y their eggs and rear
the offspring. Whatever the object may be for which these retreats are
built, they constitute altogether various manifestations of the same
industry, and I will class them, not according to the uses which they
are to serve, but according to the amount of art displayed by the
architect.

In this series, as in those which we have already studied, we shall
find every stage from that of beings provided for by nature, and
endowed with a special organ which secretes for them a shelter, up to
those who are constrained by necessity to seek in their own
intelligence an expedient to repair the forgetfulness of nature. These
productions, so different in their origin, can only be compared from
the point of view of the part they play; there are analogies between
them but not the least homology.

_Animals naturally provided with dwellings._--Nearly all the Mollusca
are enveloped by a very hard calcareous case, secreted by their
mantle: this shell, which is a movable house, they bear about with
them and retire into at the slightest warning.

Caterpillars which are about to be transformed into chrysalides weave
a cocoon, a very close dwelling in which they can go through their
metamorphosis far from exterior troubles. It is an organic form of
dwelling, or produced by an organ. It is not necessary to multiply
examples of this kind; they are extremely numerous. In the same
category must be ranged the cells issuing from the wax-glands which
supply Bees with materials for their combs in which they enclose the
eggs of the queen with a provision of honey.

I do not wish to insist on creations of this kind which are
independent of the animal's will and reflection. Near these facts must
be placed those in which animals, still using a natural secretion, yet
endeavour to obtain ingenious advantages from it unknown by related
species.

[Illustration: FIG. 18.]

There is, for example, the _Macropus viridi-auratus_, or
Paradise-fish, which blows air bubbles in the mucus produced from its
mouth. This mucus becomes fairly resistant, and all the bubbles
imprisoned and sticking aside by side at last form a floor. It is
beneath this floating shelter that the fish suspends its eggs for its
little ones to undergo their early development.

_Animals who increase their natural protection by the addition of
foreign bodies._--Certain tubicolar Annelids, whose skin furnishes
abundant mucus which does not become sufficiently hard