1 62 PHYSIOLOGY AND MORPHOLOGY OF ANIMALS. 



ular ones, b b\ are far removed from the axis on the 

 temporal side, and ^o situated that lines drawn through 



them and through the pu- 

 pils are parallel. These 

 axes can be converged on 

 a given point, and doubt- 

 v less corresponding points 

 are symmetrically ar- 

 ranged about them and 

 not about the other. The 

 central or monocular 

 fovea is the most distinct, 

 and therefore the monoc- 

 ular vision is better than 

 the binocular. This is the 

 reason why birds — for ex- 



FiG. ioo.— • Section of bird's head (after ample, the domestic fowl 



Slonaker): VV\ monocular visual _j n l 00 king attentively 



lines ; «, binocular visual lines ; & J 



a a', b b\ central and temporal foveae turn the head and look 



respectively. . , 



with one eye. 



Below birds, except in some lizards, nothing like a 

 distinct fovea is found. 



It seems certain, therefore, that binocular vision in 

 its most perfect form is found only in man and the higher 

 apes, and thence becomes gradually less and less per- 

 fect until it disappears entirely in the lowest verte- 

 brates. It is almost needless to add that it is not found 

 at all in invertebrates. 



INVERTEBRATES. 



In all that follows we are compelled to be very brief, 

 touching only most salient points. Some of these points 

 will come up again under Evolution of the Eye. 



We pass over the arthropods, because in most of 

 them the structure of the eye is so different from what 



