RECAPITULATION 229 



the proximal part of the feather remains tubular and 

 does not split, and the vascular tissue within dies, 

 shrivels, and dries up, forming the pith of the quill. 

 When the papilla recommences to grow the old 

 feather is pushed out, and this process causes the 

 moult. It would appear, therefore, that the feather 

 must have been evolved, not by a continuous modi- 

 fication from the scale but by a development of a 

 new kind between the scales. I have been unable 

 to discover hitherto any evidence suggesting an 

 external stimulus which could cause this remarkable 

 process of development in feathers, or indicating 

 that the function of flight would involve such a 

 stimulus. For the present, therefore, we must con- 

 clude that feathers are not an adaptation, and not 

 due to somatogenic modification, but must be the 

 result of a gametogenic mutation. 



Feathers, having been evolved, served in the 

 wings and tail as important organs of flight. There 

 is reason to believe that, once present, the size and 

 growth of feathers was modified greatly by the 

 degree of stimulation applied to the papillae at their 

 roots by the movement and bending strain of the 

 feathers. The modification of the bones and muscles 

 of the wing, shoulders, and sternum by the functional 

 stimuli involved in flying are obviously adaptations, 

 and in my opinion are only to be explained as the 

 hereditary effects of functional stimulation, like all 

 skeleto-muscular adaptations. The strains produced 

 in bones by muscular contraction produce hyper- 

 trophy of the part of the bone to which the muscles 

 are attached, and thus we can understand the origin 

 of the carina of the sternum in flying birds, and its 

 absence in flightless forms. In bats and in ptero- 



