CuAP. VI. AND COMPLICATION. I79 



of the lowest organisms, in Avbich 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 

 j)arent-form, in order to see what gradations arc possible, and 

 for the cliance of some gradations liaving 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 hy 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 trans- 

 lucent skin, but without any lens or other refractive body. AVe 

 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 starfishes, small depressions in the 

 layer of pigment which surrounds tlie nerve are filled, as de- 

 scribed by the author just quoted, Avith 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 raj's and render 

 their perception more easy. In this concentration of the rays 

 we gain the first and by far the most important step toward 

 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 concentrat- 

 ing ap]iaratus, and an image will be formed on it. 



In the great class of tlie Articulata, we may start from an 

 optic nerve simply coatccl with pigment, the latter sometimes 

 forming a sort of pupil, but destitute of a lens or other optical 

 contrivance. "NVitii insects it is now known that the numer- 

 ous facets on the cornea of the great comjiound eyes form 

 true lenses, and that the cones include curiously-modified 

 nervous filaments. But these organs in the Articulata are so 



