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." 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. Guenther has actually seen a
mouse suspend itself by the tail (_Origin_, p. 189).

Again, Mr. Lawson Tait has called attention to the use of the tail in
the cat, squirrel, yak, and many other animals as a means of preserving
the heat of the body during the nocturnal and the winter sleep. He says,
that in cold weather animals with long or bushy tails will be found
lying curled up, with their tails carefully laid over their feet like a
rug, and with their noses buried in the fur of the tail, which is thus
used exactly in the same way and for the same purpose as we use
respirators.[43]

Another illustration is furnished by the horns of deer which, especially
when very large, have been supposed to be a danger to the animal in
passing rapidly through dense thickets. But Sir James Hector states,
that the wapiti, in North America, throws back its head, thus placing
the horns along the sides of the back, and is then enabled to rush
through the thickest forest w