EVIDENCES OF EVOLUTION. 110 



But this recapitulation may be observed, not 

 only in the organisms themselves, but likewise in 

 their constituent parts. A striking illustration is 

 afforded in the development of the eye of the loligo, 

 one of the higher cephalopoda, as compared with 

 the rudimentary eyes of various species of mollusca. 

 Thus, as the late Mr. Marshall tells us: " In 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 of 

 the narrowing of the mouth of the pit. This is a 

 simple change but a very important step forward, 

 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. The gain here is two-fold, 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 quan- 

 tity of light, a larger pencil of rays from each part 

 of the object, to reach the corresponding part of 



