kes with poisonous fangs often have some
characteristic either of rattle, hood, or unusual colour, which
indicates that they had better be left alone.

But there is yet another form of coloration, which consists in special
markings--bands, spots, or patches of white, or of bright colour, which
vary in every species, and are often concealed when the creature is at
rest but displayed when in motion,--as in the case of the bands and
spots so frequent on the wings and tails of birds. Now these specific
markings are believed, with good reason, to serve the purpose of
enabling each species to be quickly recognised, even at a distance, by
its fellows, especially the parents by their young and the two sexes by
each other; and this recognition must often be an important factor in
securing the safety of individuals, and therefore the wellbeing and
continuance of the species. These interesting peculiarities will be more
fully described in a future chapter, but they are briefly referred to
here in order to show that the most common of all the characters by
which species are distinguished from each other--their colours and
markings--can be shown to be adaptive or utilitarian in their nature.

But besides colour there are almost always some structural characters
which distinguish species from species, and, as regards many of these
also, an adaptive character can be often discerned. In birds, for
instance, we have differences in the size or shape of the bill or the
feet, in the length of the wing or the tail, and in the proportions of
the several feathers of which these organs are composed. 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