6 THE BIOLOGY OF THE SEA-SHORE 



being usually the better defined and more coherent, have 

 generally a marked influence in determining and modifying 

 the animal associations occurring with, and to a large extent 

 depending on, them. Thus it is that many of the large 

 animal communities are named after the plant association 

 with which they occur, since although the nature of the plants 

 depends upon physical conditions, it is the plants (or their 

 absence) which gives the characteristic appearance to a 

 region. We speak in consequence of the fauna of the tropical 

 or coniferous forest, of the Savana, prairie or desert, each 

 of which has its distinctive features. A study, however, 

 in order to be associational need not deal with such a wide 

 area ; in fact, the smaller the area under study the greater 

 probably the amount of ecological agreement. The possible 

 subdivisions of any wide natural area are very numerous, 

 and we may speak with perfect accuracy of the association of 

 a cockle or mussel bed, of a rubbish-heap, or of a rock pool. 

 Any particular association contains forms which, in 

 course of time, have become more or less inter- dependent. 

 Many of the animals, as we have seen, depend entirely upon 

 the plants present ; others prey upon these vegetarians or 

 upon other carnivorous forms, so that a balance is estab- 

 lished which is only altered by some change in the environ- 

 ment, or by the introduction of some fresh factor or form. 

 This balance is relative, not fixed, the centre of gravity 

 swinging to and fro within varying hmits. From time to 

 time some local catastrophe, such, for instance, as a river 

 shifting its course, or a volcano suddenly becoming active, 

 will destroy the old order, and a new balance will slowly 

 be established in its place. Man, particularly, is a disturbing 

 agent and may himself become involved in the confusion 

 resulting from his action. The mongoose in Jamaica, the 

 English sparrow in America, and the rabbit in Australia are 

 striking examples of the overturning of the local balance of 

 nature by the sudden introduction of a completely new 

 factor. 



In considering the many different haunts of life as a whole 



