350 ANIMAL AGGREGATIONS 



larly valuable in that it dramatizes the possibilities of co-operation 

 under group organization, even in non-human communities. 



Restating the general argument of the present chapter, we may 

 say that it seems quite possible that sex arose originally from the 

 beneficial stimulation received as a result of the aggregation of two, 

 or more, simple asexual organisms. Sex, once originated, became 

 one of the integrating factors in further social development. In sex- 

 conditioned society the offspring of one pair or of one parent may 

 have remained in association with their parents for the immediate 

 mutual benefit of all concerned, or there may have intervened a 

 sexually promiscuous horde life from which the consociation of off- 

 spring with their individual parents arose as a further protective 

 evolution. When these lengthened associations of parent and off- 

 spring continued long enough, a division-of-labor t\pe of society 

 could evolve. At first this would occur between parents and mono- 

 morphic offspring; later, dimorphic and even polymorphic offspring, 

 as in ants and termites, might develop. However it arose, the family 

 and the highly integrated division-of-labor society which may origi- 

 nate through it is only one type of expression of the fundamental 

 tendency of animals to aggregate. There are other social phases of 

 animal life which have developed on this same foundation of animal 

 aggregations as a result of forces not centering about sex; these 

 have produced social units of importance in animal life. 



A part of the difficulties we have encountered in discussing the 

 role of different types of animal aggregations in the evolution of 

 social groupings may be avoided if we recognize that there are 

 many levels of social organization and that these overlap. Among 

 the groups which we may fairly call "social" there are: (i) those 

 that show their social habit merely through the toleration of the 

 close proximity of other similar individuals in the same restricted 

 space — these may exist without any positive mutual attraction and 

 may be called the toleration level; (2) those that form groups 

 which react more or less definitely as units — the group integration 

 level; (3) those which show physiological division of labor; finally 

 (4), those that show morphologically distinct castes, each associated 

 with some phase of the division of labor. The animals on the higher 



