THE FUNCTION OF THE EYE To man the eye is by far the most important of all the sense organs. With the help of our eyes we recognise our fellow men, animals and plants, the things we use in daily life, in short all the world around us. He who studies the sight of animals is apt to make the mistake of supposing that their eyes function like ours.
Such perception of form as permits us, from the appearance of an object, to draw conclusions as to its other qualities, and thus to understand its nature, is only to be found in the highest ani mals, mammals, birds, and, perhaps, some reptiles. Even the frog does not recognise a motionless fly as desirable food, but is quite indifferent to it ; it is, so to speak, "intellectually blind." This applies in a still greater degree to the invertebrate or lower animals. Of these, so far as is known at present, only bees and their relatives show a definite sense of form. Von Frisch demon strated that bees can be trained to distinguish forms. Food was given them in a closed box, into which they were cibliged to crawl through a hole. Around this hole was glued the picture of a flower, while round a hole in another box, which was placed beside the first, but was empty, was gummed the picture of another flower. Af ter food had been given thus for several days, the boxes, which could be distinguished only by the difference in the pictures of the flowers, were placed before the bees empty. A much greater number of visits was paid to the box which had contained food (see fig. II). Bees, therefore, are able to distin guish between two flowers according to their shape. Similar experiments may be made also by using simple geometrical figures.
Perception of Movement.—As far as is known at present, none of the other lower animals are capable of such a perform ance. It is easy to guess for what purpose they use their eyes if we consider the behaviour of some mammals. The hare takes not the least notice of the sportsman standing motionless at the edge of the wood, but as soon as he makes the slightest move ment, it takes to flight. It, therefore, exhibits in a marked degree the power of perception of movement. Spontaneous movement can take place only in living organisms. Since the animal heeds
only the movement, and not the motionless form, it is able, in the simplest way, to distinguish between "living" and "lifeless." The power of perceiving movement is very distributed. Schrader has made interesting experiments with falcons. These intelligent birds, under normal conditions also, are able to dis tinguish motionless prey as such. If, however, the cerebrum is removed, they notice movement only. A falcon from which the cerebrum has been removed, pursues and strikes a living, active mouse just as skilfully as would a normal bird; as soon as the mouse is dead, however, it takes no more interest in it. In gen eral, frogs and salamanders snap only at moving prey, and every angler knows that fishes act in the same way. Among the lower animals interesting observations have been made on insects, crustaceans and molluscs. The scallop, Pecten jacobaeus, has, on the edges of its mantle, a large number of eyes of complex structure. On account of the poor development of its brain, how ever, it is unable to distinguish its enemies by means of sight, and is able only to perceive movement. If another animal crawls slowly towards it, as soon as the scallop perceives the movement, it extends the long, thread-like tentacles, which are its olfactory organs. By means of these it determines whether the approach ing animal is friend or foe. If it be the former, the scallop quickly becomes quiet again ; if the latter, it swims away hur riedly. A corresponding co-operation between eyes and feelers is often to be seen in decapod crustaceans. These animals react to a moving object by holding the antennae straight out towards it. (See fig. 12.) Uexkuell showed that the house-fly finds its mate by means of this perception of movement. If a small, black bead, about the size of a fly, is fastened to a thread, and drawn fairly rapidly through the air, the male flies immediately throw themselves upon it. If the surface of the bead be smeared with a sticky substance, it makes an ideal fly-trap. That dragon flies and other predatory insects only pursue moving prey need hardly be mentioned.