FOODS. 315 



them in the simpler compounds, carbon dioxide, water, and 

 urea; which are incapable of either yielding energy or build- 

 ing tissue for any other animal and so of serving it as food. 

 The question immediately suggests itself How, since animals 

 are constantly breaking up these complex bodies and cannot 

 again build them, is the supply kept up ? For example, the 

 supply of proteids. substances which cannot be made arti- 

 ficially by any process which we know, and yet are necessary 

 foods for all animals, and daily destroyed by them. 



The Food of Plants. As regards our own Bodies the 

 question at the end of the last paragraph might perhaps be 

 answered by saying that we get our proteids from the flesh 

 of the other animals which we eat. But, then, we have to 

 account for the possession of them by those animals; since 

 they cannot make them from urea and carbon dioxide and 

 water any more than we can. The animals eaten get them, 

 in fact, from plants which are the great proteid formers of 

 the world, so that the most carnivorous animal really depends 

 for its most essential foods upon the vegetable kingdom ; the 

 fox that devours a hare in the long-run lives on the proteids 

 of the herbs that the hare had previously eaten. All animals 

 are thus, in a certain sense, parasites; they only do half of 

 their own nutritive work, just the final stages, leaving all the 

 rest to the vegetable kingdom and using the products of its 

 labor ; and plants are able to meet this demand because they 

 can live on the simple compounds of carbon, hydrogen, and 

 nitrogen eliminated by animals, building up out of them new 

 complex substances \vhich animals can use as food. A green 

 plant, supplied with ammonium salts, carbon dioxide, water, 

 and some minerals, will grow and build up large quantities 

 of proteids, fats, starches, and similar things; it will pull the 

 stable compounds eliminated by animals to pieces, and build 

 them up into complex unstable bodies, capable of yielding 

 energy when again broken down. However, to do such work, 

 to break up stable combinations and make from them less 

 stable, needs a supply of kinetic energy which disappears in 

 the process, being stored away as potential energy in the new 

 compound ; and we may ask whence it is that the plant gets 

 the supply of energy which it thus utilizes for chemical con- 

 struction, since its simple and highly oxidized foods can yield 

 it none. It has been proved that for this purpose the green 

 plant uses the energy of sunlight : those of its cells which con- 



