378 PHYSIOLOGICAL DEVELOPMENT. 



which escape through the lungs, there are waste products 

 which escape through the skin, the kidneys, the liver. The 

 blood has separated from it in each of these structures, the 

 particular product which this structure has become adapted 

 to separate ; leaving the other products to be separated by the 

 other adapted structures. How have these special adapta- 

 tions been made possible? By union of the organs as 

 recipients of one circulating mass of blood. While there is no 

 efficient apparatus for transfer of materials through the body, 

 the waste products of each part have to make their escape 

 locally; and the local channels of escape must be competent 

 to take off indifferently all the waste products. But it 

 becomes practicable and advantageous for the differently- 

 localized excreting structures to become fitted to separate 

 different waste products, as soon as the common circulation 

 through them grows so efficient that the product left unex- 

 creted by one is quickly carried to another better fitted to 

 excrete it. So that the integration of them through a 

 common vascular system, is the condition under which only 

 they can become differentiated. Perhaps the clearest 



idea of the way in which differentiation leads to integration, 

 and how, again, increased integration makes possible still 

 further differentiation, will be obtained by contemplating the 

 analogous dependence in the social organism. While it has 

 no roads, a country cannot have its industries much special- 

 ized: 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 industries, there tends 

 ever to arise some exchange of the commodities they can 

 respectively produce with least labour. This exchange leads 

 to the formation of channels of communication. The cur- 

 rents 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 



