Origin of Species Variorum

→ as subversive of the theory. 1872

real. 1859 1860 1861 1866 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

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

→ OMIT 1869 1872

in each case 1859 1860 1861 1866

→ other species and genera 1866 1869 1872

species 1859 1860 1861

→ OMIT 1869 1872

from the earlier stages of descent, 1859 1860 1861 1866

↑ 1 blocks not present in 1869 1872; present in 1859 1860 1861 1866

Amongst existing Vertebrata, we find but a small amount of gradation in the structure of the eye, and from fossil species we can learn nothing on this head.

→ without any nerves, and resting 1872

but which rest 1866

but without any nerve, and resting 1869

→ projecting 1869 1872

and this projects outwardly 1866

→ OMIT 1869 1872

look for gradations, we 1866

→ the latter sometimes forming 1869 1872

which sometimes forms 1866

→ facets on the cornea of their 1872

cones surrounded by pigment, which form the 1866

facets on the cornea of the 1869

→ form true lenses, and that the cones include curiously modified nervous filaments. 1869 1872

are filled with transparent refractive matter, and these cones produce images; but in addition, in certain beetles the facets of the cornea are slightly convex both externally and internally,—that is, are lens-shaped. 1866

→ But these organs in the Articulata are so much diversified 1872

Altogether so diversified is the structure of the compound eyes, 1861

perfect and complex eye could

have been

formed by natural selection, though insuperable by our imagination,

can hardly

cannot

should not

be considered →as subversive of the theory. How a nerve comes to be sensitive to light, hardly concerns us more than how life itself

first

....

originated; but I may remark

that

that,

→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.

looking

searching

for the gradations

through

which

any

organ in any species has been perfected, we ought to look exclusively to its lineal

ancestors;

progenitors;

but this is scarcely ever possible, and we are forced →OMIT to look to →other species and genera 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 →OMIT in an unaltered or little altered

condi- tion.

condition.

But the state of the

organ even

same organ

in distinct classes may incidentally throw light on the steps by which it has been

perfected

any

one

species.

↑

The simplest organ which can be called an eye consists of an optic nerve, surrounded by

pigment-cells,

pigment-cells

covered

and 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

....

organ

organs

of vision, →without any nerves, and resting merely on sarcodic

tissue

not

furnished

with

any

nerve.

Eyes of the above simple nature are not capable of distinct vision,

but

and

serve

merely

only

to distinguish light from darkness. In certain star-fishes, small depressions in the layer of pigment which

sur- rounds

surrounds

the nerve are filled, as described by the author just quoted, with transparent gelatinous matter, →projecting 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.

easy.

In this concentration of the rays we gain the first and by far the most important step towards the formation of a

true

true,

....

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

body,

and in some near the surface, at the right distance from the concentrating apparatus, and an image

must

will

be formed on it.

In the great class of the Articulata,

....

we →OMIT may start from an optic nerve simply coated with pigment, →the latter sometimes forming a sort of pupil, but

....

destitute of a lens or other optical contrivance. With insects it is now known that the numerous →facets on the cornea of their great compound

eyes,

eyes

→form true lenses, and that the cones include curiously modified nervous filaments. →But these organs in the Articulata are so much diversified