OTHER THEORIES OF SEXUAL COLOURING 329 



to the production or change of colour bear such a 

 relation to the vital energy expended in their develop- 

 ment, that we can judge of the amount of the 

 expenditure by the degree of admiration excited in 

 ourselves.^ 



The effects are only explicable by a theory of 

 selective breeding 



We are also required to believe that these hetero- 

 geneous elements are combined by the same means 

 into an elaborate and harmonious whole. A process 

 of selective breeding, like that of Sexual Selection, 



* A white peacock in the Zoological Gardens, shown to me by Mr. 

 F. E. Beddard, appears at first sight to support Mr. Wallace's views ; 

 for the • pigment ' colours and * structural ' colours are alike absent 

 (see p. 11). Closer examination reveals the fact that regions in 

 which ' structural ' colours usually appear are readily recognisable, 

 the white being of a different quality. The ' eyes ' on the train, for 

 example, are quite distinct, coming out like the pattern on a white 

 damask table-cloth. 



Dr. Gadow informs me that the same fact is true of white ducks 

 and drakes, the wing coverts, which are blue in normally pigmented 

 individuals, exhibiting a pecuUar sheen or gloss, dilfering from the 

 rest of the plumage. Dr. Gadow states that the structural colours 

 are absent, because the existence of a pigment beneath the super- 

 structure is necessary in order to show them off ; and he points out 

 that the ancestors of birds with such structural colours cannot well 

 have been white, because the effect depends in part upon pigment. 



Mr. Gotch and I found that the ' eyes ' of the white peacock do 

 not regain the normal appearance in any of the colours of the 

 spectrum, nor when examined by monochromatic light. 



Inasmuch as we can trace the form and distribution of all structural 

 markings in an albino animal, it is clear that the physical cause of 

 the appearance is not affected by albinism, in the same manner as 

 the cause of pigment colours. 



