nto layers of different densities
and thicknesses, placed at different distances from each other, and
with the surfaces of each layer slowly changing in form. Further we
must suppose that there is a power always intently watching each slight
accidental alteration in the transparent layers; and carefully selecting
each alteration which, under varied circumstances, may in any way, or in
any degree, tend to produce a distincter image. We must suppose each new
state of the instrument to be multiplied by the million; and each to
be preserved till a better be produced, and then the old ones to
be destroyed. In living bodies, variation will cause the slight
alterations, generation will multiply them almost infinitely, and
natural selection will pick out with unerring skill each improvement.
Let this process go on for millions on millions of years; and during
each year on millions of individuals of many kinds; and may we not
believe that a living optical instrument might thus be formed as
superior to one of glass, as the works of the Creator are to those of
man?

If it could be demonstrated that any complex organ existed, which
could not possibly have been formed by numerous, successive, slight
modifications, my theory would absolutely break down. But I can find
out no such case. No doubt many organs exist of which we do not know
the transitional grades, more especially if we look to much-isolated
species, round which, according to my theory, there has been much
extinction. Or again, if we look to an organ common to all the members
of a large class, for in this latter case the organ must have been first
formed at an extremely remote period, since which all the many members
of the class have been developed; and in order to discover the early
transitional grades through which the organ has passed, we should have
to look to very ancient ancestral forms, long since become extinct.

We should be extremely cautious in concluding that an organ could not
have been formed by transitional gradations of some kind. Numerous cases
could be given amongst the lower animals of the same organ performing
at the same time wholly distinct functions; thus the alimentary canal
respires, digests, and excretes in the larva of the dragon-fly and in
the fish Cobites. In the Hydra, the animal may be turned inside out, and
the exterior surface will then digest and the stomach respire. In such
cases natural selection might easily specialise, if any advant