14 INTRODUCTION. 
grown animal, would be the mbryology of the Horse; and 
its Morphology, the special study of the form of the adult 
animal and of its internal organs. 
Thus far we have been looking, as it were, at a steam- 
engine, with the fires out, and nothing in the boiler; but 
the body of the living Horse is a beautifully formed, active 
machine, and every part has its different work to do in the 
working of that machine, which is what we call its life. The 
science of such operations as the grinding of the food in the 
complex mill of the mouth; its digestion in the laboratory of 
the stomach; the pumping of the blood through a vast system 
of pipes over the whole body; its purification in the lungs; 
the process of growth, waste, and repair; and that wondrous 
telegraph, the brain, receiving impressions, and sending mes- 
sages to the muscles, by which the animal is endowed with 
voluntary locomotion—this is Physiology. But Horses are 
not the only living creatures in the world; and if we compare 
the structures of various animals, as the Horse, Zebra, Dog, 
Monkey, Eagle, and Codfish, we shall find more or less re- 
semblances and differences, enough to enable us to classify 
them, 2nd give to each a description which will distinguish it 
from all others. This is the work of Systematic Zoology. 
Moreover, the Horses now living are not the only kinds that 
have ever lived; for the examination of the earth’s crust—the 
great burial-ground of past ages—reveals the bones of numer- 
ous horse-like animals: the study of this pre-adamite race 
belongs to Paleontology. The chronological and geographical 
distribution of species is the department of Distributive Zo- 
ology. Speculations about the origin of the modern Horse, 
whether by special creation, or by development from some 
allied form now extinct, are kept aloof from demonstrative 
science, under the head of Theoretical Zoology. 
2. History.—The Greek philosopher Aristotle (B.c. 384- 
322) is called the “Father of Zoology.” Certainly, he is the 
only great representative in ancient times, though his frequent 
allusions to familiar works on anatomy show that something 
had been done before him. His “History of Animals,” in 
nine books, displays a wonderful knowledge of external and 
internal structure, habits, instincts, and uses. His deserip- 
tions are incomplete, but generally exact, so far as they go. 
