Little need be said of the vision of am phibians and reptiles, which is probably rather dull and restricted. These animals, like many fishes, depend more on the sense of smell than on sight to guide their actions, and have, in addition, good hearing Vision of Birds.— The power of vision at tains its highest development among birds ; and nowhere but in human faces is the eye so ex pressively beautiful or so exquisitely adapted to its service as an optical instrument. Birds possess the keenest and most farsighted vision, and also extraordinary power of swiftly alter ing the focus of the eye to changing distances, accompanied by astounding acuteness in mental calculation. A swallow or nighthawk sweeping and dodging in the air is catching insects almost invisible to our eyes and excessively agile; a hawk chasing a swift-winged and quick-turning sparrow or a leaping grasshopper must have eyes that can follow Exactly every movement. More wonderful is the work of an eagle or vulture, which while soaring so high in the sky that it appears to us a mere speck sees and de fines an object on the ground that we could hardly notice or recognize at a hundred yards, and darts down upon it with the speed of a bullet. Imagine the farsighted keenness of this act and then the perfection of the apparatus by which the focus of the eye is changed in accord with the speed of the bird's descent, keeping the object always in clear view and stopping at precisely the right instant to escape collision. A hummingbird will dash past one's
eyes like a gleam of light and halt at rest on a twig without fail. A woodcock or partridge will rush at high speed through a tangled wood and never touch a twig. These feats are evi dence of the perfection of birds' eyes, especially in the faculty of accommodation to quickly varying distances.
Eyesight among mammals is relatively less important than among birds, or even to man kind, for their alert hearing and keen sense of smell give them much information. Vision among them varies with their manner of life, and is restricted to their needs in each case. The most farsighted and useful vision, prob ably, is that of the large grazers on the plains —deer, antelopes, giraffes, horses, etc., al though they use but one eye at a time. The beasts of the chase, like owls among birds, have eyes in front, so that they observe with both at once, and of these perhaps the wolves, foxes and wild hunting dogs are best endowed. Here as elsewhere structure of the eye and quality of vision are in adaptation to the habitual needs of each kind of animal, and are always correlated with the power of loco motion.