e
imagination; but I have felt the difficulty far to keenly to be
surprised at others hesitating to extend the principle of natural
selection to so startling a length.

It is scarcely possible to avoid comparing the eye with a telescope.
We know that this instrument has been perfected by the long-continued
efforts of the highest human intellects; and we naturally infer that the
eye has been formed by a somewhat analogous process. But may not this
inference be presumptuous? Have we any right to assume that the Creator
works by intellectual powers like those of man? If we must compare the
eye to an optical instrument, we ought in imagination to take a thick
layer of transparent tissue, with spaces filled with fluid, and with a
nerve sensitive to light beneath, and then suppose every part of this
layer to be 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