THE SEXUAL SELECTION THEORY 145 



in thickness — they are longer than those of the female, 

 but resemble them in appearance, and further, the whole 

 stature is greatly increased, but it is at the same time 

 conspicuously less massive, particularly at the neck and 

 forequarters. In eunuchs it results in immense stature 

 and the loss of the more characteristic male features, such 

 as the beard and the bass voice. The removal of the 

 testes in birds is always a difficult operation and is rarely 

 successfully performed. Hence the accounts of changes in 

 plumage consequent on this operation are inconclusive. 

 It has generally been supposed that whenever, either by 

 removal or by disease, the testes are rendered inoperative 

 the plumage, when normally of a resplendent type, assumes 

 the coloration of the female. This is probably an 

 erroneous supposition, but what happens is a failure to 

 secrete the more intense pigments and the more special- 

 ized forms of feathers, so that the resultant dress answers 

 to the juvenile male dress. It is not a case of " reversion " 

 to this livery, but a failure to assume the latest acquire- 

 ments of the species. These, as has already been shown, 

 are only very gradually developed. The intensity of 

 pigmentation, or concentration of pigmentation, which 

 results in sharply defined areas of colour, is a cumulative 

 process. As it loses in intensity at any given moult, so 

 the individual tends to reproduce the phases of the earlier 

 and vanishing livery. Sooner or later, however, this 

 earlier livery disappears more or less completely : is 

 eliminated from the system, so to speak : and what is 

 commonly called lack of " vigour " results, not in a return 

 to the earlier, sombre dress, but in the later-acquired, 

 resplendent plumage lacking intensity. The seasonal, 

 temporary secondary sexual character has become, as 

 some say, a " somatic " character. Highly probable as 



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