ultaneous 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. 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
chances are against the preservation of any one 'sport' (_i.e._ sudden,
marked variation) in a numerous tribe. The vague use of an imperfectly
understood doctrine of chance has led Darwinian supporters, first, to
confuse the two cases above distinguished; and, secondly, to imagine {58}
that a very slight balance in favour of some individual sport must lead to
its perpetuation. All that can be said is that in the above example the
favoured sport would be preserved once in fifty times. Let us consider what
will be its influence on the main stock when preserved. It will breed and
have a progeny of say 100; now this progeny will, on the whole, be
intermediate between the average individual and the sport. The odds in
favour of one of this generation of the new breed will be, say one and a
half to one, as compared with the average individual; the odds in their
favour will, therefore, be less than that of their parents; but owing to
their greater number, the chances are that about one and a half of them
would surviv