348 THE RECAPITULATION THEORY 



exposed to accidental injury. The narrowing of 

 the mouth of the pit in the next stage is a simple 

 change, but a very important step forwards. Up 

 to this point the eye has served to distinguish 

 light from darkness, but the formation of an image 

 has been impossible. Now, owing to the smallness 

 of the aperture, and the pigmentation of the walls 

 of the pit, which accompanies the change, light 

 from any one part of an object can only fall on one 

 particular part of the inner wall of the pit or retina, 

 and so an image, though a dim one, is formed. 

 This type of eye is permanently retained in the 

 Nautilus. The closing of the mouth of the pit by 

 a transparent membrane will not affect the optical 

 properties of the eye, and will be a gain, as it will 

 prevent the entrance of foreign bodies into the 

 cavity of the eye. The formation of the lens by 

 deposit of cuticle is the next step. The gain here 

 is increased distinctness 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 corre- 

 sponding part of the retina. The eye is now in 

 the condition in which it remains throughout life 

 in the snail and other gasteropods. Finally the 

 formation of the folds of skin known as iris and 

 eyelids provide for the better protection of the eye, 

 and is a clear advance on the somewhat clumsy 

 method of withdrawal seen in the snail. 



The development of the vertebrate liver is 

 another good but simpler example. The most 



