206 VllYSIOLOGlCAL DEVELOPMENT. 



individuals Laving them. That survivals so caused, if thej 

 ever occurred at all, should have occurred with the frequency 

 requisite to' establish and increase the variation, is hardly 

 siipposable. And if we reject, as also unlikely, the repro- 

 duction of these callosities de novo in each individual, 

 there remains only the inference that they have arisen 

 by the transmission and accumulation of functional adapta- 

 tions. Another case which seems interpretable 

 only in an analogous way, is that of the spurs that are 

 developed on the wings of certain birds — on those of the 

 Chaja screamer for example. These are weapons of offence 

 and defence. It is a familiar fact that many birds strike 

 with their wings, often giving severe blows ; and in the 

 birds named, the blows are made more formidable by the 

 horny, dagger^shaped growths standing out from those points 

 on the wings which deliver them. Are these spurs directly 

 or indirectly adaptive ? To conclude that natural selection 

 of spontaneous variations has caused them, is to conclude 

 that, without any local stimulus, thickenings of the skin 

 occurred symmetrically on the two wings at the places 

 required ; that such thickenings, so localized, happened to 

 arise in birds given to using their wings in fight ; and that 

 on their first appearance the thickenings were decided enough 

 to give appreciable advantages to the individuals distinguished 

 by them — advantages in bearing the reactions of the blows if 

 not in inflicting the blows. But to conclude this is, I think, 

 to conclude against probability. Contrariwise, if we assume 

 that the thickening of the epidermis produced by habitual 

 rough usage is inheritable, the development of these struc- 

 tures presents no difilculty. The points of impact would 

 become indurated in wings used for striking with unusual 

 frequency. The callosities of surface thus generated, render- 

 ing the parts less sensitive, would enable the bird in which 

 they arose to give, without injury to itself, more violent blows 

 and a greater number of them — so, in some cases, helping it 

 to conquer and survive. Among its descendants, inheriting 



