84 ILLUSTRATIONS OF THE ACTION 



greyhound-like form, whicli 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 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 inter- 

 crossing 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 ani- 

 mals, _ 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 organ- 

 isms. 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 favorable variation; still, as the Eeviewer 

 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 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 individual perpet- 

 uating its kind to the exclusion of the common form- but 



