"To suppose," he says, "that the eye with all its inimitable
contrivances for adjusting the focus to different distances, for
admitting different degrees of light, and for the correction of
spherical and chromatic aberration, could have been formed by natural
selection, seems, I freely confess, absurd in the highest degree." (p.
222) Nevertheless he attempts to explain the process. "It is scarcely
possible," he says, "to avoid comparing the eye with the telescope. We
know that this instrument has been perfected by the long continued
efforts of the highest of 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, tends to
produce a distinct image.
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