ARISTOTLE, the greatest philosopher of antiquity, was born at Stagira, in Macedonia, and lived from 384 to 322 B.C. He wrote most excellent works on all branches of human knowledge, and was the founder of Natural History and Comparative Anatomy. His acquaintance with anatomy as illustrated principally in his treatise _On the Different Parts of Animals_, is absolutely extraordinary for the time in which he lived. One chapter of this work[96] is altogether dedicated to the study of the teeth; but he also speaks of these organs in many other of his works, particularly in his _History of Animals_, which is a real and proper treatise on zoölogy, wherein the author records a great number of notes about the peculiarities presented by the dental system, in the different classes of animals. In spite of the great errors into which he has fallen, his ideas about the teeth are, taken as a whole, quite worthy of attention, especially when one considers the remote epoch in which this great philosopher wrote. We will here give a brief notice of the most important of his observations relating to the dental organs. The form, the disposition, the number of the teeth, varies in animals, according to the quality of their food and according to whether the teeth serve merely to divide and to chew the alimentary substances, or as instruments of offence and defence as well. In man, the teeth serve principally for mastication, but the front ones have, besides, another most important office, namely, that of assisting in the articulation of words, in the pronunciation of certain letters. In those animals in which the teeth also serve as weapons, it is to be observed either that some of them protrude like those of the boar, or that they are sharp and saw-like in their disposition, as in the lion, the panther, the dog, etc. No animal possesses at the same time protruding and saw-like teeth. The teeth are not always equal in number in both jaws; the animals provided with horns have no teeth in the front of the upper jaw; this, however, is also to be observed in animals without horns, as for example, in the camel. Among the animals provided with horns there are none which have protruding or saw-like teeth. In general, the front teeth are pointed and the back ones broad. Nevertheless, all the teeth of the seal are pointed, with a saw-like disposition, perhaps because this animal marks the transition from the quadruped to the fish, all of which, with few exceptions, have their teeth formed in that way. Animals with saw-like teeth have generally very large mouths. No animal has ever more than one row of teeth in each jaw; however, says Aristotle, if Ctesias[97] is to be believed, there is an animal in India, named marticora, which has a triple row of teeth. The molar teeth are never changed either in man or in any known animal; the pig never changes its teeth.