io BIRTH: GROWTH. 



that all force is but the representative of some other force 

 with which it is in exact correlation, and into which it can 

 be re-converted. Thus, in the language of modern science, 

 the motion of a steam-engine is the expression in another 

 form of just so much expansive power in the steam which 

 moves the piston ; and this power, again, is the trans- 

 formed expression and exact correlate of the heat produced 

 by the combustion of so much fuel. Coal and wood, again, 

 represent that amount of force, in the shape of the light and 

 heat of the sun, which was expended in their production ; 

 and when they are burnt, give out again just the same 

 amount. Thus, the light and heat of the sun, in this 

 instance, are transformed, so to speak, into chemical 

 affinity, then into heat and light again, then motive 

 power. And, again, were this motive power interfered 

 with as motion, it might be made to re-appear as heat 

 and light. We may use the term transformation of force, 

 or say that it is the same force throughout, variously 

 manifesting itself as heat, light, motion, etc. It matters 

 not so that we remember that no force can be exercised 

 where no force has been previously expended. It cannot 

 spring up from nothing, any more than can the matter 

 through which it acts. It is, equally with this, measur- 

 able and indestructible. 



It has been said in the last chapter, that life manifests 

 itself in those beings that have it, by birth, growth, de- 

 velopment, decay, and death. To understand the nature 

 of life, we must take these events in succession, and con- 

 sider what series of actions each of them represents, and 

 whether there is any reason to suppose that life is the 

 correlative expression of any other force. 



The term birth, it need scarcely be said, is applied to 

 that period of an animal's life at which, having arrived 

 at a fit state for a more independent existence, it leaves 

 the body of the parent, or, as the phrase goes, is " brought 

 into the world." 



