78 ORGANIC EVOLUTION — THE FACTORS 



probable one, since it implies that those slight extra 

 thicknesses of skin on the knuckles with which we 

 must suppose the selection to have commenced, were 

 so advantageous as to cause survivals of the individuals 

 having them. That survivals so caused, if they ever 

 occurred at all, should have occurred with the frequency 

 requisite to establish and increase the variation, is 

 hardly supposable. And if we reject, as also unlikely, 

 the reproduction of these callosities de novo in each 

 individual, there remains only the inference that they 

 have arisen by the transmission and accumulation of 

 functional adaption. 



" Another case which seems interpretable only in an 

 analogous way, is that of the spurs which 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 on those 

 points of 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 reaction of the blows, if not 

 in inflicting the blows. But to conclude this is, I think, 

 to conclude against probability. Contrariwise, if we as- 

 sume that the thickenings of the epidermis produced by 

 habitual rough usage is inheritable, the development 

 of these structures presents no difficulty. The points 

 of impact would become indurated in wings used for 

 striking with unusual frequency. The callosities of 

 the surface thus generated, rendering the parts less 

 sensitive, would enable the bird in which they arose 

 to give, without injuiy to itself, violent blows and a 



