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 "Natural Selection," 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.