Origin of Species Variorum

Comparison with 1869

Text in this page (from paragraph 3200, sentence 130, word 17 to paragraph 3200, sentence 300, word 46) is not present in 1869

When we reflect on these facts, here given 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 the forms now living 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 great Articulate Class.

He who will go thus far, ought not to hesitate to go one step further, if he finds on finishing this volume that large bodies of facts, otherwise inexplicable, can be explained by the theory of descent with modification; he ought to admit that a structure even as perfect as ... an eagles eye might be formed by natural selection, although in this case he does not know .. the transitional states. It has been objected that in order to modify the eye and still preserve it as a perfect instrument, many changes would have to be effected simultaneously, which, it is assumed, could not be done through natural selection; but as I have attempted to show in my work on the variation of domestic animals, it is not necessary to suppose that all the modifications were simultaneous, if they were extremely slight and gradual. ↑ 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. ↑

It is scarcely possible to avoid comparing the eye with

cluding divergent pencils of light, usually adhere to the cornea, but not rarely they are separate from it, and have their free ends convex; and in this case they must act as converging lenses. Altogether so diversified is the structure of the compound eyes, that Müller makes three main classes, with no less than seven sub-divisions of structure; he makes a fourth main class, namely, "aggregates" of stemmata, and he adds that "this is the transition-form between the mosaic-like compound eyes unprovided with a concentrating apparatus, and organs of vision with such an apparatus." ↑ With these facts, here .. too briefly and imperfectly given, which show how much graduated diversity there is in the eyes of our existing crustaceans, and bearing in mind how small the number of living animals is in proportion to those which have become extinct, I can see no very great difficulty (not more than in the case of many other structures) in believing that natural selection has converted the simple apparatus of an optic nerve merely coated with pigment and invested by transparent membrane, into an optical instrument as perfect as is possessed by any member of the great Articulate class.

He who will go thus far, if he find on finishing this treatise that large bodies of facts, otherwise inexplicable, can be explained by the theory of descent, ought not to hesitate to go further, and to admit that a structure even as perfect as the eye of an eagle might be formed by natural selection, although in this case he does not know any of the transitional grades. ↑ His reason ought to conquer his imagination; 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 such startling lengths.

It is scarcely possible to avoid comparing the eye to