Other animals, more respectful of property, avoid using another’s dwelling until it is abandoned by its proprietor, and no reproach of indelicacy can be addressed to the Gobius minutus, a fish which lives on our coasts at the mouth of rivers. The female lays beneath overturned shells, remains of Oysters, or Cardium shells. The valve is buried beneath several centimetres of sand, which supports it like a vault. It forms a solid roof, beneath which the eggs undergo their evolution. Sometimes the male remains by the little chamber to watch over their fate. It is possible to distinguish the two holes of entrance and exit which mark his habitual passage.
The Hermit-crab perhaps knows best how to take advantage of old clothes. ([Fig. 19.]) He collects shells of Gasteropods, abandoned flotsam, the first inhabitant of which has died. The Hermit-crab (Pagurus Bernhardus) is a Decapod Crustacean — that is to say, he resembles a very small Crab. But his inveterate habit during so many generations of sheltering his abdomen in a shell prevents this part from being encrusted with lime and becoming hard. The legs and the head remain in the ordinary condition outside the house, and the animal moves bearing it everywhere with him; on the least warning he retires into it entirely. But the Crustacean grows. When young he had chosen a small shell. A Mollusc, in growing, makes his house grow with him. The Hermit-crab cannot do this, and when his dwelling has become too narrow he abandons it for one that is more comfortable.
At first enclosed in the remains of a Trochus, he changes into that of a Purpura; a little later he seeks asylum in a Whelk. Beside the shelter which these shells assure to the Crustacean, they serve to mask his ferocity, and the prey which approaches confidently what it takes to be an inoffensive Mollusc, becomes his victim.
The Great Horned Owl likewise does not construct a nest; but takes possession of the dwellings abandoned by others. These birds utilise for laying their eggs sometimes the nest of a Crow or a Dove, sometimes the lair which a Squirrel had considered too dilapidated. The female, without troubling about the bad state of these ruins, or taking pains to repair them, lays her eggs here and sits on them.
Classification of artificial shelters. — It is time to turn to animals who have more regard for comfort, and who erect dwellings for themselves or their offspring. These dwellings may be divided into three groups: (1) Those which are hollowed in earth or in wood; (2) those which in the simplest form result from the division of material of any kind; then, as a complication, of materials bound together; then, as a last refinement, of delicate materials, such as blades of grass or threads of wool woven together; such are the nests of certain birds and the tents of nomads; (3) those which are built of moist earth which becomes hard on drying; the perfection of this method consists of piling up hard fragments, pieces of wood or ashlar, the moist earth being only a mortar which unites the hard parts together. Animals exercise with varying success these different methods, all of which Man still practises.
Hollowed dwellings — Rudimentary burrows. — We will first occupy ourselves with the dwelling hollowed in the earth. It is the least complicated form. The number of creatures who purely and simply bury themselves thus to obtain shelter is incalculable; I will only mention a few examples, and pass on from simple combinations to the more perfected industries, of which they present the first sketch.
It is known that at a certain epoch of the year Crabs abandon their hard carapaces. This phenomenon is known by the name of the moult; they remain in this condition for some time; it is the period during which they grow; then their integuments are encrusted anew with lime and again become resistant. While they are thus deprived of their ordinary protection they are exposed to a crowd of dangers, and they are so well aware of this that they remain hidden beneath rocks and pebbles. A crab of Guadeloupe, called Gecarinus ruricola, escapes the perils of this situation, thanks to its kind of life and its habit of hollowing out a burrow to live in while it is deprived of its habitual defence. This Crustacean lives on the earth, at a distance of about ten or twelve kilometres from the sea-shore, and nourishes itself on animal and vegetable remains. It approaches the water only at the period of laying eggs, turning towards the coast in the months of February and March. This migration does not take place, like some others, in compact bands; each follows the road in independence, and preserves a certain amount of liberty with regard to the path and the epoch of the journey. They lead an aquatic life till May or June; then the female abandons her little ones, who had begun their development attached to her claws, and they return to land. The moult takes place in August. At the approach of this dreaded crisis each hollows a hole between two roots, supplies it with green leaves, and carefully stops up the entrance. These labours accomplished, the crab is entirely sheltered; it undergoes the moult in safety, and does not emerge from its retreat until it is again capable of facing enemies, and of seizing food with its claws, which have become hard again. This seclusion appears to last a month. Here is, then, an example of a temporary dwelling rendered necessary by special conditions of defect for external life. We are here still in the infancy of the art.