ANIMAL AGGREGATIONS AND SOCIAL LIFE 341 



nizes these asexually conditioned groupings. Among the homotypic 

 categories alone are to be found such aggregations as the following: 

 all collections in a favorable locality of limited extent; hibernating 

 groups; animals collected about food; individuals joined in migrat- 

 ing bands; aggregations brought together by tropistic reactions 

 which lead the responding organisms into a limited space; collec- 

 tions due to unfavorable conditions, whether passive, as in drift 

 lines of beetles formed by wind action, or active, due to the moving 

 together of stimulated individuals. From heterotypic categories one 

 finds such social mutualism as exists between flocks of cowbirds 

 and herds of cattle; one animal living upon the shell or covering of 

 another without being parasitic; two different species occupying the 

 same runways, even though these runways are made by only one 

 of the species involved; the well-known relationship between ants 

 and aphids, where the former feed on excretions of the latter and 

 in turn afford them some protection; the same sort of relationship 

 where narcotic material is supplied rather than food; the relation- 

 ship between so-called "robber guests" and their hosts; or that of 

 harmless guests which feed upon fragments dropped by a larger or 

 more active species; or of the animals which make their small nests 

 in the large nest of a larger individual; or of the small individuals 

 which remain in the neighborhood of a larger one without being 

 attacked, and thus avoid attacks from others; or the cases of ani- 

 mals cemented into the built-up covering of another, as caddis-fly 

 larvae use small mussels; or those which live within the body of 

 other animals without becoming parasitic; and, finally, the different 

 types of parasitism. 



All the foregoing list must be dismissed, wholly or in part, as fall- 

 ing outside the range of social phenomena, if the latter is to be 

 regarded as limited solely to those relations which depend upon 

 some sort of a sexual or family basis. While there are many who 

 would be willing to dismiss a part of these as falling without the 

 field of the social life of animals, I know of no student of social life 

 who would dismiss them all. From such considerations as these we 

 are drawn to the conclusion that, important as sex and the family 



