372 PHYSIOLOGICAL DEVELOPMENT. 



the same time functionally integrated by a network of roads, 

 and finally railways, along which rapidly circulate the cur- 

 rents severally sent out and received by the localities. And 

 it will be manifest that in individual organisms a like corre- 

 lative 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- 

 wise differentiated, it must equally happen that as fast as 

 they become channels along which molecular disturbances 

 travel, the parts they connect become physiologically in- 

 tegrated, in so far that a change in one initiates a change in 

 the other. We may dimly perceive that if portions of what 

 was originally a uniform mass having a common function, 

 undertake sub-divisions of the function, the molecular 

 changes going on in them will be in some way complemen- 

 tary to one another : that peculiar form of molecular motion 

 which the one has lost in becoming specialized, the other has 

 gained in becoming specialized. And if the molecular motion 

 that was common to the two portions while they were undiffer- 

 entiated, becomes divided into two complementary kinds of 

 molecular motion ; then between these portions there will be a. 

 contrast of molecular motions such that whatever is plus in 

 the one will be minus in the other ; and hence there will be a 

 special tendency towards a restoration of the molecular equili- 

 brium between the two : the molecular motion continually 

 propagated away from either will have its line of least resist- 

 ance in the direction of the other. If, as argued 

 in the last chapter, repeated restorations of molecular equili- 

 brium, always following the line of least resistance, tend ever 



