2 OUTLINES OF NATURAL HISTORY. 



If vital actions did not take place at the expense of the 

 already existing substance of the body, then an animal, 

 when once produced, might go on Living for an indefinite 

 period, "without any necessity of eating. As it is, the 

 process of living is attended with a constant destruction 

 of the living matter of the body, and if this destruction 

 were not counterbalanced in some way, an animal woidd 

 rapidly waste away and die from the incessant losses of 

 matter, caused by its movements and other vital processes. 

 This residt, however, is prevented by the fact that every 

 animal is constantly taking in " food," for the purpose 

 of repairing the losses caused by living, and in this the 

 process of " nutrition " (Latin, nutrio, I nourish), essen- 

 tially consists. 



Difi"erent animals Live upon different kinds of food, 

 but in all cases the food must contain materials which 

 are capable of taking their place in the structure of the 

 animal ; otherwise it is not food in the proper sense of 

 the term. And, as a matter of fact, the food of all ani- 

 mals, whether it consist of vegetables or of flesh, or of 

 both combined, can be shown to consist of essentially the 

 same constituents, and to be capable of entering into the 

 body of the animal to be nourished. The first condition 

 of nutrition, therefore, is that the animal should be able 

 to get food containing materials which can be built up 

 into its own tissues. 



As a general rule, the food which an animal eats cannot 

 be built up directly and tvithout change into the new struc- 

 tures of the body. On the contrary, the food has to 

 undergo certain changes before it can be employed in 

 making new tissues. These changes are effected by what 

 is commonly called the "digestion" of the food, but what 

 is more properly and in a more general sense known as 

 " assimilation " (Latin, cissimilo, I make like to). In 

 other words, the food has to be reduced to a common 

 basis having a certain likeness to the tissues which it 

 is intended to replace, before the animal is in a position 

 to make use of it. 



In most animals, the process of assimilation is com- 



