110 ILLUSTEATIOXS OF THE ACTION OF [Chap. IV. 



and transmitted to the males alone or to both sexes at 

 corresponding ages ; but I have not space here to enter 

 on this snbject. 



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, colour, or ornament, such differences 

 have been mainly caused by sexual selection : that is, 

 by individual males having had, in successive genera- 

 tions, 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 becoming attached to the male sex, which apparently 

 have not been augmented through selection bv man. 

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

 cannot 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. 



Illustrations of the Action of Natural Selection, or the 



Survival of the Fittest 



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

 selection acts, I must beg permi<-i>ai 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 fleetness; and let us 

 suppose that the fleetest prey, a deer for instance, had 

 from any change in the country increased in numbers, 

 or that other prey had decreased in numbers, during that 

 season of the year when the wolf was hardest pressed 



