Very characteristic of the mammals are, further, their teeth, for whereas the teeth of the reptiles are indefinite in number, and generally very numerous, those of the mammal are relatively few, and each species has a definite normal number. Moreover, the teeth of the reptile are all of a kind, and they may be replaced almost any number of times during the animal's life, whereas those of the mammal show differentiation according to their respective functions, and are only once changed, that is, when the milk teeth are replaced by the permanent set. The teeth of the mammal are of four kinds: incisors, or chisel-like cutting teeth, canines, which are especially well developed in carnivorous species, and which are used in the tearing up of flesh, etc., and two groups of grinders—premolars, which are replaced during the animal's life, and molars, which occur only as permanent teeth.
Fig. 102.—Pareiosaurus Baini. Fossil skeleton from the Permian of South Africa.
Fig. 103.—Skull of Galesaurus, from the Permian of South Africa.
A, From side; B, from below; C, from above; D, back tooth.
Fig. 104.—Skull of Tritylodon from below.
The mammals in all probability arose from a particular group of reptiles which flourished in the Permian and Triassic periods, and which disappeared very shortly afterwards. These pass under the name of the Theromorpha, and a typical specimen, Pareiosaurus, is illustrated in Fig. 102. The skull of another, illustrated in Fig. 103, shows distinct indications of a mammal-like differentiation of the teeth. The Tritylodon, whose skull is shown in Fig. 104, is an intermediate type between this group and the mammals, some zoologists regarding it, as it were, as the last reptile, others as the first of the mammals.
As to how the special mammalian features arose, or what special conditions called them into existence, we are of course without definite knowledge, for neither hair nor mammary glands are recognisable in fossils. But it seems likely that the warm blood and the hairy covering evolved in correlation with one another, and as adaptions to meet a gradual cooling of the climate. It is certain at all events that the present-day mammals (and also the birds) are far better adapted for a cold environment than the reptiles, which very easily get frozen to death; and it is also known that ice periods occurred in South Africa, where many fossil Theromorpha and the Tritylodon are found, at the time when these creatures existed; both of which facts support the theory indicated.