INTRODUCTION. 13 
animal kingdom. His great primary divisions are founded 
upon differences in the fundamental details of structure. 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 determine what points of structure are 
essential and fundamental, and what are only caused, by physio- 
logical differences. Such, too, is the constitution of the human 
mind, that different observers 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 morphological 
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 discharged by the 
various definite parts or organs of which most animals are 
composed. All these various functions come under three great 
heads: 1. Munctions of Nutrition, comprising all those func- 
tions by means of which an animal is able to live, grow, and 
maintain its existence as an individual. 2. Puzctions of Repro- 
duction, comprising all the functions by which fresh individuals 
are produced and the perpetuation of the sfeczes insured. 3. A 
series of functions which are known by the somewhat mis- 
leading name of the Functions of Relation or of Correlation. 
Under this term are included all those functions by means of 
which external objects are brought into ve/aézonx with the or- 
ganism, and by which it, in turn, reacts upon the outer world. 
The functions of nutrition and reproduction are often spoken 
of collectively as the functions of “organic” or “vegetative” 
life, as being common to animals and plants alike. The func- 
tions 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, but are manifested to a 
greater or less extent by various plants, 
