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 perpetuated. 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
favourable variation; still, as the Reviewer goes onto show, the young
would have only a slightly better chance of surviving and breeding; and
this chance would go on decreasing in the succeeding generations. 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.
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