6 INTRODUCTION. 



The differences between animals and plants in this respect may be roughly 

 stated as follows : 



1. Plants live upon purely dead or inorganic substances, such as water, 

 carbonic acid, and ammonia — and they have the power of making out of 

 these true organic substances, such as starch, cellulose, sugar, etc. Plants, 

 therefore, take as food very simple bodies, and manufacture them into 

 much more complex substances, so that plants are the great producers in 

 nature. 



2. Plants in the process of digestion break up carbonic acid into the two 

 elements of which it is composed — namely, carbon and oxygen, keeping the 

 carbon and setting free the oxygen. As carbonic acid occurs always in the 

 air in small quantities, the result of this is that plants remove carbonic acid 

 from the atmosphere and give out oxygen. 



3. Animals, on the other hand, have no power of living on dead or in- 

 organic matters, such as water, carbonic acid, and ammonia. They have no 

 power of converting these into the complex organic substances of which 

 their bodies are composed. On the contrary, annuals require to be supplied 

 with ready-made organic compounds if their existence is to be maintained. 

 These they can only get in the first place from plants, and therefore ani- 

 mals are all dependent upon plants for food either directly or indirectly. 

 Animals, therefore, differ from plants in requiring as food complex organic 

 bodies which they ultimately reduce to very much simpler inorganic bodies. 

 While plants, then, are the great manufacturers in nature, animals are the 

 great consumers. Another distinction arising from the nature of their food 

 is, that while plants decompose carbonic acid, keeping the carbon and setting 

 free the oxygen, animals absorb oxygen and give out carbonic acid, so that 

 their reaction upon the atmosphere is the reverse of that of plants. 



As regards these general distinctions between plants and animals, there 

 are three points which should be remembered : 



1. That, even if universally true, these distinctions can often not be ap- 

 pUed in practice to the ambiguous microscopic organisms about which alone 

 any doubt can be entertained. 



2. These general laws are certainly not of universal appUcatlon in the 

 case of plants. Some fungi ^are known which in the matter of food are ani- 

 mals — that is to say, they camiot live upon inorganic materials alone, but 

 require ready-made organic products for their support. 



[ 3. Recent researches have rendered it not unlikely that some of the 

 I lower animals have the power of acting as plants, and of manufacturing 

 I organic compounds out of inorganic materials. 



3. Classification. 



By the term classification is understood the arrangement 

 of a number of dissimilar objects of any kind into larger or 

 smaller groups according as they exhibit more or less likeness 

 to one another. The number of different animals is so enor- 

 mous that it was long ago perceived that some classification 

 of them, or method of arranging them into groups, was abso- 

 lutely indispensable. Without some such arrangement it would 

 have been utterly impossible to have ever acquired a clear 

 notion of the animal kingdom as a whole. In the older 



