Origin of Species Variorum

Comparison with 1861

conditions of life, then the difficulty of believing that a perfect and complex eye could be formed by natural 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.

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.

Text in this page (from paragraph 3100, sentence 110 to paragraph 3110, sentence 400, word 12) is not present in 1861

conditions of life, then the difficulty of believing that a perfect and complex eye could be formed by natural selection, though insuperable by our imagination, cannot 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, 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.

In searching for the gradations through which an organ in any species has been perfected, we ought to look exclusively to its lineal progenitors; but this is scarcely ever possible, and we are forced ... 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 ... in an unaltered or little altered condition. 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. ↑

The simplest organ which can be called an eye consists of an optic nerve, surrounded by pigment-cells 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 an organ of vision, but without any nerve, and resting merely on sarcodic tissue. .. .. .. .. .. Eyes of the above simple nature are not capable of distinct vision, and serve only to distinguish light from darkness. In certain star-fishes, small depressions in the layer of pigment which sur- rounds