brain can be educated to discriminate between red and green even when
the impressions have not the normal degree of difference.

The tests for taste, smell, muscular sense, touch, and memory cannot be
made with much thoroughness or satisfaction till two years of age,
though observation will show a recognition by taste and smell of that
which is agreeable and that which is disagreeable. Accurate tests of
hearing cannot be made till the child is three or four, but it is
possible when he is twelve months old to determine whether the hearing
is normal or is seriously impaired, and it is very desirable that this
should be done.

The expression "seriously impaired," when applied to the hearing of a
little child, must be given an entirely different interpretation than it
would have if used with reference to an adult who had previously had
normal hearing. A degree of impairment that would be unimportant in an
adult is a very serious matter in the case of a child. This is because
the ear is the natural teacher of speech and language. If the sounds of
speech are not clearly heard the imitation of them will always be
imperfect, and the acquisition of language will be impeded. If deafness
is so great that spoken words are not heard at all, then the child will
not learn to speak and to understand when spoken to unless specially
taught. A much slighter degree of deafness will prevent the proper
acquisition of speech and language than would in later life prevent the
comprehension of conversation in a familiar language. As even the child
of fifteen months would benefit from some modifications of the ordinary
treatment of a baby, if his hearing was not normally acute, it is to his
advantage to have the fact of his deafness known at once by those in
charge of him.