CHAPTER VI. DIFFICULTIES OF THE THEORY.
4. ORGANS OF EXTREME PERFECTION AND COMPLICATION. (continued)
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 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.