CHAP. XIII MAMMALIA 437 



control and co-ordination of complex movement of the body. The 

 second feature is the unrivalled height to which viviparity has evolved 

 in the Mammalia, the early, relatively helpless, stages being passed 

 within the body of the mother, the young individual forming as it were 

 a constituent part of an adult individual with its full equipment for 

 holding its own in the struggle for existence. The third feature is that 

 within the limits formed by the general characteristics of the group there 

 exists a remarkable variety of specialization for very different modes of 

 life. These speciaUzations are associated more especially with different 

 types of movement — running (Ungulates), burrowing (Mole), cHmbing 

 (Sloths, Monkeys), flying (Bats), swimming (Sirenia, Cetacea). 



What may be termed the normal form of body of the mammal (Fig. 

 i77j C, p. 403) does not differ widely from that of the lower Tetrapoda 

 but an important advance is seen in that the mammal carries its 

 body well clear of the ground instead of merely shuffling along its 

 surface. 



As in Reptiles and Birds the skin is provided with special develop- 

 ments of the horny layer, in this case hair. On the other hand the 

 mammal differs from the Reptile or Bird and resembles rather the 

 Amphibian in the fact that its skin is richly provided with epidermal 

 glands. 



A hair is a thin fibre-like extension of the homy layer of the epidermis. 

 To give it firmness the underlying portion of the deep, active, growing, 

 layer of the epidermis (Malpighian layer) becomes sunk down into the 

 dermis so that the hair projects from a deep socket or follicle. The 

 main function of the hair is to serve, with the air intervening between 

 the individual hairs, as a non-conducting coat to lessen the loss of heat 

 from the surface of the body. It serves also an important function in 

 giving the mammal the colouration characteri^ic of the species. As a 

 rule this is of such a kind as to render the animal inconspicuous when 

 in its natural environment and under natural illumination from the sky. 

 The pigmentation of the individual hairs is commonly such as to counter- 

 act the brighter illumination of the upper surface of the body and the 

 less bright illumination of the lower surface, and in this way to flatten 

 out the relief of the body when seen under normal illumination (Thayer's 

 principle). The continuity of surface may also be broken up by strongly 

 marked parti-colouring — as in the Zebra — while in the smaller mammals 

 the fur helps to blur the sharpness of outline of the body. 



The hairs, projecting out beyond the general surface of the body, 

 form as it were outposts for the reception of tactile impressions and we 

 findj as we should expect, that their bases are provided with sensory 



