Page:The Zoologist, 4th series, vol 5 (1901).djvu/364

336 no pictures, and hence there is no power of visual discrimination between objects. They probably recognize their right host by the aid of organs of taste, and at any rate they are often able to distinguish their host from closely allied species."

It is a long jump from a Leech to a Rhinoceros, but the principle is the same, though the animals in a developmental sense are so widely divided. Dr. Livingstone describes the Rhinoceros as having such dimness of vision as to make it charge past a man who has wounded it, if he stands perfectly still, in the belief that its enemy is a tree. Dr. Livingstone, however, adds that this imperfect sight probably arises from the horn being in the line of vision, "for the variety named Kuabaoba, which has a straight horn directed downwards away from that line, possesses acute eyesight, and is much more wary." Mr. Scott Elliot feels sure that the East African Rhinoceros cannot see clearly for more than about fifty yards. Mr. Blanford, writing on the Abyssinian Rhinoceros, states that "they are easily eluded by turning, as they are not quick of sight, and, like most mammals, they never look for enemies in trees; consequently a man two or three feet from the ground will remain unnoticed by them if he keeps quiet." Elephants are reported to have a most defective power of sight, and, generally speaking, among mammals, as a rule, the world, as known by their senses, is probably if we judge by vision alone, a much more circumscribed one than that cognized by ourselves. With other senses much more developed, and with the addition of some of which we are totally ignorant, nature may be to them revealed beyond our imagination.

According to Dr. Günther, fishes, in the range of their vision and acuteness of sight, are very inferior to the higher classes of vertebrates; yet, at the same time, it is evident that they perceive their prey or approaching danger for a considerable distance; and it would appear that the visual powers of a Periophthalmus, when hunting insects on mud-flats of the tropical coasts,