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.

We have now noticed the chief characteristics of instinct: its
existence prior to all experience or instruction; its incapacity of
improvement, except within the narrow sphere of domestication; its
limitation to a few objects, and the certainty of its action within
these limits; the distinctness and permanence of its character for each