INTRODUCTORY 



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 quad- 

 rate 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 terres- 

 trial 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 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 infra-uterine life between the 

 circulatory systems of the parent and the fcetus, the young are 

 born in an exceedingly imperfectly developed condition. For their 



