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 u