f some gradations having
been transmitted in an unaltered or little altered condition. But the
state of the same organ in distinct classes may incidentally throw light
on the steps by which it has been perfected.

The simplest organ which can be called an eye consists of an optic
nerve, surrounded by pigment-cells and covered by translucent skin, but
without any lens or other refractive body. We may, however, according
to M. Jourdain, descend even a step lower and find aggregates of
pigment-cells, apparently serving as organs of vision, without any
nerves, and resting merely on sarcodic tissue. Eyes of the above simple
nature are not capable of distinct vision, and serve only to distinguish
light from darkness. In certain star-fishes, small depressions in the
layer of pigment which surrounds the nerve are filled, as described by
the author just quoted, with transparent gelatinous matter, projecting
with a convex surface, like the cornea in the higher animals. He
suggests that this serves not to form an image, but only to concentrate
the luminous rays and render their perception more easy. In this
concentration of the rays we gain the first and by far the most
important step towards the formation of a true, picture-forming eye; for
we have only to place the naked extremity of the optic nerve, which in
some of the lower animals lies deeply buried in the body, and in
some near the surface, at the right distance from the concentrating
apparatus, and an image will be formed on it.

In the great class of the Articulata, we may start from an optic nerve
simply coated with pigment, the latter sometimes forming a sort of
pupil, but destitute of lens or other optical contrivance. With insects
it is now known that the numerous facets on the cornea of their great
compound eyes form true lenses, and that the cones include curiously
modified nervous filaments. But these organs in the Articulata are so
much diversified that Muller formerly made three main classes with seven
subdivisions, besides a fourth main class of aggregated simple eyes.

When we reflect on these facts, here given much too briefly, with
respect to the wide, diversified, and graduated range of structure in
the eyes of the lower animals; and when we bear in mind how small the
number of all living forms must be in comparison with those which have
become extinct, the difficulty ceases to be very great in believing that
natural selection may have converted the simple