CHAPTER VII 



Parasitism, Commensalism, and Symbiosis on the 

 Sea-shore 



We have seen, in the early part of this book, that, under 

 the influence of environmental factors, plants and animals 

 become sorted out into numerous more or less distinct 

 groupings or communities, the members of any one com- 

 munity being, generally speaking, related to one another in 

 virtue of the similar nature of the problems set them by 

 their surroundings. In not a few^ cases, however, the 

 relationships between certain members of a community are 

 of a more intimate kind than is normal. These cases fall, 

 for the most part, under three headings : (i) commensalism ; 

 (2) parasitism ; and (3) symbiosis ; but they cannot all be 

 defined with equal exactitude, nor is the significance of any 

 one of these associations to the forms concerned in it always 

 of the same importance. 



Such associations are undoubtedly a result of the stimulus 

 of competition, and the frequency of their occurrence on the 

 sea-shore is eloquent testimony to the intensity of the 

 struggle for existence in this area. They are the expression 

 of a general tendency of evolutionary processes, acting in 

 conjunction with or superposed upon the struggle for 

 existence, to link lives together. The same peculiar con- 

 ditions which have led to the growth of a numerous, varied, 

 and resourceful population on the sea-shore have at the 

 same time been the spur to the formation of the peculiar 

 types of association we are about to discuss. The conditions 

 we have in mind are, more especially, the narrow width of 

 the tidal zone, the usually broken nature of its surfaces, the 



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