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.—Zodlogy, or the study of 
animals, is divided for convenience into several branches 
or phases. The study of the classification of animals is 
called systematic zodlogy; 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 zodlogy is 
so great, some zodlogists limit themselves exclusively to one 
of these phases of zodlogical 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 
ith 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 
