribution of marine animals, unless the differ- 

 ences of pressure are very great indeed. 



6. The Amount of Light which penet7xitcs to the Sea-hottom. The 

 extent to which light penetrates the sea-water is one of the most 

 important factors in determining the distribution of marine life, since 

 on it depends the existence of the plant life upon which animals ulti- 

 mately depend for their food supply. 



Berthold has shown that different species of marine algae require 

 different light-intensities in order to flourish at their best. Some 

 grow only where the light is very strong, the majority, however, attain 

 their maximum development in situations which are to some extent 

 shaded, or in which the intensity of the light is modified by the depth 

 of water through which it has to pass. 



The amount of light which penetrates the sea-water and becomes 

 available for bringing about the assimilation of plants depends, in the 

 first place, upon the amount of light which strikes the surface of the 

 sea, and will vary therefore with the particular climate and meteoro- 

 logical conditions prevailing in each locality. The depth at which 

 marine algiu will be found will be greater in localities where the sky 



