4 LIFE BY THE SEASHORE. 



haunted by these. A little experience on the shore will 

 soon convince you that shore animals are not quite so 

 sharply differentiated from one another as regards food as 

 this description seems to suggest, for some forms seem to 

 indulge in a mixed diet; but at the same time it may be 

 helpful at first to look at the food-supply in this way. 



So far we have seen that the shore area is above all 

 distinguished by its abundant food-supply, but it must not 

 be supposed on this account that life within this area is 

 necessarily easy. It is indeed rather the reverse that is 

 true. In the first place the abundant food-supply has led 

 to a great increase of population, and a consequent increase 

 in the intensity of the struggle for existence among the 

 shore animals, and in the second place the physical environ- 

 ment is so variable as to make heavy demands on the 

 adaptability of the organism. Look at the wreckage which 

 almost every tide strews upon the beach, and you will 

 realise how fierce is the struggle against inorganic nature 

 which goes on in the shore area. 



Let us look for a little at the special peculiarities of the 

 physical environment of shore animals. Kound our coasts 

 one of the most striking of the natural phenomena of the 

 littoral region is the daily ebb and flow of the tide. Twice 

 in each twenty-four hours the waters retreat and leave bare 

 a great stretch of the shore, twice they return, the breakers 

 thundering on the rocks as they advance. As everyone 

 who has had anything to do with the sea knows well, not 

 only does the extent of the rise vary according to the locality, 

 but for the same locality it varies from day to day. Twice 

 in every lunar month occurs the phenomenon of spring tides, 

 when the water rises to an unusual height and sinks to a 

 correspondingly low level. Even these spring tides are, 

 however, not constant, certain tides in spring and autumn 

 rising to a much greater height than the ordinary springs. 

 Later, we shall discuss the importance of these facts to the 

 naturalist, at present we are concerned merely with their 

 importance to the shallow-water animals. The shore area is 

 populated by truly marine animals from high-tide mark 

 downwards ; indeed, certain periwinkles seem to live above 

 the level of all but the highest spring tides. If we begin 

 with these hardy forms and pass downwards to the region 



