XIII MORPHOLOGY AND PHYSIOLOGY 217 



allied species, whereas if there is a complete series of grada- 

 tions 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, firstly, 

 from the point of view of its stn/ciure, 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 foundations of a scientific Zoo- 

 logy 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 embryology of animals, their 

 classification may be attempted : that is, we may proceed to 

 arrange them in groups and sub-groups, each capable of 

 accurate definition. In doing so we must be careful to dis- 

 tinguish between homologous parts, or those which correspond 

 structurally (compare p. 39), and analogous 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 — i.e., are formed on a similar plan 

 to — the wings of a bird, although differing from them in 

 function ; they are only analogous to those of an insect, 

 for though the function is similar in both, the structure is 

 widely different. In the same way the wing of the bird 

 is only analogous to that of an insect. 



