FORMATION OF ANIMAL AGGREGATIONS 43 



optimum space, aggregations may form from either of these two 

 reaction methods, the method used depending in part on the nature 

 of the stimulus emanating from the favorable locality, but, in the 

 main, on the reaction system of the animals involved. If the condi- 

 tions are such that directive stimuli are absent, aggregations, if 

 formed, will result only from the methf: d of "trial." This apparently 

 happens many times in nature and in lx^c laboratory. 



Land isopods (Allee, 1926) tend to collect in aggregations in the 

 hot, dry summer and in the cold, and often physiologically dry, 

 winter. These aggregations are frequently such as might result when 

 shelter is limited, provided there is a tolerance for the presence of 

 other similar animals; but at times these animals collect in much 

 closer units than can be entirely explained on this basis. That is to 

 say, the isopods do not occupy all the available and apparently 

 equally desirable space, but clump together in one part of this. 



When the method of formation of the aggregations is studied in 

 the laboratory, the grouping is found to be brought about by the 

 "selection of random movement" type of behavior. Usually the iso- 

 pods wander over the surface of their container, preferably around 

 the margin, and come to rest in the position in which they are ap- 

 parently less stimulated. Downs (Allee, 1926) made a long series of 

 observations in an attempt to find the method of formation of ag- 

 gregations when conditions were as nearly uniform in all parts of the 

 container as they could be made. Under these uniform environ- 

 mental conditions the land isopods usually wandered about until 

 one came to rest for some reason or other. Sometimes inequalities 

 developed in an originally uniform environment; at other times the 

 isopod apparently stopped for internal reasons. After one became 

 quiet, there was a distinct tendency for others to come to rest near- 

 by. These might or might not be in physical contact with the first; 

 frequently they had crawled over it immediately before stopping. In 

 their incipient stages these bunches were frequently quite loose. The 

 isopods would then alternate periods of rest and of motion. During 

 the latter, many, or perhaps all, might start up again; but often a 

 nucleus remained, consisting of the original individual and one or 

 more others. Around such a nucleus the isopods would again gather, 



