Chap. IV.] NATURAL SELECTION. 71 



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 surviv- 

 ing, 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 fleetuess of his greyhouuds 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 grey- 

 hound-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 auy 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 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 occasional 

 deviation of structure, such as a monstrosity, would be a rare event ; 

 and that, if at first preserved, it would generally be lost by subse- 

 quent intercrossing with ordinary individuals. Nevertheless, 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 perpetuated. 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 pro-create their kind. This is rather 

 an extreme estimate for most of the higher animals, but by no means 

 so for many of the lower organisms. 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 stiongly against its survival. Supposing it tc 

 survive and to breed, and that half its young inherited the favour- 

 able variation ; still, as the Reviewer goes on to show, the young 



