v.] VALUE OF NATURAL HISTORY SCIENCES. 63 



course, upon the nature of the subject-matter of Biology ; and I 

 think a few preliminary considerations will place before you in a 

 clear light the vast difference which exists between the living 

 bodies with which Physiological science is concerned, and the 

 remainder of the universe ; between the phenomena of Number 

 and Space, of Physical and of Chemical force, on the one hand, 

 and those of Life on the other. 



The mathematician, the physicist, and the chemist contem 

 plate things in a condition of rest ; they look upon a state of 

 equilibrium as that to which all bodies normally tend. 



The mathematician does not suppose that a quantity will 

 alter, or that a given point in space will change its direction 

 with regard to another point, spontaneously. And it is the same 

 with the physicist. When Newton saw the apple fall, he con 

 cluded at once that the act of falling was not the result of any 

 power inherent in the apple, but that it was the result of the 

 action of something else on the apple. In a similar manner, all 

 physical force is regarded as the disturbance of an equilibrium 

 to which things tended before its exertion, to which they will 

 tend again after its cessation. 



The chemist equally regards chemical change in a body as 

 the effect of the action of something external to the body 

 changed. A chemical compound once formed would persist for 

 ever, if no alteration took place in surrounding conditions. 



But to the student of Life the aspect of Nature is reversed. 

 Here, incessant, and, so far as we know, spontaneous change is 

 the rule, rest the exception the anomaly to be accounted for. 

 Living things have no inertia, and tend to no equilibrium. 



Permit me, however, to give more force and clearness to these 

 somewhat abstract considerations, by an illustration or two. 



Imagine a vessel full of water, at the ordinary temperature, 

 in an atmosphere saturated with vapour. The quantity and the 

 Jigure of that water will not change,- so far as we know, for ever. 



Suppose a lump of gold be thrown into the vessel motion and 

 disturbance of figure exactly proportional to the momentum of 

 the gold will take place. But after a time the effects of this 



