66 ILLUSTRATIONS OF THE ACTION OF [CHAP. IV. 



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 com- 

 pelled 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 uncon-j 

 scious 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 subsequent intercrossing with ordinary indi- 

 viduals. 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 strongly against its survival. Supposing it to survive 

 and to breed, and that half its young inherited the favourable 

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

 would have only a slightly better chance of surviving and breed- 

 ing ; and this chance would go on decreasing in the succeeding 

 generations. The justice of these remarks cannot, I think, be 

 disputed. If, for instance, a bird of some kind could procure its 

 food more easily by having its beak curved, and if one were born 

 with its beak strongly curved, and which consequently flourished, 

 nevertheless there would be a very poor chance of this one indi- 



