TEGUMENTAL ORGANS 11 



such as climate, domestication, natural and artificial selection, 

 influence the hairy covering. Further, the development of this 

 may be in inverse ratio to the thickness of the integument, and 

 particularly of the epidermis (Leydig), the hair and the epidermis 

 supplementing one another in the work of protecting the body. 

 This is illustrated, on the one hand, by animals which have a 

 delicate epidermis and thin skin and a thick covering of wool or 

 fur; and on the other by animals' like the Rhinoceroses, Hippo- 

 potami, some Armadillos, and Scaly Ant-Eaters, in which, while 

 the epidermis is so thickened as to form a hard carapace, the 

 hair is very scanty. 



I cannot leave this subject without touching upon the question of the 

 origin of the Mammalia, especially as this chapter in morphology has recently 

 been ably dealt with by Max Weber, who deduces reasons for taking up the 

 following position. The first Mammals, as descendants from primitive scaly 

 Reptiles, were covered with scales, differing from those of the Reptiles only in 

 minor points. Behind the scales of the primitive Mammals there first 

 appeared a few small hairs, the origin of which it is difficult to explain with 

 certainty. By degrees, as a constant temperature was maintained by the 

 body, the covering of hair attained a greater development and the scales 

 degenerated. Scales, somewhat specialised, are still retained as a covering 

 for the mammalian body in a few cases, e.g. Armadillos and Scaly Ant-Eaters. 

 Among other Mammals they are found, as a rule, only on the tail and limbs. 

 The recurrent arrangement of the hairs, however, due to their original 

 development behind scales, has very generally persisted, and on this basis 

 hairs may be considered to imply the earlier presence of scales. 



NAILS 



The nails of the fourth and fifth fingers (and especially the 

 latter) most nearly suggest the claws of the lower animals, in being 

 decidedly arched from side to side. As the thumb is approached 

 the nails become more and more flat, and the like is true of the 

 great toe as compared with the four lesser toes. This condition 

 commences with the Lemuroidea [although among the lower 

 Mammalia the Squirrels, for example, bear a flattened nail upon 

 the pollex]. 



On the under edge of the nail, between it and the ball of the 

 finger, is found the last vestige of a structure which in the Apes 

 is covered with a thickened layer of epidermis. 1 This structure 

 undergoes considerable degeneration, even during intra- uterine 

 life, through the advancing development of the ball of the finger 

 (Gegenbaur). 



1 This structure is most conspicuous in the Ungulata, and it is there known as the 

 "frog." 



