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.
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