e little frogs. Instead
of swallowing it, however, the duck instantly threw it out of its mouth,
and went about jerking its head as if trying to get rid of some
unpleasant taste.[114]

The power of predicting what will happen in a given case is always
considered to be a crucial test of a true theory, and if so, the theory
of warning colours, and with it that of mimicry, must be held to be well
established. Among the creatures which probably have warning colours as
a sign of inedibility are, the brilliantly coloured nudibranchiate
molluscs, those curious annelids the Nereis and the Aphrodite or
sea-mouse, and many other marine animals. The brilliant colours of the
scallops (Pecten) and some other bivalve shells are perhaps an
indication of their hardness and consequent inedibility, as in the case
of the hard beetles; and it is not improbable that some of the
phosphorescent fishes and other marine organisms may, like the
glow-worm, hold out their lamp as a warning to enemies.[115] In
Queensland there is an exceedingly poisonous spider, whose bite will
kill a dog, and cause severe illness with excruciating pain in man. It
is black, with a bright vermilion patch on the middle of the body; and
it is so well recognised by this conspicuous coloration that even the
spider-hunting wasps avoid it.[116]

Locusts and grasshoppers are generally of green protective tints, but
there are many tropical species most gaudily decorated with red, blue,
and black colours. On the same general grounds as those by which Mr.
Belt predicted the inedibility of his conspicuous frog, we might safely
predict the same for these insects; but we have fortunately a proof that
they are so protected, since Mr. Charles Home states that one of the
bright coloured Indian locusts was invariably rejected when offered to
birds and lizards.[117]

* * * * *

The examples now given lead us to the conclusion that colours acquired
for the purpose of serving as a danger-signal to enemies are very
widespread in nature, and, with the corresponding colours of the species
which mimic them, furnish us with a rational explanation of a
considerable portion of the coloration of animals which is outside the
limits of those colours that have been acquired for either protection or
recognition. There remains, however, another set of colours, chiefly
among the higher animals, which, being connected with some of the most
interesting and most disp