geneous” substances. He traces here, according to his own ideas, the ascent from the inorganic to the organic world: out of heat, cold, wetness, and dryness the four elements are compounded; out of the four elements are formed the homogeneous substances or tissues; out of these are formed the organs, and out of the organs the organised being. All this served as a provisional theory, until superseded by the discoveries of chemistry. Aristotle laid it down as a principle of method (‘Parts of An.,’ I. i. 4), that all which was common to the various species of living beings should be discussed before entering upon their specific differences. Therefore (2) the treatise ‘On the Soul’ followed next in order, and traced out the vital principle through its successive ascending manifestations. To this was appended (3) the ‘Parva Naturalia’ or ‘Physiological Tracts,’ which dealt with some of the functions of living creatures, whether common or special, such as sensation, memory, dreaming, and also with the following pairs of opposites: waking and sleeping, youth and old age, inspiration and expiration, life and death. It was added that there is another pair still to be treated of—namely, health and sickness. The Stagirite, as was natural from his family traditions, always appears to have looked forward to composing a philosophical work on Medicine. But there is no trace of this ever having been achieved.
The 4th book on the list kept still to generalities. This was the short treatise ‘On the Locomotion of Animals,’ which showed how various organs in the various creatures are adapted by nature for this pur-
