NATURAL SELECTION 1493 



Some well-known laws, with respect to the plumage 

 of male and female birds, in comparison with the 

 plumage of the young, can partly be explained 

 through the action of sexual selection on variations 

 occurring at different ages, and transmitted to the 

 males alone or to both sexes at corresponding ages. 



Thus it is, as I believe, that when the males and 

 females of any animal have the same general habits 

 of life, but differ in structure, color, or ornament, 

 such differences have been mainly caused by sexual 

 selection: that is, by individual males having had, in 

 successive generations, some slight advantage over 

 other males, in their weapons, means of defence, or 

 charms, which they have transmitted to their male 

 offspring alone. Yet, I would not wish to attribute 

 all sexual differences to this agency: for we see in 

 our domestic animals peculiarities arising and be- 

 coming attached to the male sex, which apparently 

 have not been augmented through selection by man. 

 The tuft of hair on the breast of the wild turkey- 

 cock can not be of any use, and it is doubtful whether 

 it can be ornamental in the eyes of the female bird ; 

 indeed, had the tuft appeared under domestication, 

 it would have been called a monstrosity. 



In order to make it clear how, as I believe, natural 

 selection acts, I must beg permission to give one or 

 two imaginary illustrations. Let us take the case of 

 a wolf, which preys on various animals, securing 

 some by craft, some by strength, and some by fleet- 

 ness; and let us suppose that the fleetest prey, a deer, 

 for instance, had from any change in the country in- 

 creased in numbers, or that other prey had decreased 



