" Hence, when we consider the life of
mice, either nocturnal or seeking their food in dark and confined
places, the length of the ears may be in each case adapted to the
particular habits and surroundings of the species. Again, the tail, in
the larger mammals, often serves the purpose of driving off flies and
other insects from the body; and when we consider in how many parts of
the world flies are injurious or even fatal to large mammals, we see
that the peculiar characteristics of this organ may in each case have
been adapted to its requirements in the particular area where the
species was developed. The tail is also believed to have some use as a
balancing organ, which assists an animal to turn easily and rapidly,
much as our arms are used when running; while in whole groups it is a
prehensile organ, and has become modified in accordance with the habits
and needs of each species. In the case of mice it is thus used by the
young. Darwin informs us that the late Professor Henslow kept some
harvest-mice in confinement, and observed that they frequently curled
their tails round the branches of a bush placed in the cage, and thus
aided themselves in climbing; while Dr.
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