continually changing slowly in density, so as to separate into 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, represented by natural
selection or the survival of the fittest, always intently watching each
slight alteration in the transparent layers; and carefully preserving each
which, under varied circumstances, in any way or degree, tends to produce a
distincter image. We must suppose each new state of the instrument to be
multiplied by the million; each to be preserved until a better is produced,
and then the old ones to be all destroyed. In living bodies, variation
will cause the slight alteration, generation will multiply them almost
infinitely, and natural selection will pick out with unerring skill each
improvement. Let this process go on for 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?

MODES Of TRANSITION.

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, around which,
according to the theory, there has been much extinction. Or again, if we
take an organ common to all the members of a class, for in this latter case
the organ must have been originally formed at a 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