organisms: by the possession of a soul or an organic life with nutritive faculty, a plant is superior to a stone; by the possession of a soul or an organic life, with sensitive, appetitive and motor faculties (besides the nutritive), an animal is superior to a plant; and by the addition of the intellectual faculty in his soul or organic life, man is supreme among animals. As another and concomitant test of excellence, Aristotle took the amount of vital heat which the animal possessed. "The more perfect are those which are hotter in their nature and have more moisture and are not earthy in their composition, and the measure of natural heat is the lung when it has blood in it, for generally those animals which have a lung are hotter than those which have it not, and in the former class again those whose lung is not spongy nor solid nor containing only a little blood, but soft and full of blood."28
These tests of excellence might be difficult to apply to the classification of animals into genera and species,—a yearning for which with a realization of its practical and logical difficulties, pervades Aristotle's biological treatises, as already said. It will be interesting to feel our way along his various avenues of approach
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