IX THE DYNAMICS OF LIFE. 93 



last chapter, constantly going on in every living organism, matter and 

 But I believe it is equally true, that a transformation of ^'^^'^^ 

 energy is also constantly going on ; and I believe that any 

 account of the vital process contains only half the truth, 

 unless it gives as much emphasis to the fact of the trans- 

 formation of energy by the action of the organism, as to 

 the fact of the transformation of matter. In the present 

 state of science, indeed, this is almost self-evident ; but it 

 could not be clearly seen until the laws of the conservation 

 of energy, and of the transformations of energy, were 

 understood. The principal relation of vegetables to energy Eelation of 

 is, as already explained, that they transform the radiance and'^of 

 that falls on their leaves into the potential energy due to animals to 

 the decomposition of the carbonic acid which is separated ^^^^^' 

 into carbon and oxygen. And one important relation of Animals 

 animals to energy is that, as already explained, they pro- Produce 

 duce heat by the oxidation of carbon, in their respiratory 

 organs and throughout their whole system. 



But it is obvious that this is not the only mode in which 

 animals transform energy. They are producers of motion and 

 as well as] of heat; and one of the most characteristic "^°*^°°" 

 peculiarities of the animal kingdom as contrasted with the 

 vegetable, is the possession of a special apparatus — the 

 nervo-muscular — of which the primary function is the 

 production of motion. But the production of motion does Motion in- 

 not depend on the presence of either nerves or muscles : dependent 

 there are animals of low organization, in which the micro- ture. 

 scope reveals no muscular structure, and indeed no struc- 

 ture at all, which have nevertheless the power of motion. 

 And the same appears to be true of the " ciliated cells " of 

 the higher animals. 



Heat and motion are the principal forms of energy Animal 

 that animals produce, but not the only ones. The glow- motion 

 worm, and some other insects, have a special apparatus for light, and 

 the production of light, and many of the simpler marine ^ ^*^*^"^^*^' 

 animals are luminous.^ And the torpedo, the gymnotus, 



1 Among the medusae and mollnsca the luminosity appears to bo due 

 to a phosphorescent secretion, and, if so, cannot be regarded as a vital 

 phenomenon. But among the marine annelids, or worms, it does not 



