PHYSIOLOGICAL INTEGRATION IN ANIMALS. 371 



possible still further differentiation, will be obtained by con- 

 templating the analogous dependence in the social organism. 

 While it has no roads, a country cannot have its industries 

 much specialized : each locality must produce, as best it can, 

 the various commodities it consumes, so long as it has no 

 facilities for barter with other localities. But the localities 

 being unlike in their natural fitnesses for the various indus- 

 tries, there tends ever to arise some exchange of the commo- 

 dities they can respectively produce with least labour. This 

 exchange leads to the formation of channels of communica- 

 tion. The currents of commodities once set up, make their 

 foot-paths and horse-tracks more permeable ; and as fast as 

 the resistance to exchange becomes less, the currents of 

 commodities become greater. Each locality takes more 

 of the products of adjacent ones, and each locality devotes 

 itself more to the particular industry for which it is naturally 

 best fitted : the functional integration makes possible a further 

 functional differentiation. This further functional differen- 

 tiation reacts. The greater demand for the special product of 

 each locality, excites improvements in production leads to 

 the use of methods which both cheapen and perfect the com- 

 modity. Hence results a still more active exchange ; a still 

 clearer opening of the channels of communication ; a still 

 closer mutual dependence. Yet another influence comes into 

 play. As fast as the intercourse, at first only between neigh- 

 bouring localities, makes for itself better roads as fast as 

 rivers are bridged and marshes made easily passable, the 

 resistance to distribution becomes so far diminished, that the 

 things grown or made in each district can be profitably carried 

 to a greater distance ; and as the economical integration is 

 thus extended over a wider area, the economical differentia- 

 tion is again increased ; since each district, having a larger 

 market for its commodity, is led to devote itself more exclu- 

 sively to producing this commodity. These actions and re- 

 actions continue until the various localities, becoming greatly 

 developed and highly specialized in their industries, are at 



