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. 
“Tt 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 bea 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 
