5 ELEMENTARY ZOOLOGY. 



the green coloring matter (chlorophyll), which we think 

 of as being confined to plants; and yet these green cells 

 have the power of definite locomotion, such as is thought 

 of as confined to animals. Such a mingling of charac- 

 teristics is very confusing. It is not possible to say that 

 these organisms are clearly plants or animals. It is 

 better just to recognize that the two kingdoms are re- 

 lated and that the relation shows most in the simplest 

 forms. 



Many people would doubtless think of a nervous system 

 and sensitiveness as being one of the features separating 

 animals from plants. But all plants are sensitive in 

 some degree to all the chief external stimuli; and many 

 animals have no more signs of a nervous system than do 

 the plants. 



Indeed, there is no single statement that is true of all 

 animals which does not include some plants; just as 

 there is no single statement that includes all plants 

 which is not equally true of some animals. 



There is this, however, in these popular distinctions: 

 it is true in a general way, that animals are more motile, 

 and more sensitive, and must have more complex foods 

 than plants. In consequence, special tissues and organs 

 are developed in most animals for these purposes, as 

 the muscular, the nervous, and the digestive systems, 

 which are relatively lacking in plants. 



67. The Relation of Plants and Animals to the Foods. 



In Section 46 was enumerated a list of the chemical ele- 

 ments which are commonly found in protoplasm. These, 

 therefore, are the elements that must be had by organisms 

 as food in order to build up protoplasm and to do the 

 work of life. All of these elements occur in such forms 



