The human mind, and the animal mind apparently, manifests itself in
three states or functions. These are intelligence, the realm of
knowledge; susceptibility, the realm or state of feelings or
emotions; will, the power or state of choice. Let us trace first the
development of intelligence or the intellect in the animal. Let us
try to discover what kinds of knowledge are successively attained
and the mode and sequence of their attainment. Hydra appears to be
conscious of its food. It recognizes it partially by touch, perhaps
also by feeling the waves caused by its approach. It seems also to
recognize food at a little distance by a power comparable to our
sense of smell. Stronger impacts cause it to contract. It neither
sees nor hears; it probably does little or no thinking. Its
knowledge is therefore limited to the recognition of objects either
in contact with, or but slightly removed from, itself. And its
recognition of the objects is very dim and incomplete, obtained
through the sense of touch and smell.
A little higher in the animal world a rude ear has developed, first
as a very delicate organ for feeling the waves caused by approaching
food or enemies; only later as an organ of hearing. Meanwhile the
eye has been developing, to perceive the subtle ether vibrations.
The eye of the turbellaria distinguishes only light from darkness,
that of the annelid is a true visual organ.
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