152 AGGREGATION, COMPETITION AND CYCLES 



tinctly social groupings occur. As in plants, such unsocial aggregation 

 is a consequence of the response of the individual to physical factors, 

 and the resulting cooperation arises out of the reaction of the group 

 upon the habitat. It differs essentially from cooperation on the social 

 level, in which coactions among the members of the group are the 

 motive forces that bring about group organization and differentiation. 

 However, the simpler type passes more or less imperceptibly into the 

 other and no hard-and-fast line can be drawn between tropistic and 

 social communities (cf. Kropotkin, 1915). 



Cooperation in the Family. Although aggregation into families 

 often involves something of the tropistic relation to a physical factor 

 or to food, it is primarily determined by sex. The simplest coopera- 

 tive unit in this category is the potential family, consisting of a single 

 male and female in which the latter is fertilized, as, e.g., in a nuptial 

 flight. A distinct advance in the nature of the social bond occurs 

 when a pair remains mated for a longer period, such as a season or 

 more. Reproduction leads to a second type of family comprising only 

 the young organisms, in which the binding force may be purely tropis- 

 tic or more or less social in nature. The first step toward parental 

 cooperation is taken when one of the parents, usually the female, takes 

 some concern for the fate of the eggs, and true cooperation results 

 when this is shared by its mate. The first step toward actual coopera- 

 tion within the family occurs when one or both parents remain to 

 care for the young for a longer or shorter time, as do many of the 

 vertebrates. However, this is realized only when the offspring as- 

 sumes a certain and often the major share in the family tasks, as is 

 best illustrated below the human family by Hymenoptera and termites 

 (cf. Forel, 1930). 



In the families of social insects, cooperation finds its chief expres- 

 sion in division of labor and in conservation of food. The former 

 may occur without the origin of castes, as in Belanogaster, in which 

 the older females lay eggs, the younger gather food and materials, and 

 the youngest feed the larvae and tend the nest. Among the social 

 wasps, a distinct worker caste first appears in the vespids, and then 

 remains more or less typical of the three families of social bees, and 

 especially of the honeybee. In certain ants, the division of labor is 

 fourfold, a soldier caste being added to the three, males or drones, 

 queens, and workers found among the social bees. A similar develop- 

 ment of castes has taken place in the termites, but has been carried 

 much further, to the point of producing eight castes, each containing 

 both mules and females, and as many as five or six of these may be 

 found in the termite family (Wheeler, 1923:252). 



