64 THE GENESIS OF SPECIES: [CHAP 



belief of Mr. Darwin an example of the unconscious action 

 of volition upon credence. A vast number of difficulties 

 similar to those which have been mentioned might easily 

 be cited those given, however, may suffice. 



There remains, however, to be noticed a very weighty 

 consideration, which was brought forward in the North 

 British Review for June 1867, p. 286, namely, the necessity 

 for the simultaneous modification of many individuals. 

 This consideration seems to have escaped Mr. Darwin, for 

 at p. 104 of his last (fifth) edition of " The Origin of Species," 

 he admits, with great candour, that until reading this 

 article he did not " appreciate how rarely single variations, 

 whether slight or strongly marked, could be perpetuated." 



The North British Review (speaking of the supposition that 

 a species is changed by the survival of a few individuals in 

 a century through a similar and favourable variation) says: 

 " It is very difficult to see how this can be accomplished, 

 even when the variation is eminently favourable indeed ; 

 and still more difficult when the advantage gained is very 

 slight, as must generally be the case. The advantage, 

 whatever it may be, is utterly outbalanced by numerical 

 inferiority. A million creatures are born ; ten thousand 

 survive to produce offspring. One of the million has 

 twice as good a chance as any other of surviving ; but the 

 chances are fifty to one against the gifted individuals being 

 one of the hundred survivors. No doubt the chances are 

 twice as great against any one other individual, but this 

 does not prevent their being enormously in favour of some 

 average individual. However slight the advantage may 

 be, if it is shared by half the individuals produced, it will 

 probably be present in at least fifty-one of the survivors, 

 and in a larger proportion of their offspring; but the 



