THE ARGUMENT FROM EMBRYOLOGY 107 



Solen we find the simplest condition of the molluscan 

 eye, merely a slightly depressed and slightly modified 

 patch of skin, which can only distinguish light from 

 darkness, and in which the sensitive cells are pro- 

 tected by being situated at the bottom of the fold of 

 skin. In Patella the next stage is found, where the 

 eye forms a pit with a widely open mouth. This is 

 a distinct advance on the preceding form, for owing 

 to the increased depth of the pit, the sensory cells 

 are less exposed to accidental injury. The next stage 

 is found in Haliotis, and consists in the narrowing of 

 the mouth of the pit. This is a simple change, but a 

 very important step forwards, for in consequence of 

 the smallness of the aperture, light from any one part 

 of an object can only fall on one particular part of the 

 pit or retina, and so an image, though a dim one, is 

 formed. The next step consists in the formation of 

 a lens at the mouth of the pit, by a deposit of 

 cuticle : this form of eye is found in Fissurella. 

 (Fig. 17.) The gain here is twofold — viz., increased 

 protection and increased brightness of the image, for 

 the lens will focus the rays of light more sharply on 

 the retina, and will allow a greater quantity of light, 

 a larger pencil of rays from each part of the object, 

 to reach the corresponding part of the retina. (Fig. 

 18.) Finally, the formation of the folds of skin 

 known as the iris and eyelids provides for the better 

 protection of the eye, and is a distinct advance on 

 the somewhat clumsy method of withdrawal seen in 

 the snail. This is found in the Cephalopoda, such 

 as Loligo. 



If now we study the actual development of the 



