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.