exceptions, circular in outline; 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; 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, articulating to the squamosal without the intervention of a quadrate bone; in having a pair of laterally placed occipital condyles 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 (usually produced alive, although in the lowest forms by means of externally hatched eggs) are nourished for some time after birth.

In common with all vertebrated animals, mammals never have more than two pairs of limbs; as the larger number live ordinarily on the surface of the earth, in the great majority of the class both pairs are well-developed and functional, and adapted for terrestrial progression. Mammals are, however, by no means limited to this situation. Thus some species spend the greater part of their lives beneath the surface, their fore limbs being specially modified for burrowing; others, again, 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; while in others which are as aquatic as fishes, the limbs assume 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 may be modified to perform various functions, as grasping boughs in climbing, or even gathering food, in the case of the prehensiletailed Monkeys and Opossums, swimming in the Cetacea, and acting as a flap to drive away troublesome insects from the skin in the Ungulata.

The state of development of the young at the time of birth varies greatly in the different groups. Thus among the Marsupials where there is no connection during intra-uterine life between the circulatory systems of the parent and the foetus, the young are born in an exceedingly imperfectly developed condition. For their

protection the mother, in a large number of cases, has a special pouch enclosing the mammæ, into which the young are transferred at birth, and in which they remain till they are well developed. Among the higher, or Placental types, however, where a connection exists between the maternal and fœtal circulations previous to birth, the young are always born in a much more highly developed state than among the Marsupials, although we meet with great variations in this respect. In those forms which habitually live in holes, like many Rodents, the young are always very helpless at birth; and the same is also true of many of the Carnivora, which are well able to defend their young from attack. In the great order of Ungulate, or Hoofed Mammals, where in the majority of cases defence from foes depends upon fleetness of foot, or upon huge corporeal bulk, the young are born in a very highly developed condition, and are able almost at once to run by the side of the parent. This state of relative maturity at birth reaches its highest development in the Cetacea, where it is evidently associated with the peculiar conditions under which these animals pass their existence. In the Primates, however, we again find the young produced in a more or less helpless condition, and requiring a long period before they attain their full development, this being more especially the case with those higher forms which approximate in structure to man.

In point of size mammals vary to a greater extent than the existing members of any one class of animals, and include the largest living inhabitants of the earth. The extremes of size are marked on the one hand by the whale known as Sibbald's Rorqual, which attains a length of eighty feet and a weight of nearly as many tons, and on the other by the Pigmy-Shrew and the minute Harvestmouse, which can climb a stem of wheat.

Of all the living creatures inhabiting our globe, mammals are by far the most important in their economic uses, since, in addition to being the only animals capable of labour for human benefit, they furnish the greater portion of the animal food of many races of man, and likewise a large amount of their clothing. In these respects the Ungulates hold the first place.

As regards employment for labour, with the exception of the Dogs used for sleighing by the Esquimaux, and those which among some European nations draw light carts, all the mammals in general use are Ungulates. Of the first importance are the Horses and Asses, which are employed as beasts of draught or burden over nearly the whole globe. Among many nations, however, cattle, as represented by the true Oxen, the Buffalos, and the Yaks of Tibet, occupy a still more important position, while in the highlands of Tibet Sheep are largely used for carrying burdens. In other regions, again, the place of the Horse and the Ass is taken by the Camels,

which are peculiarly fitted for traversing parched and arid deserts, while in the Andes we find the Llamas serving the same office. In Lapland and other parts of the northern regions the Reindeer is the main agent employed in draught. Lastly, we must not omit to mention the Indian Elephant, which, from its vast strength, is so useful in transport through the wilder parts of its native country.

As regards food, we again find the Ungulates, and more especially the Artiodactyle division, taking the foremost place; and in this connection we have only to mention, among animals kept in a domestic condition, Swine, Cattle, Sheep, and Goats-the three latter affording not only their flesh, but also milk and its resulting cheese and butter. To many races, however, Mares and Camels are the chief milk producers, while the Laps make use of the milk of the Reindeer. The Rodents, as represented by Hares and Rabbits, occupy a minor position as furnishers of food.

