Many wonders there be, but naught more wondrous than man: * * * * * The light-witted birds of the air, the beasts of the weald and the wood He traps with his woven snare, and the brood of the briny flood. Master of cunning he: the savage bull, and the hart Who roams the mountain free, are tamed by his infinite art. And the shaggy rough-maned steed is broken to bear the bit. Sophocles, _Antigone_, verses 342 ff. (Translation of F. Storr.) It is thus not surprising that our first systematic treatment of animals is in a practical medical work, the περι διαιτης {peri diaitês}, _On diet_, of the Hippocratic Collection. This very peculiar treatise dates from the later part of the fifth century. It is strongly under the influence of Heracleitus (_c._ 540-475) and contains many points of view which reappear in later philosophy. All animals, according to it, are formed of fire and water, nothing is born and nothing dies, but there is a perpetual and eternal revolution of things, so that change itself is the only reality. Man's nature is but a parallel to that of the universal nature, and the arts of man are but an imitation or reflex of the natural arts or, again, of the bodily functions. The soul, a mixture of water and fire, consumes itself in infancy and old age, and increases during adult life. Here, too, we meet with that singular doctrine, not without bearing on the course of later biological thought, that in the foetus all parts are formed simultaneously. On the proportion of fire and water in the body all depends, sex, temper, temperament, intellect. Such speculative ideas separate this book from the sober method of the more typical Hippocratic medical works with which indeed it has little in common.