244 THE DIRECTION OF MOTION. 



the walls of the capillaries, varies according as change of 

 position changes the effect of gravity in different parts of the 

 body. 



It may be well in passing just to note the bearing of the 

 principle on the development of species. From a dynamic 

 point of view, " natural selection " implies structural 

 changes along lines of least resistance. The multiplication 

 of any kind of plant or animal in localities that are favour- 

 able to it, is a growth where the antagonistic forces are less 

 than elsewhere. And the preservation of varieties that suc- 

 ceed better than their allies in coping with surrounding con- 

 ditions, is the continuance of vital movement in those direc- 

 tions where the obstacles to it are most eluded. 



79. Throughout the phenomena of mind the law 

 enunciated is not so readily established. In a large part of 

 them, as those of thought and emotion, there is no percepti- 

 ble movement. Even in sensation and volition, which show 

 us in one part of the body an effect produced by a force ap- 

 plied to another part, the intermediate movement is infer- 

 ential rather than visible. . Such indeed are the difficulties 

 that it is not possible here to do more than briefly indicate 

 the proofs which might be given did space permit. 



Supposing the various forces throughout an organism to 

 be previously in equilibrium, then any part which becomes 

 the seat of a further force, added or liberated, must be one 

 from which the force, being resisted by smaller forces 

 around, will initiate motion towards some other part of the 

 organism. If elsewhere in the organism there is a point at 

 which force is being expended, and which so is becoming 

 minus a force which it before had, instead of plus a force 

 which it before had not, and thus is made a point at which 

 the re-action against surrounding forces is diminished ; then, 

 manifestly, a motion taking place between the first and the 

 last of these points is a motion along the line of least resist- 

 ance. Now a sensation implies a force added to, or evolved 



