SPECIAL PROBLEMS OF SHORE LIFE 6i 



entirely different habitats, the area between this and the 

 lowest limit of spring tides is one in which marine and 

 terrestrial influences predominate alternately. In other 

 words, the tidal area is a sort of " Tom Tiddler's ground " 

 where land and marine animals may play alternately, pro- 

 vided they can find some means of passing the intervening 

 periods. This is not difficult for the roving higher forms 

 like mammals, birds, and fishes, but the lower forms which, 

 as it happens, form the bulk of the shore population, are 

 mostly incapable of rapid displacements and require special 

 adaptations of structure, function, and behaviour if they 

 are to resist the set of conditions which is inimical to their 

 particular case. 



The problem for animals breathing atmospheric oxygen 

 is the reverse of that which confronts typically marine 

 organisms breathing the oxygen dissolved in water. And 

 since the overwhelming majority of shore animals are of 

 this latter class, the problem for us to consider is how 

 animals breathing dissolved oxygen can endure protracted 

 periods of drought. We shall not, however, omit to discuss 

 the not infrequent cases of the reverse type, particularly 

 as these afford fine examples of evolutionary progress. 



Importance of Rock Pools and of Algae. — To some extent, 

 the problem of the tides, so far as the typically marine forms 

 are concerned, is solved by the permanent occupation of 

 pools ; this is particularly the case on a rocky shore. More- 

 over, even on retreat of the tide, a considerable amount of 

 moisture is retained by sand, stones, and especially algae, the 

 function of the latter being equivalent to the soil mulch 

 adopted by agriculturists in regions of low rainfall. Need- 

 less to say the greatest advantage is taken of this feature of 

 the environment. One has only to turn over a mass of wet 

 Fucus, at low tide, to disclose countless numbers of Gam- 

 marus hiding in the damp weed, and the removal of a flat 

 stone will give a similar result. While sedentary animals 

 have been constrained to respond to the tide-factor by 

 suitable modifications of structure, the effect on actively 

 moving forms has been rather to influence behaviour. The 



