CHAPTER XXI. 



ON THE RELATION OF LIFE TO OTHER FORCES. 



AH enumeration of theories concerning the nature of life would be 

 beside the purpose of the present chapter. They are interesting as marks 

 of the way in which various minds have been influenced by the mystery 

 which has always hung about vitality; their destruction is but another 

 warning that any theory we can frame must be considered only a tie for 

 connecting present facts, and one that must yield or break on any addi- 

 tion to the number which it is to bind together. 



Before attention had been drawn to the mutual convertibility of the 

 various so-called physical forces heat, light, electricity, and others and 

 until it had been shown that these, like the matter through which, they 

 act, are limited in amount, and strictly measurable; that a given quantity 

 of one force can produce a certain quantity of another and no more; that 

 a given quantity of combustible material can produce only a given quan- 

 tity of steam, and this again only so much motive-power; it was natural 

 that men's minds should be satisfied with the thought that vital force 

 was some peculiar innate power, unlimited by matter, and altogether in- 

 dependent of structure and organization. The comparison of life to a 

 flame is probably as early as any thought about life at all. And so long 

 as light and heat were thought to be inherent qualities of certain material 

 which perished utterly in their production, it is not strange that life 

 also should have been reckoned some strange spirit, pent up in the 

 germ, expending itself in growth and development, and finally declining 

 and perishing with the body which it had inhabited. 



With the recognition, however, of a distinct correlation between the 

 physical forces, came as a natural consequence a revolution of the com- 

 monly accepted theories concerning life also. The dictum, so long ac- 

 cepted, that life was essentially independent of physical force began to 

 be questioned. 



As it is well-nigh impossible to give a definition of life that shall be 

 short, comprehensive, and intelligible, it will be best, perhaps, to take its 

 chief manifestations, and see how far these seem to be dependent on 

 other forces in nature, and how connected with them. 



1 This chapter is a reprint, with some verbal alterations, of an essay contributed to 

 St. Bartholomew' 's Hospital Reports, 1867, by W. Morrant Baker. 



