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 if,a]petum) 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. Instinct 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- 
