eful to its possessor, can certainly be
shown to exist;' since, as certainly, slight variations of the eye do
occur, and are inherited, and since of these variations there cannot but
be some which are useful to the animal exhibiting them under changing
conditions of life, the difficulty of believing in the formation of a
perfect and complex eye by natural selection can be little else than a
prejudice of the imagination. He proceeds to indicate some probable
stages in the assumed process. Some of the lowest organisms, in which no
trace of nerves can be detected, are known to be sensible to light,
owing, probably, to the presence in the _sarcode_ of which they are
mainly composed, of certain elements which, in organisms somewhat higher
in the scale, become aggregated and developed into nerves specially
endowed with the same sensibility. An optic nerve thus formed,
surrounded by pigment cells, and covered by translucent skin, is the
simplest organ that can be called an eye, but it is an eye incapable of
distinct vision, and serving only to distinguish light from darkness. In
certain star-fishes, small depressions in the layer of pigment-cells
are filled with transparent gelatinous matter projecting with a convex
surface like a rudimentary cornea, and this, it has been suggested, may
serve, not only to form an image, but to concentrate the luminous rays.
In insects, the numerous facets in the cornea of their great compound
eyes have now been ascertained to form true lenses, the cones, moreover,
having been discovered to include curiously modified nervous filaments.
It is impossible not, in this series of changes, to perceive the
appearance of graduation, nor ought there to be much difficulty in
believing the apparent graduation to be real, when we consider how few
comparatively are the still living forms in which the changes cited have
been observed, and how far more numerous the extinct forms by which
intermediate changes may have been presented. If there be no
extravagance in supposing that natural selection may have occasioned
these early steps, neither is there any in supposing that, by continued
progress in the same direction, it may at length have fabricated the
most perfect optical instrument possessed by any member of the
_articulata_. And, if credited so far, why not still further? why not
with competence to form a man's or an eagle's eye? So far I am as
completely at one with Mr. Darwin in respect to the eye as in r