The alertness and general intellectual superiority of mammals to all animals lying below them in the series (with the exception of the birds, which are in their way almost on a level with the Mammalia) are seen by their active and continuous movements. The lengthy periods of absolute motionlessness, so familiar to everybody in such a creature as the Crocodile, are unknown among the more typical Mammalia except indeed during sleep. This mental condition is clearly shown by the proportionate development of the external parts of all the organs of the higher senses. The Mammalia as a rule have well-developed, often extremely large, flaps of skin surrounding the entrance to the organ of hearing, often called "ears," but better termed "pinnae." These are provided with special muscles, and can be often moved and in many directions. The nose is always, or nearly always, very conspicuous by its naked character; by the large surface, often moist, which surrounds the nostrils; and again by the muscles, which enable this tract of the integument to be moved at will. The eyes, perhaps, are less marked in their predominance over the eyes of lower Vertebrates than are the ears and nose; but they are provided as a rule with upper and lower eyelids, as well as by a nictitating membrane as in lower Vertebrates. The apparent predominance of the senses of smell and hearing over that of sight appears to be marked in the Mammalia, and may account for their diversity of voice as well as of odour, and for the general sameness of coloration which distinguishes this group from the brilliantly-coloured birds and reptiles. The head, too, which bears these organs of special sense, is more obviously marked out from the neck and body than is the case with the duller creatures occupying the lower branches of the Vertebrate stem. [Illustration] FIG. 1.--A, Section of human skin. _Co_, Dermis; _D_, sebaceous glands; _F_, fat in dermis; _G_, vessels in dermis; _GP_, vascular papillae; _H_, hair; _N_, nerves in dermis; _NP_, nervous papillae; _Sc_, horny layer of epidermis; _SD_, sweat gland; _SD_^1, duct of sweat gland; _SM_, Malpighian layer. B, Longitudinal section through a hair (diagrammatic). _Ap_, Band of muscular fibres inserted into the hair-follicle; _Co_, corium (dermis); _F_, external longitudinal; _F_^1, internal circular, fibrous layer of follicle; _Ft_, fatty tissue in the dermis; _GH_, hyaline membrane between the root-sheath and the follicle; _HBD_, sebaceous gland; _HP_, hair-papilla with vessels in its interior; _M_, medullary substance (pith) of the hair; _O_, cuticle of root-sheath; _R_, cortical layer; _Sc_, horny layer of epidermis; _Sch_, Hair shaft; _SM_, Malpighian layer of epidermis; _WS_, _WS_^1, outer and inner layers of root-sheath. (From Wiedersheim's _Comparative Anatomy_.) [Illustration]