FOOD OF ANIMALS. 9 



from increase in size of a non-living body (e g.^ a crystal), 

 which is merely an addition 10 the surface and is called 

 growth by accretion. We have seen that efficiency of the 

 vital functions depends upon the relationship of surface to 

 bulk in the organism, for alimentation and excretion depend 

 upon this proportion. But increase in bulk involves a 

 reduction of the proportion between surface and bulk to 

 the detriment of the former. Here we have a definite 

 limit to the bulk of an organism beyond which it cannot 

 go without further differentiation. 



2. Reproduction. — Further growth necessitates an 

 increase of surface by division of the organism. Division 

 results in the production of two organisms from the 

 former one, usually termed Reproduction. Reproduction 

 alternating with growth are the two vital phenomena which 

 result in life on this earth presenting itself as a series 

 of organisms or individuals, which have a common origin 

 in primeval protoplasm. This perpetual organic continuity 

 of protoplasm throughout the animal kingdom is a most 

 important principle in connection with the problems of 

 heredity and descent. 



Food of Animals. — The foods of animals and their 

 nature have an important bearing on structure and function. 

 We may distinguish four kinds : — 



1. Proteids. — These form the most important foods. We have 

 already seen that they are highly organised, that they contain carbon, 

 hydrogen, oxygen, nitrogen and sulphur, and enter into the very com- 

 position of protoplasm. White-of-egg or albumen is a common example.. 



2. Carbohydrates. — Carbohydrates differ in many respects from 

 proteids. Not the least is their chemical composition, into which 

 carbon, hydrogen and oxygen alone enter. Starches and sugars are 

 familiar examples. 



3. Fats. — Fats are complex compounds of glycerine and some 

 fatty acid. Tliey contain only carbon, oxygen and hydrogen. Dilute 

 alkalies decompose them into glycerine and soap. 



4. Minerals. — The minerals include water and nmnerous mineral 

 salts in solution, such as common salt and phosphates of lime. 



The three first kinds of food are mostly, by the very nature of things, 

 insoluble, and the process of digestion consists essentially in reducing 

 them to a soluble state. If this occurred at the surface of the organism 

 the soluble substances would be largely lost, hence the insoluble food 

 has to be taken within the organism. Here we may say in a very 



