CHARACTERISTICS OF ASSOCIATIONS 3 



which is here the hub of the microcosm." This is the 

 study of aggregate ecology. A third branch of ecology is 

 that which deals with the inter-relations of animals occurring 

 together in the same habitat and regarded as a unit quite 

 apart from any question of hereditary or taxonomic relation- 

 ship among the component members. Every grouping of 

 this kind, in spite of the complexity consequent on the 

 innumerable reciprocal activities of the animals within it 

 and their responses to the environment, will be found on 

 analysis to have a constitution which is specific and which 

 differentiates it from other groupings. The study of such 

 a grouping is the business of associational ecology. It is 

 largely with this aspect of ecology that the present work 

 is to deal. 



Animal Associations. — Dwelling on the earth are a 

 vast number of plant and animal forms which vary greatly 

 as to their relative abundance and distribution. Some 

 have a wide range ; some are restricted to more or less 

 limited areas, others again are rare. Each of these forms 

 has a relationship to the other forms among which it dwells 

 and with which it is associated, hence it is that those forms 

 which live together habitually have come to be termed an 

 association. 



Any group of forms normally occupying the same 

 environment may be regarded as an association, and it is 

 usual to name the association after the most obvious feature 

 of that environment, e.g. the association of the tidal zone. 

 The fact of a number of unrelated animals occurring together 

 in the same habitat implies considerable equivalence of 

 form, function, and habit. The word equivalence is used 

 in contrast to identity, for though the problems presented 

 by the environment may be identical, the means by which 

 they are solved often vary markedly. The working out of 

 these analogies and contrasts forms one of the most interest- 

 ing portions of our study. 



According to Shelf ord (191 5) animals select their environ- 

 ments by one of three methods : (i) by the wide dispersal 

 of the reproductive elements and selective survival, (2) by 



