EXTERNAL FEATURES 29 



like the primordia of Reptilian scales, and there seems to be 

 no difficulty here. On the body of the grass-snake and of 

 the chick, and on the feet of the chick, there are very similar 

 structures, passing without definite limits into one another, 

 forming a sort of rippling and due to proliferation of both 

 dermis and epidermis. When feathers are going to develop 

 there is on the second day a proliferation of epidermis at 

 the apex and steep side of the papillae, and the feather 

 primordia becomes well-defined as if the rippling was 

 smoothed out. The foot of the chick and the fore limb of 

 the blackbird show embryonic down feathers on scales, 

 and at a very early stage these down primordia are seen 

 arising as special differentiations on the scale primordia. 

 These facts seem on the whole to point to the conclusion 

 that a scale does not correspond to more than the base of a 

 feather. 



The very opposite view is suggested by Bornstein 

 (191 1) who studied the foot of the capercailzie, where 

 feathers and scales occur in close association. The con- 

 clusion he arrived at is that a feather corresponds not to an 

 entire scale, but only to. part of a scale, the rest being 

 suppressed. 



There is a geometrical precision about the disposition 

 of the feathers, which is interpreted by S. Schaub (1907) 

 in correlation with strains on the skin. He also points out 

 that while there may have been a primitive diffuseness of 

 distribution from which the geometrically orderly arrange- 

 ments have evolved, a secondary diffuseness may arise in 

 the definite plumage. 



§ 4. Moulting of Feathers 



The advantage of moulting, which is normally annual, 

 is obvious, for it means a replacement of worn feathers. 

 But it is not clear to what precise stimulus the activation of 

 the folhcle is due. Moulting usually takes place after the 

 fatigue of the breeding season and before the autumnal 

 migration, but in swallows, diurnal birds of prey, and some 



