The subject of our discussion is now narrowed down to the group of the mammals. The mammals are characterised by two very obvious features: a body-covering of hair, and a set of special glands in the female which secrete milk for the nourishment of the young. These are constant characters, and neither is ever found in any other group. As to how the hair originated, nothing definite is known; but (while there are certain difficulties in regard to the theory) it is on the whole reasonable to suppose that the mammalian hair arose, as the bird's feather undoubtedly did, as a modification of the reptile's scale. The mammary gland appears to represent a modification of other skin glands, either of sweat glands or more probably of the oil glands which exist in connection with the hairs. Another important character, already mentioned at the end of the last chapter, is the diaphragm, a muscular partition separating the body cavity into a thoracic and an abdominal portion. The diaphragm has important functions in connection with the mechanism of breathing. By means of it the thoracic cavity can be increased or diminished in size, and air thus drawn into or expelled from the lungs. It is interesting to observe that a similar partition, with the same function, occurs in the Crocodiles, but this has different relations to the abdominal organs, and has evidently evolved quite independently. 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. [Illustration: FIG. 102.--_Pareiosaurus Baini._ Fossil skeleton from the Permian of South Africa.] [Illustration: FIG. 103.--Skull of Galesaurus, from the Permian of South Africa. _A_, From side; _B_, from below; _C_, from above; _D_, back tooth.] [Illustration: FIG. 104.--Skull of Tritylodon from below.]