NATURAL SELECTION 587 



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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 CHAPTEB 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 



