46 THE GENETIC AND THE OPERATIVE EVIDENCE 



can protect their nests if attacked. As a further support of his 

 view, Wallace points out that in the few cases where the female is 

 more highly colored than the male (as the dotterel, species of phalarope, 

 an Australian creeper) the male incubates the eggs. 



Wallace's suggestion still leaves unexplained the ornamentation of 

 the male, which he tries to account for as the direct result of the greater 

 vitality of the male. He tries to show that excessive ornaments and 

 high coloration develop especially in those parts of the body to which 

 there is an unusual supply of blood or where nerves and blood-vessels 

 emerge to go to the skin or to the muscles. 



"If we have found a vera causa for the origin of ornamental appendages of 

 birds and other animals in a surplus of vital energy, leading to abnormal 

 growths in those parts of the integument where muscular and nervous action 

 are greatest, the continuous development of these appendages will result from 

 the ordinary action of natural selection in preserving the most healthy and 

 vigorous individuals, and the still further selective agency of sexual struggle 

 in giving to the very strongest and most energetic the parentage of the next 

 generation. And, as all the evidence goes to show that, so far as female birds 

 exercise any choice, it is for 'the most vigorous, defiant, and mettlesome male,' 

 this form of sexual selection will act in the same direction, and help to carry 

 on the process of plume development to its culmination. That culmination 

 will be reached when the excessive length or abundance of the plumes begins 

 to be injurious to the bearer of them; and it may be this check to the further 

 lengthening of the peacock's train that has led to the broadening of the 

 feathers at the ends, and the consequent production of the magnificent eye- 

 spots which now form its crowning ornament. 



"The display of these plumes will result from the same causes which led to 

 their production. Just in proportion as the feathers themselves increased in 

 length and abundance, the skin-muscles which serve to elevate them would 

 increase also; and the nervous development as well as the supply of blood to 

 these parts being at a maximum, the erection of the plumes would become a 

 habit at all periods of nervous or sexual excitement. The display of the 

 plumes, like the existence of the plumes themselves, would be the chief external 

 indication of the maturity and vigor of the male, and would, therefore, be 

 necessarily attractive to the female. We have, thus, no reason for imputing 

 to her any of those esthetic emotions which are excited in us, by the beauty of 

 form, color, and pattern of these plumes; or the still more improbable esthetic 

 tastes, which would cause her to choose her mate on account of minute differ- 

 ences in their forms, colors, or patterns." 



Wallace says, referring to the immense tuft of golden plumage in 

 the best known birds of paradise (Paradisea apoda and P. minor) that 

 springs from a very small area on the side of the breast, that Mr. Frank 

 E. Beddard, who has kindly examined a specimen, says that "this 

 area lies upon the pectoral muscles, and near to the point where the 

 fibers of the muscle converge towards their attachment to the humerus. 

 The plumes arise, therefore, close to the most powerful muscle of the 

 body, and near to where the activities of that muscle would be at a 

 maximum. Furthermore, the area of attachment of the plumes is just 



