.144 Organs of extreme Perfection. Chap. VI. 



could be formed by natural selection, though insuperable by our 

 imagination, 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 originated ; but I may remark 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. 



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 same organ in distinct classes may incidentally 

 throw light on the steps by which it has been perfected. 



The simplest organ which can be called an eye consists of an 

 optic nerve, surrounded by pigment-cells 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 organs 

 of vision, without any nerves, 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 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 serves not 

 to form an image, but only to concentrate the luminous rays and 

 render their perception more easy. In this concentration of the 

 rays we gain the first and by far the most important step towards 

 the formation of a 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, and in some near 

 the surface, at the right distance from the concentrating apparatus, 

 and an image will be formed on it. 



In the great class of the Articulata, we 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 form true lenses, and that 

 the cones include curiously modified nervous filaments. But these 



