Books five to seven deal with the subject of generation.
The eighth book contains a variety of details respecting animals, their food, migrations, hibernation, and diseases; with the influence of climate and locality upon them.
The ninth book describes the habits and instincts of animals. The details are interesting; but there is, as usual, very little attempt at classification. Disjointed statements and sudden digressions occur, the subjects being treated in the order in which they presented themselves to the author. Such curious statements as the following are met with: "The raven is an enemy to the bull and the ass, for it flies round them and strikes their eyes." "If a person takes a goat by the beard, all the rest of the herd stand by, as if infatuated, and look at it." "Female stags are captured by the sound of the pipe and by singing. When two persons go out to capture them, one shows himself, and either plays upon a pipe or sings, and the other strikes behind, when the first gives him the signal." "Swans have the power of song, especially when near the end of their life; for they then fly out to sea, and some persons sailing near the coast of Libya have met many of them in the sea singing a mournful song, and have afterwards seen some of them die." "Of all wild animals, the elephant is the most tame and gentle; for many of them are capable of instruction and intelligence, and they have been taught to worship the king."
In the work "On the Parts of Animals," the author considers not only the phenomena of life exhibited by each species, but also the cause or causes to which these phenomena are attributable. After a general introduction, he proceeds to enumerate the three degrees of composition, viz.:—
(1) "Composition out of what some call the elements, such as air, earth, water, and fire," or "out of the elementary forces, hot and cold, solid and fluid, which form the material of all compound substances."
(2) Composition out of these primary substances of the homogeneous parts of animals, e.g. blood, fat, marrow, brain, flesh, and bone.
(3) Composition into the heterogeneous parts or organs. These parts he describes in detail, considering those belonging to sanguineous animals first and most fully.
These divisions correspond roughly to the threefold study of structure which we nowadays recognize as chemical, histological, and anatomical.
As examples of Aristotle's method of treatment, his descriptions of blood, the brain, the heart, and the lung may be considered.
Of the blood he says, "What are called fibres are found in the blood of some animals, but not of all. There are none, for instance, in the blood of deer and of roes, and for this reason the blood of such animals as these never coagulates.... Too great an excess of water makes animals timorous.... Such animals, on the other hand, as have thick and abundant fibres in their blood are of a more choleric temperament, and liable to bursts of passion.... Bulls and boars are choleric, for their blood is exceedingly rich in fibres, and the bull's, at any rate, coagulates more rapidly than that of any other animal.... If these fibres are taken out of the blood, the fluid that remains will no longer coagulate."