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 prehensile-tailed 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 fœtus, 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 Harvest-mouse, 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.