586 EVOLUTION, GENETICS, AND EUGENICS 



selection : that is, by individual males having had, in successive gen- 

 erations, 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 peculi- 

 arities arising and becoming attached to the male sex, which appar- 

 ently 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 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 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, and 

 the other more bulky, with shorter legs, which more frequently attacks 

 the shepherd's flocks. 



It should be observed that, in the above illustration, I speak of the 

 slimmest individual wolves, and not of any single strongly marked 

 variation having been preserved. In former editions of this work I 

 sometimes spoke as if this latter alternative had frequently occurred. 

 I saw the great importance of individual differences, and this led me 

 fully to discuss the results of unconscious selection by man, which 



