278 The Evolution of Optics. 



globular. In timid animals, and in those who defend them- 

 selves by kicking, we find the eyes lateral and prominent, 

 which enables the animal to watch for the approach of a 

 possible enemy from behind. The retractor muscle of the 

 eyeball is present in all mammals up to the Quadrumana. 

 In the catarrhine Quadrumana and in man we have that 

 specialization in the function of the retina which gives us 

 what we know as the macula ; this pit is not developed in 

 the embryo, and is inconspicuous in childhood. 



In some of the mammalia, especially in the carnivora, 

 there is on the inner surface of the choroid a patch of 

 brilliant pigment of metallic luster called the tapitum. 

 This assists the vision of the animal in a feeble light by 

 reflecting forward the rays of light as from a mirror. 



"We find in the eye many interesting examples of retro- 

 gression and atrophy from disuse. Very frequently, when 

 an animal is freely moving in its larval state, the eyes which 

 it then possesses are lost later on with its power of locomo- 

 tion, as, for example, in the ascidian. The rudimentary 

 character of the eyes of animals living in dark caverns is 

 well known. In those forms living at a great depth beneath 

 the surface of the water, where little light can penetrate, 

 one of two adaptations usually occurs: the eyes either 

 atrophy, the animal finding it safer to depend on the sense 

 of touch alone, or else they become very large, to catch all 

 the rays of light possible. Sometimes these deep-sea ani- 

 mals possess their own luminous organs, which are useful 

 not only in the search for food, but likewise as weapons of 

 defense, and which, like dark lanterns, they flash upon their 

 frightened enemies. Again, other species hang out their 

 lanterns to tempt their unwary prey within easy reach. In 

 certain lizards there are found concealed beneath a scale in 

 the center of the head the atrophied remains of wjiat was a 

 perfect median eye in the now extinct ancestral forms. 

 This eye is connected with that curious and unexplained 

 organ the pineal gland, which has at least served a useful 

 purpose as a target for theory makers. It was in this organ, 

 you will remember, that Descartes located the seat of the 

 soul, and we now find it connected with a once functional 

 window of that elusive phantom. This pineal eye has been 

 studied in several reptilian forms by Mr. Spencer, who con- 

 cluded that it is identical with the median eye of the ich- 

 thyosaurus and allied extinct reptilia> in which it attained 

 large size. 



