64 . ANIMAL LIFE 



plishment of the principal functions. While many of the 

 lower animals have no eyes and no ears, and trust to more 

 primitive means to discover food or avoid enemies, the 

 higher animals have extraordinarily complex organs for 

 seeing and hearing, two functions which are accessory only 

 to such a principal function as food-taking. 



38. Differentiation of structure, We have seen, in our 

 study of the slightly complex animals, how the body be- 

 comes more and more complex in proportion to the degree 

 in which the different life processes are divided or assigned 

 to different parts of it for performance. With the gradu- 

 ally increasing division of labor the body becomes less 

 homogeneous in structure; a differentiation of structure 

 becomes apparent and gradually increases. The extent of 

 the division of labor and the extent of the differentiation 

 of structure, or division of the body into distinct and dif- 

 ferent parts and organs, go hand in hand. An animal in 

 which the division of labor is carried to an extreme is an 

 animal in which complexity of structure is extreme. 



39. Anatomy and physiology. Zoology, or the study of 

 animals, is divided for convenience into several branches 

 or phases. The study of the classification of animals is 

 called systematic zoology; the study of the development 

 of animals from their beginning as a single cell to the time 

 of their birth is called animal embryology ; the study of 

 the structure of animals is called animal anatomy, and the 

 study of the performance of their life processes or functions 

 is called physiology. Because the whole field of zoology is 

 so great, some zoologists limit themselves exclusively to one 

 of these phases of zoological study, and those who do not 

 so definitely limit their study, at least give their special at- 

 tention to a single phase, although all try to keep in touch 

 with the state of knowledge in other phases. In earlier 

 days the study of the anatomy of animals and of their 

 physiology were held to be two very distinct lines of in- 

 vestigation, and the anatomists paid little attention to 



