INTRODUCTION 21 



Carbon dioxide and water are taken into the plant and are 

 changed into starch by means of a green substance known as 

 chlorophyll. Light is necessary for this process. A by-product 

 of photosynthesis is oxygen. 



The qualities that are usually cited as being peculiarly char- 

 acteristic of animals are locomotion and nervous activity. With 

 the exception of a few extremely sensitive species of which the 

 common sensitive plant, Mimosa pudica, is the most familiar 

 example, plants respond very slowly to external stimuli, and their 

 power of transmitting impulses is poorly developed. Locomo- 

 tion is impossible except in a few simple forms and free swimming 

 reproductive cells. 



7. Classification 



It is natural when a large number of dissimilar objects are 

 collected to attempt to place them in groups according to the 

 presence or absence of certain characteristics. This is known as 

 classification. Animals are not infinitely variable, since only 

 about five hundred thousand species have been described, and 

 they may be classified in several ways. 



By artificial classification we mean the grouping of animals 

 according to some resemblance in structure, color, habitat, etc. 

 For example, certain animals may be said to be aquatic because 

 they live in the water; others terrestrial, because they live on 

 land. Or certain animals are said to be carnivorous because they 

 eat flesh, others herbivorous because they live on vegetable food, 

 and still others omnivorous because they devour both animal 

 and vegetable matter. 



It is often convenient to use an artificial classification, but 

 for all scientific work the natural classification is employed. 

 This is an attempt to seek out the relationships of animals and 

 to group them, not because of superficial resemblances, but on 

 a basis of their similarity in structure and probable kinship. A 

 number of large divisions, known as phyla, are recognized by 

 zoologists. Each phylum is again divided into classes, each 



