INTRODUCTION. 7 



other. The number of different animals is so enormous, that it was 

 long ago perceived that some classification of them, or method of 

 arranging them into groups, was absolutely indispensable. With- 

 out some such arrangement it would have been utterly impossible 

 to have ever acquired a clear notion of the animal kingdom as a 

 whole. In forming such a classification, we might take some one 

 character, such as the possession of wings, the number of legs, or 

 the like ; just as we might divide mankind into groups according 

 to the colour of the hair, the shape of the nose, or some other trivial 

 feature. Classifications of this kind, however, are "artificial," and 

 do not express the real relationships of animals ; hence they are now 

 universally discarded by naturalists. The true method of forming 

 a classification is to compare different animals with one another as 

 regards all the features which they present ; and in this way we 

 obtain what is called a " natural " classification. It is to be remem- 

 bered, however, that a thoroughly natural classification of animals 

 would imply that our knowledge of all their characters was com- 

 plete ; and as that is not the case, no known classification can be 

 regarded as more than approximately true. 

 (a natural classification, then, is founded upon a comparison of| 

 different animals with one another as regards all their essential char-j 

 acters ; and these characters are of two kinds. \Firstly, we have 

 to look to the laws, form, and ai-rangement of the structures of the 

 organism^ This constitutes what is called " Morphology," or the 

 science of form and structure (from the Greek words morphe, shape 

 or form ; and logos, a discourse). Secondly, we have to study the 

 vital actions performed by the living organism, and the functions'- 

 discharged by each portion of the body. This constitutes the science 

 of " Physiology." 



Morphology. — As we have just seen. Morphology, as a general 

 science, deals with the mere form and structure of the organism, 

 irrespective of function or ttsc; but it admits of a more or less 

 artificial division into subordinate departments. Thus the study of 

 the form and structure of living beings in their fully developed \ 

 condition constitutes the science of Anatomy. The term Human 

 Anatomy is given to the morphology of man, and the term Com- 

 parative Anatomy is applied to the study of the structure of animals 

 in general, as compared with one another and with man. 



Morphology, again, not only deals with the organism as a fully 

 developed whole, but is also concerned with the changes through 

 which every living being has to pass in reaching its mature or 

 fully-grown condition. This constitutes a special branch of the sub- 

 ject to which the term " Development " is applied. 



Finally, a separate department of the subject is sometimes consti- 



