ACTION OF NATURAL SELECTION 103 



structure, colour, 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 differ- 

 ences 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 

 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 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 selec- 

 tion 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 for food. Under such circumstances the 

 swiftest and slimmest wolves would have the best chance of 

 surviving and so be preserved or selected, — provided always 

 that they retained strength to master their prey at this or 

 some other period of the year, when they were compelled to 

 prey on other animals. I can see no more reason to doubt 

 that this would be the result, than that man should be able to 

 improve the fleetness of his greyhounds by careful and 

 methodical selection, or by that kind of unconscious selection 

 which follows from each man trying to keep the best dogs 

 without any thought of modifying the breed. I may add, 

 that, according to Mr. Pierce, there are two varieties of the 

 wolf inhabiting the Catskill Mountains, in the United States, 

 one with a light greyhound-like form, which pursues deer, 



