| Comparison with 1860 |
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selection, though insuperable by our imagination, can hardly
be considered real. How a nerve comes to be sensitive to light, hardly concerns us more than how life itself first
originated; but I may remark that
several facts make me suspect that any sensitive nerve may be rendered sensitive to light, and likewise to those coarser vibrations of the air which produce sound. |
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In looking
for the gradations by
which an
organ in any species has been perfected, we ought to look exclusively to its lineal ancestors;
but this is scarcely ever possible, and we are forced in each case
to look to species
of the same group, that is to the collateral descendants from the same original
parent-form, in order to see what gradations are possible, and for the chance of some gradations having been transmitted from the earlier stages of descent,
in an unaltered or little altered condition.
↑| 1 blocks not present in 1859 1860 1861 1866; present in 1869 1872 | | But the state of the organ even
in distinct classes may incidentally throw light on the steps by which it has been perfected
in any one species.
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| 6 blocks not present in 1859 1860 1861; present in 1866 1869 1872 | | The simplest organ which can be called an eye consists of an optic nerve, surrounded by pigment-cells,
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 an
organ
of vision, but which rest
merely on sarcodic tissue
not furnished with any nerve. Eyes of the above simple nature are not capable of distinct vision, but
serve merely
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, and this projects outwardly
with a convex surface, like the cornea in the higher animals.
He suggests that this structure
serves not to form an image, but only to concentrate the luminous rays and render their perception more perfect.
In this concentration of the rays we gain the first and by far the most important step towards the formation of a true
or
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 must
be formed on it.
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In the Articulata we can commence a series with an optic nerve merely coated with pigment, and without any other mechanism; and from this low stage, numerous gradations of structure, branching off in two fundamentally different lines, can be shown to exist, until we reach a moderately high stage of perfection. ↑| 1 blocks not present in 1859 1860 1866 1869 1872; present in 1861 | | In the great kingdom of the Articulata, we can start from an optic nerve, simply coated with pigment, which sometimes forms a sort of pupil, but is destitute of a lens or any other optical mechanism. From this rudimentary eye, which can distinguish light from darkness, but nothing else, there is an advance towards perfection along two lines of structure, which Müller thought were fundamentally different; namely,— firstly, stemmata, or the so-called "simple eyes," which have a lens and cornea; and secondly, "compound eyes," which seem to act mainly by excluding all the rays from each point of the object viewed, except the pencil that comes in a line perpendicular to the convex retina.
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