It is doubtless true that in some animals the senses are immediately connected with instincts which assist and extend their operation. Metaphysicians and physiologists are agreed that the perception of distance is an acquired knowledge. The sense of sight by itself principally makes us conversant with extension only. The painting upon the retina of the eye presents all external things with flat surfaces and at the same distance. Before we can have any correct ideas of distance, we must be able to compare the result of the sense of sight with the result of the sense of feeling. By experience we in time come to judge something of distance by the size of the image which an object makes upon the retina, but more by our acquired knowledge of the form and color of external things. It is true that the eyes of many animals are constructed like those of man; but they do not learn to judge of distance by the same slow process. It is known from experiment that some animals have a perfect conception of distance at the moment of their birth; and the young of the greater part of animals possess some instinctive perception of this kind. "A flycatcher, for example, just come out of its shell, has been seen to peck at an insect with an aim as perfect as if it had been all its life engaged in learning the art." And so when the hen takes her chickens out into the field for the first time to feed, they seem to perceive very distinctly the relative distance of all objects about them, and will run by the straightest course when she calls them to pick up the little grains which she points out to them. Without this instinctive power of determining the relative distance and figure of objects, the young of most animals would perish before their sense of sight could be perfected, as ours is, by experience.