All these
differences in the organs on which the very existence of birds depends,
which determine the character of flight, facility for running or
climbing, for inhabiting chiefly the ground or trees, and the kind of
food that can be most easily obtained for themselves and their
offspring, must surely be in the highest degree utilitarian; although in
each individual case we, in our ignorance of the minutiae of their
life-history, may be quite unable to see the use. In mammalia specific
differences other than colour usually consist in the length or shape of
the ears and tail, in the proportions of the limbs, or in the length and
quality of the hair on different parts of the body. As regards the ears
and tail, one of the objections by Professor Bronn relates to this very
point. He states that the length of these organs differ in the various
species of hares and of mice, and he considers that this difference can
be of no service whatever to their possessors. But to this objection
Darwin replies, that it has been shown by Dr. Schoebl that the ears of
mice "are supplied in an extraordinary manner with nerves, so that they
no doubt serve as tactile organs.
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