184 COMPARATIVE ZOOLOGY. 



fibrous, cellular, and granular. The most sensitive part is 

 the surface lying next to the black pigment. And here 

 is a peculiarity of the vertebrate eye : the nerve-fibres, en- 

 tering from behind, turn back and look towards the bot- 

 tom of the eye, so that vision is directed backward; while 

 invertebrate vision is directly forward. In Vertebrates 

 only, the optic nerves cross each other (decussate) in pass- 

 ing from the brain to the eyes ; so that the right side of 

 the brain, e. g., receives the impressions of objects on the 

 left side of the body. 97 



Generally, the eyes of Vertebrates are on opposite sides 

 of the head ; but in the Flat-fishes both are on the same 

 side. Usually, both eyes see the same object at once ; but 

 in most Fishes the eyes are set so far back, the fields of 

 vision are distinct. The cornea may be flat, and the lens 

 globular, as in Fishes ; or the cornea very convex, and the 

 lens flattened, as in Owls. Purely aquatic animals have 

 neither eyelids nor tears, but nearly all others (especially 

 Birds) have three lids. 98 The pupil is usually round ; but 

 it may be rhomb-shaped, as in Frogs ; vertically oval, as 

 in Crocodiles and Cats : or transversely oval, as in Geese, 

 Doves, Horses, and Ruminants. Many Quadrupeds, as the 

 Cat, have a membrane (tapetum) lining the bottom of the 

 eyeball, with a brilliant metallic lustre, usually green or 

 pearly : it is this which makes the eyes of such animals 

 luminous in the dark. 



2. Instmct and Intelligence. 



The simplest form of nervous excitement is mere sensa- 

 tion. Above this we have sensation awakening conscious- 

 ness, out of which come those voluntary activities grouped 

 together under the name of Instinct; and, finally, Intelli- 

 gence. 



The lowest forms of life are completely under law, for 

 their movements seem to be due solely to their organiza- 



