sy
recognition would be impossible among numerous closely allied forms.[83]
The wonderful diversity of colour and of marking that prevails,
especially in birds and insects, may be due to the fact that one of the
first needs of a new species would be, to keep separate from its nearest
allies, and this could be most readily done by some easily seen external
mark of difference. A few illustrations will serve to show how this
principle acts in nature.

My attention was first called to the subject by a remark of Mr. Darwin's
that, though, "the hare on her form is a familiar instance of
concealment through colour, yet the principle partly fails in a closely
allied species, the rabbit; for when running to its burrow it is made
conspicuous to the sportsman, and no doubt to all beasts of prey, by its
upturned white tail."[84] But a little consideration of the habits of
the animal will show that the white upturned tail is of the greatest
value, and is really, as it has been termed by a writer in _The Field_,
a "signal flag of danger." For the rabbit is usually a crepuscular
animal, feeding soon after sunset or on moonlight nights. When disturbed
or alarmed it makes for its burrow, and the white upturned tails of
those in front serve as guides and signals to those more remote from
home, to the young and the feeble; and thus each following the one or
two before it, all are able with the least possible delay to regain a
place of comparative safety. The apparent danger, therefore, becomes a
most important means of security.

The same general principle enables us to understand the singular, and
often conspicuous, markings on so many gregarious herbivora which are
yet, on the whole, protectively coloured. Thus, the American prong-buck
has a white patch behind and a black muzzle. The Tartarian antelope, the
Ovis poli of High Asia, the Java wild ox, several species of deer, and a
large number of antelopes have a similar conspicuous white patch behind,
which, in contrast to the dusky body, must enable them to be seen and
followed from a distance by their fellows. Where there are many species
of nearly the same general size and form inhabiting the same region--as
with the antelopes of Africa--we find many distinctive markings of a
similar kind. The gazelles have variously striped and banded faces,
besides white patches behind and on the flanks, as shown in the woodcut.
The spring-bok has a white patch on the face and one on the sides, wit