xm MORPHOLOGY AND PHYSIOLOGY 217 



allied species, whereas if there is a complete series of 

 graduations between two forms, they will be considered 

 to form a single variable species. 



In the previous chapters it will have been evident that 

 an animal may be studied from two chief points of view : 

 first, from the point of view of its structure, and 

 secondly from that of the functions performed by its 

 various parts and the way in which these work together 

 for the welfare of the whole. The branch of zoology 

 dealing with the former is known as morphology, and 

 with the latter physiology. 



It is evident that a knowledge of morphology is 

 necessary as a preliminary to the study of physiology, 

 and also that, as animals have to be distinguished from 

 one another largely by structural characters, the founda- 

 tions of a scientific zoology must be laid in morphology, 

 which, as we have seen, deals not only with the external 

 characters and the anatomy and histology of the adult 

 animal, but also with the changes undergone during 

 the development of the egg into the adult form, i.e., 

 with embryology. Given a sound knowledge of the 

 anatomy, histology, and emb^ology of animals, their 

 classification may be attempted : that is, we may pro- 

 ceed to arrange them in groups and sub-groups, each 

 capable of accurate definition. In doing so we must be 

 careful to distinguish between homologous parts, or those 

 which correspond structurally (compare p. 39), and 

 a-i'.ilogous parts, which have merely a similar function, 

 and are of no value for purposes of classification. Thus 

 the fore-limbs of a frog are homologous with t..,are 

 formed on a similar plan to the wings of a bird, although 

 differing from them in function ; they are only analogous 

 to the limbs of an insect, for though the function is similar 

 in both, the structure is widely different. In the same \vay 

 the wing of the bird is only analogous to that of an insect. 



