apparatus of an optic
nerve, coated with pigment and invested by transparent membrane, into
an optical instrument as perfect as is possessed by any member of the
Articulata class.

He who will go thus far, ought not to hesitate to go one step further,
if he finds on finishing this volume that large bodies of facts,
otherwise inexplicable, can be explained by the theory of modification
through natural selection; he ought to admit that a structure even as
perfect as an eagle's eye might thus be formed, although in this case
he does not know the transitional states. It has been objected that in
order to modify the eye and still preserve it as a perfect instrument,
many changes would have to be effected simultaneously, which, it is
assumed, could not be done through natural selection; but as I have
attempted to show in my work on the variation of domestic animals, it is
not necessary to suppose that the modifications were all simultaneous,
if they were extremely slight and gradual. Different kinds of
modification would, also, serve for the same general purpose: as Mr.
Wallace has remarked, "If a lens has too short or too long a focus, it
may be amended either by an alteration of curvature, or an alteration of
density; if the curvature be irregular, and the rays do not converge
to a point, then any increased regularity of curvature will be an
improvement. So the contraction of the iris and the muscular
movements of the eye are neither of them essential to vision, but only
improvements which might have been added and perfected at any stage of
the construction of the instrument." Within the highest division of
the animal kingdom, namely, the Vertebrata, we can start from an eye
so simple, that it consists, as in the lancelet, of a little sack of
transparent skin, furnished with a nerve and lined with pigment, but
destitute of any other apparatus. In fishes and reptiles, as Owen has
remarked, "The range of gradation of dioptric structures is very
great." It is a significant fact that even in man, according to the high
authority of Virchow, the beautiful crystalline lens is formed in the
embryo by an accumulation of epidermic cells, lying in a sack-like fold
of the skin; and the vitreous body is formed from embryonic subcutaneous
tissue. To arrive, however, at a just conclusion regarding the
formation of the eye, with all its marvellous yet not absolutely perfect
characters, it is indispensable that the reason should conquer th