form and colouration, and sometimes
even the internal structure of animals.

As illustrating this latter point, I may refer to the remarkable hooked,
branched, or star-like spiculae in many sponges, which are believed to
have the function chiefly, of rendering them unpalatable to other
creatures. The Holothuridae or sea-cucumbers possess a similar
protection, many of them having anchor-shaped spicules embedded in their
skin, as the Synapta; while others (Cuviera squamata) are covered with a
hard calcareous pavement. Many of these are of a bright red or purple
colour, and are very conspicuous, while the allied Trepang, or
Beche-de-mer (Holothuria edulis), which is not armed with any such
defensive weapons, is of a dull sand-or mud-colour, so as hardly to be
distinguished from the sea bed on which it reposes. Many of the smaller
marine animals are protected by their almost invisible transparency,
while those that are most brightly coloured will be often found to have
a special protection, either in stinging tentacles like Physalia, or in
a hard calcareous crust, as in the star fishes.

_Females of some Groups require and obtain more Protection than the
Males._

In the struggle for existence incessantly going on, protection or
concealment is one of the most general and most effectual means of
maintaining life; and it is by modifications of colour that this
protection can be most readily obtained, since no other character is
subject to such numerous and rapid variations. The case I have now
endeavoured to illustrate is exactly analogous to what occurs among
butterflies. As a general rule, the female butterfly is of dull and
inconspicuous colours, even when the male is most gorgeously arrayed;
but when the species is protected from attack by a disagreeable odour,
as in the Heliconidae, Danaidae and Acroeidae, both sexes display the same
or equally brilliant hues. Among the species which gain a protection by
imitating these, the very weak and slow-flying Leptalides resemble them
in both sexes, because both sexes alike require protection, while in the
more active and strong-winged genera--Papilio, Pieris, and Diadema--it
is generally the females only that mimic the protected groups, and in
doing so often become actually more gay and more conspicuous than the
males, thus reversing the usual and in fact almost universal characters
of the sexes. So, in the wonderful Eastern leaf-insects of the genus
Phyllium, it is the fema