In like manner birds perceive the
approach of rain, and are all busy oiling and smoothing their feathers
in preparation for it; and then, before the clouds break away, they
come out from their retreats and joyfully hail the return of fair
weather. So, by some analogous sense, the birds of passage are informed
of the approach of winter and the return of spring.
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.
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