8 ZOOLOGY 



leaves, flowers, &c. they closely resemble ordinary plants. 

 Similarly there are some minute living beings which are green 

 and which can live on quite simple chemical compounds like 

 plants, but which are nevertheless reckoned as animals because 

 of their likeness to other living things which are undoubtedly 

 animals. It must be admitted, however, that there is a great 

 deal more doubt about the true position of these "plant- 

 animals" than there is about the position of the "animal- 

 plants" referred to above a doubt which is emphasized by 

 the fact that the same group of beings is lectured on by 

 professors of both botany and zoology, and described in text- 

 books which deal with both sciences. In many cases where 

 animals of some size have a green colour and are apparently 

 able to subsist on simple chemical substances, this appearance 

 has been shown to be due to the fact that their bodies 

 are the home of multitudes of minute plants, which grow in 

 them and give them their colour by shining through the more 

 or less transparent substance of the body, but which sooner or 

 later are digested by the animals in which they live and serve 

 as their food.. The only plant-animals which are left un- 

 accounted for are extremely minute microscopic forms, in the 

 case of many of which, the question of whether they shall be 

 ranked as plants or animals can only be decided by a careful 

 balance of the evidence for and against. 



For all practical purposes the definition of an animal as a 

 living thing or, to use the shorter and more convenient term, 

 an organism which can move and take in solid food which 

 must contain proteid is a good definition, and certainly ex- 

 presses the idea which rises in the mind of a scientific man 

 when he names the word animal. 



CHAPTER II 



THE PRACTICAL IMPORTANCE OF ZOOLOGY. 



HAVING in the previous chapter given the reader some idea of 

 what the science of zoology is about, we propose in the present 

 one to attempt to give him some conception of the enormous 



