THE LAW OF EVOLUTION CONCLUDED. 401 



depend more and more on the due amount of nervous dis- 

 charge. 



When we trace up the functions of external organs the 

 same truth discloses itself. Microscopic creatures are 

 moved through the water by oscillations of the cilia cover- 

 ing their surfaces; and various larger forms, as the Turbel- 

 laria, progress by ciliary action over solid surfaces. These 

 motions of cilia are, in the first place, severally very minute ; 

 in the second place they are homogeneous; and in the third 

 place there is but little definiteness in them individually, or 

 in their joint product, which is mostly a mere random 

 change of place not directed to any selected point. Con- 

 trasting this ciliary action with the action of developed loco- 

 motive organs of whatever kind, we see that instead of in- 

 numerable small or unintegrated movements there are a few 

 comparatively large or integrated movements; that actions 

 all alike are replaced by actions partially unlike; and that 

 instead of being very feebly or almost accidentally co-ordi- 

 nated, their co-ordination is such as to render the motions of 

 the body as a whole, precise. - A parallel contrast, 



less extreme but sufficiently decided, is seen when we pass 

 from the lower types of creatures with limbs to the higher 

 types of creatures with limbs. The legs of a Centipede have 

 motions that are numerous, small, and homogeneous; and 

 are so little integrated that when the creature is divided 

 and sub-divided, the legs belonging to each part propel 

 that part independently. But in one of the higher Annu- 

 losa^ as a Crab, the relatively few limbs have motions that 

 are comparatively large in their amounts, that are consid- 

 erably unlike one another, and that are integrated into com- 

 pound motions of tolerable definiteness. 



143. The last illustrations are introductory to illustra- 

 tions of the kind we class as psychical. They are the physio- 

 logical aspects of the simpler among those functions which, 

 under a more special and complex aspect, we distinguish as 



