AN INTRODUCTION TO ZOOLOGY. 



structure of the molecule is more complex, and it 

 would seem that the animal body has not the power 

 ■of building up complex substances unless part of the 

 work is done for it ; it can alter, as it were, but not 

 i»ia7ce the necessary chemical structure. 



The organic compounds referred to are of three 

 classes. I. Proteids, or quaternary compounds (t.e. 

 compounds made up of four factors, viz., carbon, 

 hydrogen, oxygen, and nitrogen); these are practically 

 identical with (dead) protoplasm, the universal sub- 

 stance of living organisms, which has been called the 

 •"physical basis of life," and of which more must be 

 •said hereafter; foods of this class may be obtained 

 ■either from plants or animals, the protoplasmic basis 

 being identical in both. II. Fats (sometimes in- 

 correctly referred to as hydrocarbons), containing 

 ■oxygen, hydrogen, and carbon ; these also may be 

 ■obtained from either plants or animals, being practi- 

 cally the same in both. III. Carbohydrates, or com- 

 pounds of carbon with the elements of water in the 

 proportion in which tliey exist in water : these are 

 ■divided into two classes, namely, starches and sugars : 

 the vegetable kingdom is the sole available source 

 of either, for in the animal body sugar only occurs 

 in rare traces, while the presence of any starchy com- 

 pound is rare and exceptional. 



The difference between the food of plants and 

 animals, then, is shortly this : the animal requires 

 its food ready made, in the shape of complex com- 

 pounds ; the plant takes up more simple compounds. 



