The Evolution of Optics. 273 



and the cells immediately subjacent might develop into 

 special nerve-tissues ; and to show that this is not imagina- 

 tive he cites eyes representing these various types, and shows 

 that the eye of a snail, in its development, passes through 

 these very stages. 



The most rudimentary form of eye with which we are 

 acquainted is such as that found in the Medusa, which we 

 can scarcely term an eye at all, but simply a nerve, which 

 responds very slowly, as shown by Mr. Romanes, to light- 

 stimulation by a contraction of the body. Such an optical 

 apparatus would seem to be of very little utility to its pos- 

 sessor ; yet the simple eye-speck of the oyster, which is su- 

 perior to this only in giving a quick response to light-stimu- 

 lus, we can readily understand to be of great value, for the 

 oyster is seen to close its shell, when the shadow of an ap- 

 proaching object falls upon it, long before the disturbance 

 of the Avater, through the sense of feeling alone, could give 

 it warning of the danger. 



Over such a simple eye-speck an advantage would be 

 gained by the thickening of the covering epidermal scales, 

 which would tend to concentrate the light upon the ex- 

 posed nerve, thus obtaining a greater intensity of stimula- 

 tion. These stages we have seen actually occurring in the 

 lower animal forms, and embryology tells us that, in our 

 own eyes, the crystalline lens is formed by such a folding in 

 of the epitheliaHayer. 



From a simple light-receiving organ it is but a short step 

 to one in which the lens serves not merely to concentrate 

 the rays of light upon terminations of the optic nerve, but 

 likewise to form some rude image of external objects. Such 

 a visual organ would no doubt tend to greatly modify the 

 habits of any animal in which it was developed. The in- 

 creased facilities for finding and selecting food would pow- 

 erfully modify its habits. It could with greater safety in- 

 crease the latitude of its excursions, and this change in 

 environment would tend to still further modification. All 

 these causes would no doubt exert a powerful influence 

 upon the development of the visual organ. 



In endeavoring to trace the development of the eye through 

 its various stages of evolution one very important fact is met 

 viz., the eye, in its progress toward perfection, has not 

 always traveled in the same lines. Under varying stimuli 

 different reactions have taken place, and we find two well- 

 marked types of the visual organ one, the invertebrate eye, 



