148 AGGREGATION, COMPETITION AND CYCLES 



of simple aggregation, comparable in a general way with that of 

 plants. The spider probably invades as a consequence of the nocturnal 

 wanderings of adults, the two species then forming an open colony as 

 a result of mixed aggregation. At this point, plants enter to give the 

 sand some small stability, the grass Ammophila in particular being 

 quickly followed by a cutworm and this by Microbembex, a solitary 

 wasp with social tendencies. With a steadily increasing number of 

 invaders, both of plants and animals, complex aggregation as- 

 sumes control of the development, to continue until the climax is 

 reconstituted. 



In the present situation, no definite rule can be laid down as to 

 the type of invaders in bare areas. In certain subseres, the soil com- 

 munity remains in possession more or less intact to constitute the 

 initial stage, which is quickly followed by the visible aggregation of 

 either plants or animals or both. In the case of a severe burn, much 

 of the soil fauna and other animals are destroyed or driven out for a 

 time, and simple or mixed aggregation may follow in a very short 

 time from the wind-blown spores of mosses and liverworts. In pri- 

 mary succession on rock, the process of aggregation concerns lichens 

 alone for many years, the soil algae and fauna and the sparse insects 

 usually appearing with the mosses. On the other hand, the hydrosere 

 exhibits a more complicated type of aggregation inasmuch as plankton 

 and larger aquatic forms are already in occupation, and complex 

 aggregation of a sort operates from the first entrance of the submerged 

 plants characteristic of the initial stage. However, the course and 

 significance of aggregation have received little detailed attention in 

 the past, and this situation will hardly be much improved until the 

 biotic approach to the study of succession becomes more or less the 

 rule. 



Kinds of Aggregation. In the present scanty knowledge of details 

 and of quantities in the process, a comprehensive analysis of aggrega- 

 tion seems to be unprofitable. As a working basis, it appears sufficient 

 to distinguish simple aggregation, which results in a family with wide 

 variation in numbers and generations. Contrasted with this is mixed 

 aggregation, in which two or more species are concerned, giving rise 

 to a colony of plants, animals, or the two combined, and also varying 

 greatly as to numbers. Less definite as to concept but of even greater 

 importance is complex aggregation, characteristic of most serai stages 

 and of climax units, in which the simple and mixed types play a con- 

 tinuous or recurring part. The distinction between simple and mixed 

 aggregation was also later recognized by Dccgener (1918) in his 

 designation of "associations" and "societies" as homotypic and hetero- 



