show the manner in which this is possible. As under domestication,
modifications of structure take place, without any continued selection,
which man finds very useful, or valuable for curiosity (as the hooked
calyx of the teazle, or the ruff round some pigeons' necks), so in a
state of nature some small modifications, apparently beautifully adapted
to certain ends, may perhaps be produced from the accidents of the
reproductive system, and be at once propagated without long-continued
selection of small deviations towards that structure{298}. In
conjecturing by what stages any complicated organ in a species may have
arrived at its present state, although we may look to the analogous
organs in other existing species, we should do this merely to aid and
guide our imaginations; for to know the real stages we must look only
through one line of species, to one ancient stock, from which the
species in question has descended. In considering the eye of a
quadruped, for instance, though we may look at the eye of a molluscous
animal or of an insect, as a proof how simple an organ will serve some
of the ends of vision; and at the eye of a fish as a nearer guide of the
manner of simplification; we must remember that it is a mere chance
(assuming for a moment the truth of our theory) if any existing organic
being has preserved any one organ, in exactly the same condition, as it
existed in the ancient species at remote geological periods.

{294} The difficulties discussed in the _Origin_, Ed. i. p. 171,
vi. p. 207, are the rarity of transitional varieties, the origin of
the tail of the giraffe; the otter-like polecat (_Mustela vison_);
the flying habit of the bat; the penguin and the logger-headed
duck; flying fish; the whale-like habit of the bear; the
woodpecker; diving petrels; the eye; the swimming bladder;
Cirripedes; neuter insects; electric organs.

Of these, the polecat, the bat, the woodpecker, the eye, the
swimming bladder are discussed in the present Essay, and in
addition some botanical problems.

{295} In the _Origin_, Ed. vi. p. 275, the author replies to
Mivart's criticisms (_Genesis of Species_, 1871), referring
especially to that writer's objection "that natural selection is
incompetent to account for the incipient stages of useful
structures."

{296} <The following sentence seems to have been intended for
insertion here> "and tha