110 ILLUSTRATIONS 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 subject. 



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

 males of any animal have the same general habits of life, 

 but differ in structure, colour, or ornament, such dif- 

 ferences have been mainly caused by sexual selection: 

 that is, by individual males having had, in suc- 

 cessive 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 dif- 

 ferences to this agency: for we see in our domestic ani- 

 mals peculiarities arising and becoming 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 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 mon- 

 strosity. 



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 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 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 



