INTRODUCTION. 13 



which a naturalist proceeds in framing a classification of the 

 animal kingdom. His great primary divisions are founded up- 

 on differences in the smaller and fundamental details of struct- 

 ure. His smaller divisions are based upon the less important 

 physiological differences with their corresponding structural 

 distinctions. Of course, in carrying out this programme of a 

 truly philosophical and natural classification, the naturalist 

 works to a great extent in the dark, and is liable to many 

 sources of error. It is by no means always easy to deter- 

 mine what points of structure are essential and fundamental, 

 and what are only caused by physiological differences. Such, 

 too, is the constitution of the human mind, that different ob- 

 servers place different values upon the same structures ; points 

 which some look upon as of essential value are regarded by 

 others as of a merely superficial nature. Nevertheless there 

 can be no doubt that the progress of Natural History as a 

 science has been strictly conterminous with the development 

 of these great principles of classification. 



In the present work an outline is given of the morpho- 

 logical differences between all the larger groups of the animal 

 kingdom, but it may be as well here to say a few words upon 

 the subject of Physiology. As already remarked, Physiology 

 treats of all the functions exercised by living bodies, or dis- 

 charged by the various definite parts or organs of which most 

 animals are composed. All these various functions come un- 

 der three great heads : 1. Functions of Nutrition, comprising 

 all those functions by means of which an animal is able to live, 

 grow, and maintain its existence as an individual. 2. Func- 

 tions of Reproduction, comprising all the functions by which 

 fresh individuals are produced and the perpetuation of the 

 species insured. 3. A series of functions which are known 

 by the somewhat misleading name of the Functions of Rela- 

 tion or of Correlation. Under this term are included all those 

 functions by means of which external objects are brought into 

 relation with the organism, and by which it, in turn, reacts 

 upon the outer world. The functions of nutrition and repro- 

 duction are often spoken of collectively as the functions of 

 " organic " or u vegetative " life, as being common to animals 

 and plants alike. The functions of relation, again, are often 

 called the functions of " animal " life, as being most highly 

 developed in animals. These functions, however, though more 

 highly characteristic of animals, are not peculiar to them, bir'- 

 are manifested to a greater or less extent by various plants. 



As regards animals, all alike, whatever their structure may 



