FOOD OF ANIMALS. 9 
from increase in size of a non-living body (eg., a crystal), 
which is merely an addition to 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 :— 
I, 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. Fars.—Fats are complex compounds of glycerine and some 
fatty acid. They contain only carbon, oxygen and hydrogen. Dilute 
alkalies decompose them into glycerine and soap. 
4. MINERALS.—The minerals include water and numerous 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 
