| Comparison with 1861 |
|
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 nerves sensitive to touch may be rendered sensitive to light, and likewise to those coarser vibrations of the air which produce sound. | several facts make me suspect that nerves sensitive to touch may be rendered sensitive to light, and likewise to those coarser vibrations of the air which produce sound. 1861 |
| 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. 1859 1860 |
| as some of the lowest organisms, in which nerves cannot be detected, are known to be sensitive to light, it does not seem impossible that certain elements in their tissues or sarcode should have become aggregated and developed into nerves endowed with special sensibility to its action. 1866 |
| as some of the lowest organisms, in which nerves cannot be detected, are known to be sensitive to light, it does not seem impossible that certain elements in the sarcode, of which they are mainly composed, should become aggregated and developed into nerves endowed with this special sensibility. 1869 |
| as some of the lowest organisms, in which nerves cannot be detected, are capable of perceiving light, it does not seem impossible that certain sensitive elements in their sarcode should become aggregated and developed into nerves, endowed with this special sensibility. 1872 |
|
|
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
condi- tion. | condi- tion. 1861 | | condition. 1859 1860 1866 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.
|
| 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 |
| eye, and from fossil species we can learn nothing on this head. 1859 1860 |
| organic world may incidentally throw light on the steps by which it has been perfected. 1866 |
|
| Text in this page (from paragraph 3110, sentence 100 to paragraph 3200, sentence 300, word 28) is not present in 1861 |
selection, though insuperable by our imagination,
can hardly | can hardly 1859 1860 1861 1866 | | cannot 1869 | | should not 1872 |
| real. 1859 1860 1861 1866 1869 |
| as subversive of the theory. 1872 |
| first 1859 1860 1861 1866 1869 | first 1872 |
| that 1859 1860 1861 | | that, 1866 1869 1872 |
| 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. 1859 1860 |
| several facts make me suspect that nerves sensitive to touch may be rendered sensitive to light, and likewise to those coarser vibrations of the air which produce sound. 1861 |
| as some of the lowest organisms, in which nerves cannot be detected, are known to be sensitive to light, it does not seem impossible that certain elements in their tissues or sarcode should have become aggregated and developed into nerves endowed with special sensibility to its action. 1866 |
| as some of the lowest organisms, in which nerves cannot be detected, are known to be sensitive to light, it does not seem impossible that certain elements in the sarcode, of which they are mainly composed, should become aggregated and developed into nerves endowed with this special sensibility. 1869 |
| as some of the lowest organisms, in which nerves cannot be detected, are capable of perceiving light, it does not seem impossible that certain sensitive elements in their sarcode should become aggregated and developed into nerves, endowed with this special sensibility. 1872 |
|
|
In
looking | looking 1859 1860 1861 | | searching 1866 1869 1872 |
| by 1859 1860 1861 | | through 1866 1869 1872 |
| an 1859 1860 1861 1869 1872 | | any 1866 |
| ancestors; 1859 1860 1861 | | progenitors; 1866 1869 1872 |
| in each case 1859 1860 1861 1866 |
| OMIT 1869 1872 |
| species 1859 1860 1861 |
| other species and genera 1866 1869 1872 |
| original 1859 1860 1861 1866 | original 1869 1872 |
| from the earlier stages of descent, 1859 1860 1861 1866 |
| OMIT 1869 1872 |
| condition. 1859 1860 1866 1869 1872 | | condi- tion. 1861 |
| 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.
|
| Amongst existing Vertebrata, we find but a small amount of gradation 1859 1860 1861 |
| But the state of the same organ even 1866 |
| structure 1859 1860 1861 |
| other main divisions 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 |
| organic world may incidentally throw light on the steps by which it has been perfected. 1866 |
|
|
| 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.
In certain crustaceans, for instance, there is a double cornea, the inner one divided into facets, within each of which there is a lens-shaped swelling. In other crustaceans the transparent cones which are coated by pigment, and which properly act only by excluding lateral pencils of light, are convex at their upper ends
|
