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. At the present time the mammals are the highest and on the whole the most successful of the vertebrate groups. They include the largest and strongest, the swiftest-footed and the most intelligent of the animal kind. They show refinements of the senses of sight, hearing, and smell such as are met with nowhere else. They range from the Equator to the coldest regions of the earth in which any food is to be found; they people alike the forest and the plain, and have their representatives both in the air and in the sea. [Illustration: FIG. 105.--The Australian Duck-mole or Duck-billed Platypus.]