NATURAL SELECTION 71 



tions. Let us take the case of a wolf which preys on various ani- 

 mals, securing some by craft, some by strength, and some by fleet- 

 ness ; 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 cir- 

 cumstances the swiftest and slimmest wolves have the best chance 

 of surviving, and so being preserved or selected, provided always 

 that they retain 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 selec- 

 tion by man, which depends on the preservation of all the more 

 or less valuable individuals, and on the destruction of the worst. I 

 saw, also, that the preservation in a state of nature of any occa- 

 sional deviation of structure, such as a monstrosity, would be a 

 rare event; and that, if at first preserved, it would generally be 

 lost by subsequent intercrossing with ordinary individuals. Never- 

 theless, until reading an able and valuable article in the North 

 British Review (1867), I did not appreciate how rarely single 

 variations, whether slight or strongly marked, could be perpetu- 

 ated. The author takes the case of a pair of animals, producing 

 during their lifetime two hundred offspring, of which, from various 

 causes of destruction, only two on an average survive to procreate 

 their kind. This is rather an extreme estimate for most of the 

 higher animals, but by no means so for many of the lower or- 

 ganisms. He then shows that if a single individual were born, 

 which varied in some manner, giving it twice as good a chance of 

 life as that of the other individuals, yet the chances would be 



