i66 THE POPULAR SCIENCE MONTHLY. 



also to do much other work besides. It is probable, however, that the 

 wants of animal activity are so immediate and urgent that, under 

 these conditions, much food would be burned for this purpose, and 

 would not reach the tissues, and the tissues would be imperfectly re- 

 paired, and would therefore waste. 



Take next the carnivorous animal full fed. In this case there can 

 be no doubt that, while a portion of the food goes to repair the tissues, 

 by far the larger portion is consumed in the blood, and passes away 

 partly as CO a and H a O through the lungs, and partly as urea through 

 the kidneys. This part is used, and can be of use only, to create force. 

 The food of carnivora, therefore, goes partly to tissue-building, and 

 partly to create heat and force. The force of carnivorous animals is 

 derived partly from decomposing tissues and partly from food-excess 

 consumed in the blood. 



11. Herbivora. — The food of herbivora and of man is mixed — 

 partly albuminoid and partly amyloid. In man, doubtless, the albu- 

 minoids are usually in excess of what is required for tissue-building ; 

 but in herbivora, probably, the albuminoids are not in excess of the 

 requirements of the decomposing tissues. In this case, therefore, the 

 whole of the albuminoids is used for tissue-making, and the whole of 

 the amyloids for force-making. In this class, therefore, these two classes 

 of food may be called tissue-food and force-food. The force of these 

 animals, therefore, is derived partly from the decomposition of the tis- 

 sues, but principally from the decomposition and combustion of the 

 amyloids and fats. 



Some physiologists speak of the amyloid and fat food as being 

 burned to keep up the animal heat ; but it is evident that the prime 

 object in the body, as in the steam-engine, is not heat, but force. Heat 

 is a mere condition and perhaps a necessary concomitant of the change, 

 but evidently not the prime object. In tropical regions the heat is 

 not wanted. In the steam-engine, chemical energy is first changed 

 into heat and heat into mechanical energy ; in the body the change is, 

 probably, much of it direct and not through the intermediation of heat. 



12. We see at once, from the above, why it is that plants cannot 

 feed on elements, viz., because their food must be decomposed in order 

 to create the organic matter out of which all organisms are built. 

 This elevation of matter, which takes place in the green leaves, of 

 plants, is the starting-point of life ; upon it alone is based the possi- 

 bility of the existence of the organic kingdom. The running down 

 of the matter there raised determines the vital phenomena of germina- 

 tion of pale plants, and even of some of the vital phenomena of green 

 plants, and all the vital phenomena of the animal kingdom. The sta- 

 bility of chemical compounds, usable as food, is such that a peculiar 

 contrivance and peculiar conditions found only in the green leaves of 

 plants are necessary for their decomposition. We see, therefore, also, 

 why animals as well as pale plants cannot feed on mineral matter. 



