28 THE SEAS 



than in any other region except the surface waters of the 

 open sea. This has an immediate effect in the distribution 

 of plant life, for plants can only exist where there is light 

 which is necessary for the " photosynthetic " action of 

 their green pigment by means of which the carbonic acid 

 gas in the atmosphere, or in solution, is combined with 

 water to form starch, which, with the addition of salts 

 obtained from the soil or from solution, forms the food of 

 the plant. The influence of this flora of sea weeds on the 

 life of the sea shore is of great importance. 



The population of the sea shore if it is to withstand these 

 very variable conditions must be extremely hardy — how 

 hardy we realize when we discover that very slight changes 

 in the temperature or salinity of the water will kill animals 

 used to the uniform conditions of the open ocean. The 

 animals must be adapted in many different ways, for pro- 

 tection, food collection, and reproduction, to mention onlj'' 

 three of the most important. Yet in spite of these diffi- 

 culties the population of the sea shore is one of the densest 

 and most varied on the surface of the earth. So dense, 

 indeed, that the most striking features of shore life are the 

 perpetual struggle for existence, the constant scramble for 

 food in which the strongest or most subtle are the con- 

 querors, innumerable devices for ensuring the continuance 

 of the race, the never ceasing pursuit by the more powerful 

 of the weaker and smaller, the latter surviving to the extent 

 to which they are able to disguise or hide themselves. 



We can distinguish very definite associations of animals 

 living on the shore. By this we mean that in different sets of 

 conditions we habitually find the same types of animals, per- 

 haps varying in species from place to place. These animals 

 may be of many different kinds — some of them worms, others 

 starfish, crustaceans, molluscs and so on — but they are all 

 adapted for life under those particular conditions ; they may 



