pigment and a
complicated eye like that of the vertebrates.--But, from the fact that
we pass from one thing to another by degrees, it does not follow that
the two things are of the same nature. From the fact that an orator
falls in, at first, with the passions of his audience in order to make
himself master of them, it will not be concluded that to _follow_ is the
same as to _lead_. Now, living matter seems to have no other means of
turning circumstances to good account than by adapting itself to them
passively at the outset. Where it has to direct a movement, it begins by
adopting it. Life proceeds by insinuation. The intermediate degrees
between a pigment-spot and an eye are nothing to the point: however
numerous the degrees, there will still be the same interval between the
pigment-spot and the eye as between a photograph and a photographic
apparatus. Certainly the photograph has been gradually turned into a
photographic apparatus; but could light alone, a physical force, ever
have provoked this change, and converted an impression left by it into a
machine capable of using it?

It may be claimed that considerations of utility are out of place here;
that the eye is not made to see, but that we see because we have eyes;
that the organ is what it is, and "utility" is a word by which we
designate the functional effects of the structure. But when I say that
the eye "makes use of" light, I do not merely mean that the eye is
capable of seeing; I allude to the very precise relations that exist
between this organ and the apparatus of locomotion. The retina of
vertebrates is prolonged in an optic nerve, which, again, is continued
by cerebral centres connected with motor mechanisms. Our eye makes use
of light in that it enables us to utilize, by movements of reaction, the
objects that we see to be advantageous, and to avoid those which we see
to be injurious. Now, of course, as light may have produced a
pigment-spot by physical means, so it can physically determine the
movements of certain organisms; ciliated Infusoria, for instance, react
to light. But no one would hold that the influence of light has
physically caused the formation of a nervous system, of a muscular
system, of an osseous system, all things which are continuous with the
apparatus of vision in vertebrate animals. The truth is, when one
speaks of the gradual formation of the eye, and, still more, when one
takes into account all that is inseparably connected wit