| → in each case 1859 1860 1861 1866 |
| OMIT 1869 1872 |
|
| → other species and genera 1866 1869 1872 |
| species 1859 1860 1861 |
|
| → from the earlier stages of descent, 1859 1860 1861 1866 |
| OMIT 1869 1872 |
|
| ↑ 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.
|
|
| → But the state of the same organ even 1866 |
| Amongst existing Vertebrata, we find but a small amount of gradation 1859 1860 1861 |
|
| → other main divisions 1866 |
| structure 1859 1860 1861 |
|
| → organic world may incidentally throw light on the steps by which it has been perfected. 1866 |
| eye, and from fossil species we can learn nothing on this head. 1859 1860 |
| eye (though in the fish Amphioxus, the eye is in an extremely simple condition without a lens), and from fossil species we can learn nothing on this head. 1861 |
|
| → but which rest 1866 |
| but without any nerve, and resting 1869 |
| without any nerves, and resting 1872 |
|
| → and this projects outwardly 1866 |
| projecting 1869 1872 |
|
|
In
for the gradations
which
organ in any species has been perfected, we ought to look exclusively to its lineal
but this is scarcely ever possible, and we are forced
→in each case
to look to
→other species and genera
of the same group, that is to the collateral descendants from the same
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
↑ →But the state of the same organ even
in the
→other main divisions
of the
→organic world 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
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
of vision,
→but which rest
merely on sarcodic
Eyes of the above simple nature are not capable of distinct vision,
serve
to distinguish light from darkness. In certain star-fishes, small depressions in the layer of pigment which
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
serves not to form an image, but only to concentrate the luminous rays and render their perception more
In this concentration of the rays we
|
