THE OUTER TISSUES OF ANIMALS. 313 



this would imply that after a thousand generations each 

 young gorilla began with knuckles having skin no thicker 

 than elsewhere), there remains only the inference that they 

 have arisen by the transmission and accumulation of func- 

 tional adaptations. 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 some 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 dis- 

 tinguished 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 structures presents no difficulty. The points of impact 

 would become indurated in wings used for striking with 

 unusual frequency. The callosities of surface thus generated, 

 rendering 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 multiply. Among its descend- 

 ants, inheriting the modification and the accompanying habit, 

 the thickening would be further increased in the same way: 

 survival of the fittest tending ever to accelerate the process. 





