_Succession._—The dentition of all mammals consists of a definite set of teeth, almost always of constant and determinate number, form, and situation, and, with few exceptions, persisting in a functional condition throughout the natural term of the animal’s life. In many species these are the only teeth which the animal ever possesses,—the set which is first formed being permanent, or, if accidentally lost, or decaying in extreme old age, not being replaced by others. These animals are called Monophyodont. But in the larger number of mammals, certain of the teeth are preceded by others, which may be only of a very transient, rudimentary, and functionless character (being in the Seals, for example, shed either before or within a few days after birth), or may be considerably developed, and functionally occupy the place of the permanent teeth for a somewhat lengthened period, during the growth and development of the latter and of the jaws. In all cases these teeth disappear (by the absorption of their roots and shedding of the crowns) before the frame of the animal has acquired complete maturity, as evidenced by the coalescence of the epiphyses of the osseous system. As these teeth are, as a general rule, present during the period in which the animal is nourished by the milk of the mother, the name of “milk-teeth” (French _dents de lait_, German _milchzähne_) has been commonly accorded to them, although it must be understood that the epoch of their presence is by no means necessarily synchronous with that of lactation. Animals possessing such teeth are called Diphyodont. No mammal is known to have more than two sets of teeth; and the definite and orderly replacement of certain members of the series is a process of quite a different nature from the indefinite succession which takes place in all the teeth continuously throughout the lifetime of the lower vertebrates. When the milk-teeth are well developed, and continue in place during the greater part of the animal’s growth, as is especially the case with the Ungulata, and, though to a less degree, with the Primates and Carnivora, their use is obvious, since taken all together they form structurally a complete epitome on a small scale of the more numerous and larger permanent set (see Fig. 3), and, consequently, are able to perform the same functions, while time is allowed for the gradual maturation of the latter, and especially while the jaws of the growing animal are acquiring the size and strength sufficient to support the permanent teeth. Those animals, therefore, that have a well-developed and tolerably persistent set of milk-teeth may be considered to be in a higher state of development, as regards their dentition, than those that have the milk-teeth absent or rudimentary.