134 THE ORIGIN OF SPECIES 



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Illustrations of the Aciioti of Natural Selection^ or the 

 Survival of the Fittest 



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

 selection acts, I must beg pcrmissiou 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 sup- 

 pose 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 slim- 

 mest 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 grey- 

 hounds 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 fre- 

 quently 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 pre- 



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