376 THE EVOLUTION OF LIFE. 



replaces the indirect mode, when these social agencies have 

 either been so long established, or have become so prevalent, or 

 both, as to modify the people's habits and ideas. Groups 

 of citizens unite into corporate bodies which quickly organ- 

 ize, because the habit of forming such combinations has so 

 far modified the thoughts and feelings of citizens, that it 

 becomes natural to them thus to arrange themselves. So 

 too, is it with the men who form a colony. The rapid as- 

 sumption by them of a social structure, as similar as circum- 

 stances permit to the structure of the mother-society, is 

 manifestly due to the fact, that the organization of the 

 mother-society has moulded the emotions and beliefs of its 

 members into conformity with itself; so that when some of 

 its members are transferred to a colony, they arrange 

 themselves directly into a structure of like type with that 

 of the mother-society : they do not repeat all the stages 

 through which the mother-society passed, because their 

 natures have been too far modified to allow of their doing 

 this. That action and reaction between a social 



organism and its units, which we here see accounts for 

 changes in modes of social development, must be paralleled 

 by the action and reaction between an individual organism 

 and its units. Various classes of phenomena compelled 

 us to conclude, that each kind of organism is composed cf 

 physiological units, having certain peculiarites which force 

 them to arrange themselves into the form of the species to 

 which they are peculiar. And in the chapters on Genesis, 

 Heredity, and Variation, we saw reason to believe, that 

 while the polarities of the physiological units determine the 

 structure of the organism as a whole; the organism as a 

 whole, if its structure is changed by incident forces, reacts 

 on the physiological units, and modifies them towards con- 

 formity with its new structure. Now this action and reac- 

 tion between an organic aggregate and its units, tending 

 ever to bring the two into absolute harmony, must be con- 

 tinually making the developmental processes more direct ; 



