| ↑ 3 blocks not present in 1859 1860 1861 1872; present in 1866 1869 |
| Even in the Vertebrata, so manifestly the
most highly organized
division of the animal kingdom, we
can start,
as in the former cases,
from an eye,
such as exists
in the fish called the lancelet, which is so simple that it consists only
of a little fold-like
sack of skin, lined with pigment and
furnished with a nerve,
but
destitute of any other apparatus,
being merely covered by transparent membrane. In the class both of
fishes and reptiles, as Owen has remarked, "the range of gradations of dioptric structures is very great." It is a significant fact that even in man, according to the high authority of Virchow, the beautiful crystalline lens is originally
formed merely
by an accumulation of cells of the epidermis,
lying in a sack-like fold of the skin; and the vitreous body is formed from embryonic sub-cutaneous tissue.
It is indeed indispensable
that the naturalist who reflects on the origin and manner of
formation of the eye, with all its marvellously perfect attributes, should make his reason conquer his imagina- tion; though
I have felt the
difficulty far too keenly to be surprised at any degree of hesitation in extending the principle of natural selection to so startling a length.
|
|
| → To arrive, however, at a just conclusion regarding the formation of the eye, with all its marvellous yet not absolutely perfect characters, it is indispensable that the reason should conquer the imagination; but 1872 |
| His reason ought to conquer his imagination; though 1859 1860 1861 |
|
| → others hesitating to extend 1872 |
| any degree of hesitation in extending 1859 1860 1861 |
|
| → spaces filled with fluid, and with a 1861 1866 1869 1872 |
| a 1859 1860 |
|
| → represented by natural selection or the survival of the fittest, always 1869 1872 |
| always 1859 1860 |
| (natural selection) always 1861 1866 |
|
|
from embryonic sub-cutaneous tissue. ↑ →To arrive, however, at a just conclusion regarding the formation of the eye, with all its marvellous yet not absolutely perfect characters, it is indispensable that the reason should conquer the imagination; but
I have felt the difficulty far too keenly to be surprised at
→others hesitating to extend
the principle of natural selection to
startling
|
|
| It is scarcely possible to avoid comparing the eye
a telescope. We know that this instrument has been perfected by the long-continued efforts of the highest human intellects; and we naturally infer that the eye has been formed by a somewhat analogous process. But may not this inference be presumptuous? Have we any right to assume that the Creator works by intellectual powers like those of man? If we must compare the eye to an optical instrument, we ought in imagination to take a thick layer of transparent tissue, with
→spaces filled with fluid, and with a
nerve sensitive to light beneath, and then suppose every part of this layer to be continually changing slowly in density, so as to separate into layers of different densities and thicknesses, placed at different distances from each other, and with the surfaces of each layer slowly changing in form. Further we must suppose that there is a
→represented by natural selection or the survival of the fittest, always
intently watching each slight
alteration in the transparent layers; and carefully
each
which, under varied circumstances,
in any
or in any degree,
to produce a distincter image. We must suppose each new state of the instrument to be multiplied by the million;
each to be preserved
a better
produced, and then the old ones to be
In living bodies, variation will cause the slight alterations, generation will multiply them almost infinitely, and natural selection will pick out with unerring skill each improvement. Let this process go on for millions
of years; and during each year on millions of individuals of many kinds; and may we not believe that a living optical instrument might thus be formed as superior to one of glass, as the works of the Creator are to those of man? |
| Modes
of
|
| If it could be demonstrated that any complex organ existed, which could not possibly have been formed by numerous, successive, slight modifications, my theory would absolutely break down. But I can find out no such case. No doubt many organs exist of which we do not know the transitional grades, more especially if we look to much-isolated species, round which, according to
theory, there has been much extinction. Or again, if we
|
