28 COMPARATIVE ANATOMY 



The contour feathers are generally not distributed irregularly 

 over the body, but are arranged in definite feather- tracts (ptcrylce) 

 separated by down-covered spaces (apteria), having a more or less 

 different arrangement in the various groups. 1 



A periodic casting of feathers, or moulting, takes place in all 

 Birds, and corresponds to the similar process of the casting of the 

 horny epidermic layer in Amphibians and Reptiles : the papilla per- 

 sists, and in connection with it the replacing feather is developed. 



The feather-covering of Birds must have been acquired in very 

 early geological periods, for Archasopteryx, found in the Jurassic 

 strata of Bavaria, possessed well-formed feathers with a delicate 

 shaft and vane (Fig. 49). 



The colours of feathers are due in part to the presence of 

 various pigments (viz., red, yellow, orange, black, and brown), and 

 in part to the phenomenon of interference, which may produce 

 white, grey, blue, and metallic or iridescent tints. 



Mammals. 



The integument of Mammals is characterised by the presence 

 of hairs, and the question as to how far scales, feathers, and hairs 

 are comparable to one another is an interesting one. No inter- 

 mediate forms are known, but there is no doubt that the 

 feather is much nearer to the scale than is the hair. The study 

 of their development, however, shows that the origin both of hair 

 and feather may be traced in the first instance to similar scale- 

 like structures, in spite of their very different final form. Thus 

 phylogenetically both are closely related to the horny scales of 

 Reptiles. 



The development of hairs, as well as their grouping and distri- 

 bution, indicates certain topographical relations to scales, and 

 also that they first arose in relation with a primitive scaly coat. 

 Secondarily they appeared on or behind the scales, which were 

 gradually reduced as the hairs underwent increasing differentia- 

 tion. Hairs, which, like feathers, are arranged in groups, are not, 

 however, individually homologous with scales, but arise from parts 

 of a scale-area, while the feather possibly corresponds to an 

 entire scale. There can be little doubt that the earliest 

 Mammals, which arose from primitive Reptiles, possessed an 

 extensive scaly covering in addition to a sparse coat of hairs. 



In development, the first essential indication of the hair is 

 seen in the epiderm, which may or may not become raised up at 



1 In some Birds bristle-like feathers occur on the head, and the foot-scales 

 or shields may bear feathers of a peculiar form. In insectivorous and nocturnal 

 forms, tactile or sinus-feathers are present around the eye and ear and at the base 

 of the beak, analogous to the sinus-hairs of Mammals (q.v.). 



