_The Insectivora._—The insect-eaters are more nearly related to the Rodentia than to the Carnivora. The form of teeth varies with the species. The incisors and canines are not especially prominent, but the molars are always serrated with numerous small-pointed eminences, or cusps, adapted to crushing insects. The three leading families of the Insectivora are the moles, the shrew-mice, and the hedgehogs. They are of small size, and are found in all countries, except in South America and Australia. _The Rodentia._—The Rodentia is a peculiar order of animals, characterised by two very long and strong teeth in each jaw, which occupy the place of the incisors and canines in other animals. Back of these there is a toothless space, and then four or five molars, which, when they have a roughened crown, indicate a vegetable, but when pointed, an insectivorous, diet. Their principal foods are grains and seeds of all kinds, and with these, often, fruits, nuts and acorns. To this order belong the families of the squirrel, marmot, all species of mice, the beaver, porcupine, hare, and others. An especial dietetic subdivision of the Rodentia is the Rhizophaga, or root-eaters, which includes some species of the Marsupials, and of mice. The food often consists exclusively of the roots of the beet, carrot, celery and onion. _The Edentata._—Occasionally, though rarely, animals of this class have rudimentary back teeth. Their food consists of leaves, blossoms, buds, and juicy stalks. Some also devour insects, especially ants. To this order belong the sloth, armadillo, pangolin, and great ant-eater. _The Omnivora._—Omnivorous animals have very distinctive teeth. The canines are markedly developed, forming regular tusks at the side of the mouth. These are used for attack and defence, and also to dig up roots, upon which these animals largely feed. The hog is typical of omnivorous animals of this character, and we all know his disposition no less than his anatomy! Animals of this class can live upon both animal and vegetable food, and _man_ is supposed to be included in this category! The great argument brought forward by those who recommend a “mixed” diet (_i.e._ one composed of flesh, vegetables, fruits, roots, grains, etc.), is that man can live on all these foods, and retain life and some degree of health, and that therefore he is omnivorous. We shall come to consider this argument somewhat at length presently. For the moment, it is enough to say that (so far as his teeth are considered, at any rate) man is in no way similar to the hog, or to any omnivorous animal whatever, but totally different from all of them. The most casual glance at the mouth and teeth of the hog should convince us that we are not in _that_ class! The other considerations we shall come to later.