OF NATURAL SELECTION. 79 



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 try- 

 irig 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 Moun- 

 tains, 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 

 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. Nevertheless, until reading an able and valua- 

 ble 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 life- 

 time 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 organisms. He then shows that if a single individ- 

 ual 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 strongly against its survival. Sup- 

 posing it to survive and to breed, and that half its young 

 inherited the favorable variation ; still, as the reviewer goes 

 on to show, the young would have only a slightly better 

 chance of surviving and breeding ; and this chance would 

 go on decreasing in the succeeding generations. The justice 



