THE INTEGUMENTARY SYSTEM 



173 



Birds, which have descended from reptihan ancestors, still have 

 scales on their feet, and even their bills and claws are presumably enlarged 

 and modified scales. 



HAIRS 



Hairs, which are characteristic of mammals, are not comparable 

 morphologically with either feathers or scales, since their development is 



CORIUM 

 PAPILLAV 



BARBULCS 



PULP^^^==7Z/^ 



C. rOLLICLE' 



Fig. 131. — Four stages in the development of a feather. A, B, and C represent 

 stages in the development of a down feather. D shows a contour feather in the feather- 

 sheath. A-B and C-D are sections of a young contour feather at the levels indicated 

 in D. In contrast with a hair the development of a feather is initiated by a corium 

 papilla. (Redrawn from Ihle, after Blitschli.) 



initiated by the epidermis and not by the corium. When, as in Manis, 

 hairs and scales occur together, the hairs are at the apices of the scales. 

 That scales are older phylogenetically than hairs, is indicated by the 

 fact that scales develop earher in the embryo; and fossil evidence demon- 

 strates that mammals have evolved from some scaly Stegocephalan-Hke 

 Cotylosaurian. But since neither the skin nor its non-bony appendages 

 are commonly fossilized, their history has to be made out chiefly from 

 embryology and comparative anatomy. 



All mammals have hair; and man's relative hairlessness is by no 

 means a distinctive human trait, since some mammals, for example the 

 whales, have less hair than man. It is well known that changes in the 

 secretion of the endocrine glands affect profoundly the growth of hair, 

 and man's loss of hair may have been thus brought about. 



