122 Evolution and Adaptation 



selection is carried out. The first example is the imaginary 

 case of a species of wolf, the individuals of which secure 

 their prey sometimes by craft, sometimes by strength, and 

 sometimes by fleetness. If the prey captured by the first 

 two methods should fail, then all the wolves would be obliged 

 to capture their food by fleetness, and consequently the fleet- 

 est alone would survive. " I can see no more reason to doubt 

 that this would be the result than that man should improve 

 the fleetness of his greyhounds." But even if the fleetness 

 of the race could be kept up in this way, it does not follow 

 that a new species of wolf would be formed in consequence, 

 as Darwin implies. His own comment on this illustration is, 

 perhaps, the best criticism that can be made. 



" 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 alter- 

 native 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 Brit- 

 ish Review (1867), I did not appreciate how rarely single 

 variations, whether slight or strongly marked, could be per- 

 petuated. 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 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 



