347 MAMMALIA MAMMALIA (French, Mammifercs ; German, Sauye- thiere) is the name invented by Linnaeus (from the Latin, mamma), and now commonly used by zoologists, for one of the classes of vertebrated animals, which, though the best known and undoubtedly the most im portant group of the animal kingdom, has never received any generally accepted vernacular designation in our language. The unity of structure of the animals compos ing this class, and their definite demarcation from other vertebrates, were not recognized until comparatively modern times, and hence no word was thought of to desig nate what zoologists now term a mammal. The nearest equivalents in common use are "beast" and "quadruped, both of which, however, cover a different ground, as they are often used to include the larger four-footed reptiles, and to exclude certain undoubted mammals, as Man, Bats, and Whales. The limits of the class as now understood by zoologists are perfectly well defined, and, although certain forms still existing on the earth (though not those mentioned above as excluded by the popular idea) are of exceedingly aberrant structure, exhibiting several well-marked characters which connect them with the lower vertebrated groups, common consent retains them in the class with which the great proportion of their characters ally them, and hitherto no traces of any species showing still more divergent or tran sitional characters have been discovered. There is thus a great interval, not bridged over by any known forms, recent or extinct, between mammals and other vertebrates. In the gradual order of evolution of living beings, mammals taken altogether are certainly the highest in organization, as they were probably the last to appear on the earth s surface, though this must be said with some reservation, pending further knowledge of the early history of the class of birds. But, as in speaking of all other large and greatly differentiated groups, this expression must not be understood in too limited a sense. The tendency to gradual perfection for their particular station in life, which all groups manifest, leads to various lines of specialization, or divergence from the common or general type, which may or may not take the direction of elevation. A too complex and sensitive condition of organization may in some circumstances of life be disadvantageous, and modification may then take place in a retrograde direction. In mammals, as in other classes, there are low forms as well as high forms, but by any tests that can be applied, especially those based on the state of development of the central nervous system, it will be seen that the average exceeds the average of any other class, that many species of this class far excel those of any other in perfection of structure, and that it contains one form which is unques tionably the culminating point yet arrived at amongst organized beings. With regard to th? time of the first appearance of mammals upon the earth, the geological record is provok- ingly imperfect. At the commencement of the Tertiary period they were abundant, and already modified into most of the leading types at present existing. It was at one time thought that they first came into being at this date, but the discovery of fragments of numerous small species has revealed the existence of some forms of the class at various periods throughout almost the whole of the age of the deposition of the Secondary rocks. This subject will be reverted to later on. It hardly need be said that mammals are vertebrated r.nimals, and possess all the characteristics common to the members of that division of the animal kingdom. They are separated from the Ichlhyopsida (fishes and amphibia) and agree with the Sauropsida (reptiles and birds) in the possession during their development of an amnion and allantois, and in never having external branchiae or gills. They differ from reptiles and resemble birds in being warm-blooded, and having a heart with four cavities and a complete double circulation. They differ from both birds and reptiles in the red corpuscles of the blood being nucleated and, with very few exceptions, circular in out line ; in the lungs being freely suspended in a thoracic cavity, separated from the abdomen by a complete muscular partition, the diaphragm, which is the principal agent in inflating the lungs in respiration ; in having but one aortic arch, which curves over the left bronchus ; in the skin being more or less clothed with hair, and never with feathers ; in the greater perfection of the commissural system of the cerebral hemispheres, which has either a complete corpus callosum, or an incomplete one associated with a very large* anterior commissure; in having no syrinx or inferior vocal organ, but a complete larynx at the upper end of the trachea ; in having a mandible of which each ramus (except in very early developmental conditions) consists of a single bone on each side, articulat ing to the squamosal, without the intervention of a quadrate bone ; in having a pair of laterally placed occipital con- dyles instead of one median one ; and in the very obvious character of the female being provided with mammary glands, by the secretion of which the young (produced alive and not by means of externally hatched eggs) are nourished for some time after birth. In common with all vertebrated animals, mammals have never more than two pairs of limbs. In the great majority of the class both are well-developed and functional, and adapted for terrestrial progression, as the larger number of mammals live ordinarily on the surface of the earth. They are, however, by no means limited to this situation. Some species spend the greater part of their lives beneath the surface, their fore limbs being specially modified for burrowing; others are habitually arboreal, their limbs being fitted for climbing or hanging to boughs of trees ; some are as aerial as birds, the fore limbs being developed into wings of a special character ; others are as aquatic as fishes, the limbs assuming the form of fins or paddles. In many of the latter the hinder extremities are either completely suppressed, or present only in a rudimentary state. In no known mammal are the fore limbs absent. The hinder extremity of the axis of the body is usually prolonged into a tail, which may be a mere pendent appendage, or modified to perform various functions, as grasping boughs in climbing, or even gathering food, in the case of the prehensile-tailed Monkeys and Opossums, swimming in the Cetacea, and acting as a flap to drive away troublesome insects from the skin in the Unyuhtta. GENERAL ANATOMICAL CHARACTERS OF THE MAMMALIA. TEGUMENTAEY STRUCTURES. The external surface of the greater number of members Hair, of the class is thickly clothed with a peculiarly modified form of epidermis, commonly called hair. This consists of hard, elongated, slender, cylindrical or tapering, filiform, unbranched masses of epidermic material, growing from a short papilla sunk at the bottom of a follicle in the derm
or true skin. Such luirs upon different parts of the same