AGGREGATION AND MIGRATION 



239 



The above illustrates the law of simple aggregation, \-iz.. that 

 lack of dissemination promotes the grouping of parent and offspring 

 into famihes, while mobility hinders it. The production of famihes, 

 moreover, takes place more readily in new and denuded habitats, 

 i.e., in open formations, than in closed ones, since in the latter 

 the indi\'iduals of various species have already become mingled 

 with each other. If all species were immobile, famiUes would be 





Fig. 97. ^lixed aggregation due to the migration of Wagnera until the 

 family includes indiA-iduals of Mentzeiia, Elymits, etc., and changes into 

 a community. 



characteristic of vegetation to a large degree. Since the great 

 majority are more or less mobile, groups of this sort are the ex- 

 ception rather than the rule. 



253. Mixed aggregation. Indi\'iduals are carried away from the 

 family group by niigraiion. and strange individuals are brought 

 into it by tlie same means. In the early gro\\th of a family, due 



