AGGREGATION AS A TROCESS 147 



Sargasso Sea being the outstanding example; and light may have a 

 distinct effect upon motile forms. 



As with plants, aggregation among animals is regularly a direct 

 outcome of reproduction in the absence of dispersive processes. Like- 

 wise, it may result from the compulsion of such factors as currents 

 of air or water, or from a more definite tropistic response to light, 

 temperature, solutes, etc. More complex and autonomic in nature is 

 aggregation in consequence of the search for shelter or for food, and 

 still more a matter of internal urge is the grouping arising from the 

 quest for mates. Such aggregation not only contains in itself a rudi- 

 ment of social grouping, but, even more important, it leads to the 

 reproduction upon which family aggregation at the various levels of 

 integration is based. 



It is evident that compulsion, tropism, and self-regulated move- 

 ment may be combined in endless variety and that they may operate 

 to produce or modify communities of all sorts, from the simplest family 

 to the biome itself. In the latter, the plan is naturally most compli- 

 cated and the pattern is to be recognized only through the analysis 

 of the coactions that have led to the integration of the innumerable 

 minor communities. This is well exemplified by the pioneer attempt 

 of Forbes to sketch the coaction bonds operating in the black-bass 

 community of fresh water (1887; Allee, 1931, a:83). 



In two illuminating chapters (1931:38-80), Allee has discussed 

 the physical factors and the animal responses concerned in the forma- 

 tion of families and colonies, as disclosed by the experimental studies 

 of a considerable number of investigators. He has also summarized 

 the results so far obtained in determining the sense directive in vari- 

 ous types of integration, for example, touch in harvestmen, odor in 

 moths, sight in catfish, and sound in beetles and ants (pp. 88-97). 



Aggregation on Land. Definite studies of the process of aggrega- 

 tion in connection with the origin or modification of terrestrial commu- 

 nities have dealt chiefly with plants and with primary reference to 

 succession (Clements, 1910, 1916). The investigation of grouping in 

 land animals has been largely incidental to other objectives, though 

 Shelford has described pioneer aggregations concerned in succession 

 (1911, a-e; 1913, a). The first organisms to invade bare areas in 

 sand dunes at the south end of Lake Michigan are the tiger beetle, 

 Cicindela lepida Dj., and the spider, Geolycosa pikei Marx. The en- 

 trance of the adult beetles may occur in autumn, or in spring when 

 the eggs are deposited in the sand at some distance from one another. 

 Upon hatching, each larva remains in position, merely drilling down- 

 ward to a depth of about 45 centimeters. This constitutes an example 



