80 ORGANIC EVOLUTION—THE FACTORS 



apparently forgets that the callosities on the hands of 

 men who do hard manual labour, and on the feet of 

 men who go barefoot, are never reproduced in their 

 infants, who were born as soft-handed and soft-soled as 

 the infants of those who do no labour, nor go barefoot. 

 As regards the Chaja screamer, Mr. Spencer, like Mr. 

 Cunninghame when writing on the evolution of horns, 

 does not correctly state the case when he says, " 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." His words imply 

 that' the thickenings were "occasional" variations, and 

 that, if we hold that spontaneous variations are alone 

 transmitted, we must believe that the evolution of 

 spurs in the Chaja screamer was due to the increased 

 rate of survival conferred by these rare variations — for 

 rare they must have been to have occurred " symmetri- 

 cally," " at points required." But, as already stated, 

 evolution does not proceed on lines of rare variations, 

 which are necessarily swamped during the process of 

 interbreeding, but on lines of normal variations. Every 

 screamer must have varied above or below a certain 

 mean as regards the thickness and denseness of the 

 tissues "at the points required," and fortunate 

 variations in this case must have been particularly 

 important, since unfavourable variations must have led, 

 in a prolonged combat or in a series of combats, to such 

 a denudation of the epidermis as rendered a renewal of 

 combat painful, and thereby caused a corresponding 

 disinclination to fight for food or females. Fitness in 