In relation to clothing, the Ungulates are likewise of paramount importance, as exemplified by the wool of the Sheep, which is so valuable on account of its peculiar property of felting. Furs, however, are mostly yielded by mammals of other orders, among which the Fur-seals are perhaps the most important at the present day. Many other Carnivores yield valuable furs, among which may be mentioned Bears, Foxes, Racoons, Skunks, Minks, Otters, and Ermines. Of less importance are certain Rodents, such as the Squirrels, Rabbits, Hares, etc., while the hair of the Beaver was formerly much sought after for the manufacture of hats. Returning to the Ungulates, we may notice the importance of horse-hair, the employment of camel's hair for brushes, and the many uses of the bristles of the pig. Some of the Monkeys yield fur which has been extensively used. Leather, again, is almost exclusively supplied by mammals, and mainly by the Ungulates.

Three other important products, namely horn, buck's-horn, and ivory, are likewise obtained solely from the same great order. Horn, as we shall notice in the sequel, is the sheath covering the bony horn-cores of the Oxen, while buck's-horn is the commercial term applied to the antlers of the Deer, which are largely used for knife-handles and other purposes. True ivory is the product of the two species of Elephant; but other kinds of ivory are obtained from the teeth of the Sperm Whale and the tusks of the Walrus and Hippopotamus, the latter kind having been extensively employed some years ago for artificial teeth. For many purposes the place of ivory is taken by bone, this being mostly obtained from Ungulates. The bones of Camels are of an especially firm texture and good colour, and are largely employed in India for inlaying. Other important uses of bones are in the form of bone-dust as manure, and also as a source of phosphoric acid. The horns of the African Rhinoceros and the hide of the Hippopotamus are occasionally

manufactured into small canes or whips. Horns and hoofs are also largely employed in the manufacture of glue.

Formerly the so-called whalebone, or more properly baleen, was much used, especially to form the ribs of umbrellas and in stiffening ladies' apparel, but the gradual destruction of the Right Whales, its only source of supply, has largely restricted its use of late years.

The Cetacea are also of great economical importance from the abundance of oil yielded by the thick layer of blubber underlying the skin. Large quantities of valuable oil are also furnished by the Walrus and the Seals. Spermaceti, which was at one time extensively used in the manufacture of candles, is obtained from a large cavity in the head of the Sperm Whale or Cachalot, and also from the Hyperoödon or Bottle-nosed Whale.

The nature of ambergris, a peculiar substance found floating on the surface of the sea and employed in perfumery, was long a matter of controversy; but it appears to be an intestinal concretion of the Sperm Whale. Other substances of more importance to the perfumer are musk, the product of the Musk-Deer of the Himalaya, and civet, which is obtained from the so-called Civet Cat and other allied Carnivores. A secretion of the Beaver has also been used in perfumery and in medicine.

CHAPTER II

GENERAL ANATOMICAL CHARACTERS

I. TEGUMENTARY STRUCTURES

Hair. The external surface of the greater number of members 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 hairs upon different parts of the same animal, or upon different animals, assume various forms, and are of various sizes and degrees of rigidity,-as seen in the delicate soft velvety fur of the Mole, the stiff bristles of the Pig, and the spines of the Hedgehog and Porcupine, all modifications of the same structures. Each hair is composed usually of a cellular pithy internal portion, containing much air, and a denser or more horny cortical part. In some animals, as Deer, the substance of the hair is almost entirely composed of the medullary or cellular substance, and it is consequently very easily broken; in others the horny part prevails almost exclusively, as in the bristles of the Wild Boar. In the Three-toed Sloth (Bradypus) the hairs have a central horny axis and a pithy exterior. Though generally nearly smooth, or but slightly scaly, the surface of some hairs is strongly imbricated, notably so in some Bats; while in the Two-toed Sloth (Cholapus) the hairs are longitudinally grooved or fluted. Though usually more or less cylindrical or circular in section, hairs are often elliptical or flattened, as in the curly-haired races of men, the terminal portion of the hair of Moles and Shrews, and conspicuously in the spines of the Rodents Xerus and Platacanthomys. Hair having a property of mutual cohesion or "felting," which depends upon a roughened scaly surface and a tendency to curl, as in domestic Sheep (in which animal this property has been especially cultivated by selective breeding), is called "wool."