As every one knows from his own experience, mammals normally have two sets of teeth, the first, temporary, or milk-dentition, in the young animal, and the second, or permanent dentition, in the adult. The milk-dentition, when fully developed, consists of incisors, canines and premolars, which usually agree in number, though often not in form, with the permanent teeth which replace them in the adult. The milk-teeth are frequently more conservative than the permanent ones and retain ancestral characters which have disappeared in the adult series, thus affording welcome information as to lines of descent and steps of evolutionary change. While there can be little doubt that the development of more than one dentition, or set of teeth, is the primitive condition among mammals and was derived by inheritance from their lower vertebrate ancestors, in which there was an indefinite succession of teeth; yet there are many mammals in which the milk-dentition is greatly reduced or altogether lost. In some, the milk-teeth are shed and replaced before birth, in others only the germs of the milk-teeth are formed and never cut the gum, while in others again all traces of the temporary series have vanished. This complete loss of the milk-teeth, like the presence of a great number of simple and similar teeth in the dolphins and porpoises, or the total absence of teeth, as in the anteaters and whalebone whales, is a secondary and derivative condition, never a primitive one. [Illustration: FIG. 44_A_.—First upper molar, right side of Deer (_Odocoileus_). On the left, the masticating surface; heavy black line, enamel. On the right, external side, showing crown and roots. _Brachyodont._] [Illustration: FIG. 45.—First upper molar, left side, of a fossil horse (_Equus sp._). On the right, external side. On the left, the grinding surface, showing two stages of wear. Heavy black line, enamel; white, dentine; shaded, cement. _Hypsodont_, roots not yet formed.]