NATURAL SELECTION 233 
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 
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 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 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 favorable 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 genera- 
tions. 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 perpetuating its kind to 
the exclusion of the common form; but there can hardly be a doubt, 
judging by what we see taking place under domestication, that this 
result would follow from the preservation during many generations of 
a large number of individuals with more or less strongly curved beaks, 
and from the destruction of a still larger number with the straightest 
beaks. 
SUMMARY OF CHAPTER ON NATURAL SELECTION 
If under changing conditions of life organic beings present indi- 
vidual differences in almost every part of their structure, and this 
cannot be disputed; if there be, owing to their geometrical rate of 
increase, a severe struggle for life at some age, season, or year, and 
this certainly cannot be disputed; then, considering the infinite com- 
plexity of the relations of all organic beings to each other and to their 
conditions of life, causing an infinite diversity in structure, constitu- 
tion, and habits, to be advantageous to them, it would be a most 
