SENSE ORGANS. l y^ 



of another and better retina. This functions as retina, 

 while the whole invertebrate eye is transformed into a 

 lens. Or (6) some low form may have had a pigmented 

 spot on each side of tie anterior part of the head. In 

 the formation of the brain and spinal cord by infolding 

 of epiderm these spots might well be carried into the 

 cerebral vesicle and thence into the optic vesicle and 

 become a retina. Meanwhile the lens was formed by in- 

 folding, as already explained. 



Further Evolution of the Vertebrate Eye. — 

 However this may be, once the vertebrate plan is estab- 

 lished the process of improvement goes on again stead- 

 ily. In fishes the position of eyes on the side of the 

 head and the absence of true chiasm show that there 

 are as yet no corresponding points, and therefore no 

 binocular vision. The ciliary muscle is also wanting, 

 and the eye can not be accommodated to accurate 

 vision for various distances in the same way as in land 

 vertebrates. The eye is no better than that of the 

 squid. 



But in land animals the lens becomes flattened to 

 double convex shape, and may now be accommodated 

 to different distances by action of the ciliary muscle. 

 A true chiasm is not formed, and therefore binocu- 

 lar vision is not evolved until we reach birds. Mean- 

 while the eyes are moved more and more to position 

 in front, with increasing capability to converge on a 

 given point; corresponding points are established in 

 the retinas; binocular vision and judgments appertain- 

 ing thereto become possible and more and more per- 

 fect. Finally, there is added a fovea, and with it the 

 ability to fix undivided attention on the objects looked 

 at, and this, in its turn, is at least one necessary con- 

 dition of the evolution of the higher faculties of the 

 mind. 



