PHYSIOLOGICAL INTEGRATION IN ANIMALS. 379 



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 differentiation 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 commodity. 

 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 neighbouring 

 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 differentiation is again 

 increased; since each district, having a larger market for its 

 commodity, is led to devote itself more exclusively to pro- 

 ducing this commodity. These actions and reactions con- 

 tinue until the various localities, becoming greatly developed 

 and highly specialized in their industries, are at the same 

 time functionally integrated by a network of roads, and 

 finally railways, along which rapidly circulate the currents 

 severally sent out and received by the localities. And it will 

 be manifest that in individual organisms a like correlative 

 progress must have been caused in an analogous way. 



308. Another and higher form of physiological integra- 

 tion in animals, is that which the nervous system effects. 

 Each part as it becomes specialized, begins to act upon the 

 rest not only indirectly through the matters it takes from 

 and adds to the blood, but also directly through the molecular 

 disturbances it sets up and diffuses. Whether nerves them- 

 selves are differentiated by the molecular disturbances thus 

 propagated in certain directions, or whether they are other- 



