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.
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