CHAPTER VIII. 



LIFE AND FOECB. 



It used to be the custom formerly to speak of life as 

 " vital force," and to suppose that this force was dis- 

 tinct in kind from the ordinary forces^ such as that of 

 heat, or chemical affinity, yet so far similar as to be 

 capable of being transmuted into terms of these forces, 

 so that it could give rise to chemical changes in the 

 same way as these give rise to heat. It is now often 

 denied that there is such a thing as " vital force " ; 

 and while some, taking a strictly materialist view, 

 would prefer to suppose that life is only a special case 

 of the action of forces already partly understood by 

 us, such as heat, or chemical affinity, or kindred to 

 these, others prefer to think of it as something of an 

 entirely different nature from any forces that we 

 understand, as something which acts on the known 

 forces somewhat as the known forces act on matter, 

 transmuting them, while not altering them in quantity. 

 But although we know so little of the nature of life, 

 a good deal has been learnt regarding the general 

 laws of its manifestation ; and something must be said, 

 even in the most elementary explanation of the func- 

 tions of animal life, of their relations with the various 



