CiiAP. IV. NATUKAL SELECTION. 93 



pressed for food. Under such cLrcumstances the swiftest and 

 slimmest wolves would have the best chance of survdving-, and 

 so be preserved or selected — provided always that they retained 

 strength to master their prey at this or at some other period 

 of the year, when they might bo compelled to prey on other 

 animals. I can see no more reason to doubt tliis, than that 

 man can improve the fleetness of his greyhounds by careful and 

 methodical selection, or by unconscious selection which results 

 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 strong- 

 Ij'-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 un- 

 conscious selection by man, which depends on the preservation 

 of the better-adapted or more 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 preserved, 

 it would generally be lost by subsequent intercrossing with 

 ordinary individuals. Nevertheless, until reading an able and 

 valuable article in the N'orth Uritish lievieio (18G7), 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, whicli produce during their lifetime 

 two hundred offspring, of which, from various causes of de- 

 struction, only two on an average survive to procreate their 

 kind. Tliis 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, whicli varied in souk* maimer, giving it twice as g'ood a 

 chance of life as that of the other individuals, yet the chances 

 would be strongly against its survival. Supposing it to sur- 

 vive and to breed, and that half its young inherited the favor- 

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

 young would liavo only a slightly-better chance of survaving 



