case of alarm
and to pass there cold days and nights.

Darwin has observed and described[82] how a little Lacertilian, the
_Conolophus subcristatus_, conducts its work of mining and digging. It
establishes its burrow in a soft tufa, and directs it almost
horizontally, hollowing it out in such a way that the axis of the hole
makes a very small angle with the soil. This reptile does not
foolishly expend its strength in this troublesome labour. It only
works with one side of its body at a time, allowing the other side to
rest. For instance, the right anterior leg sets to work digging, while
the posterior leg on the same side throws out the earth. When
fatigued, the left legs come into play, allowing the others to repose.

[82] _Voyage of the Beagle_.

Other animals, without building their cavern with remarkable skill,
show much sagacity in the choice of a site calculated to obtain
certain determined advantages. In Egypt there are dogs which have
become wild. Having shaken off the yoke of man, which in the East
affords them little or no support, they lead an independent life.
During the day they remain quiescent in desert spots or ruins, and at
night they prowl about like jackals, hunting living prey or feeding on
abandoned carcasses. There are hills which have in a manner become the
property of these animals. They have founded villages there, and allow
no one to approach. These hills have an orientation from north to
south, so that one slope is exposed to the sun from morning to mid-day
and the other from mid-day to evening. Now, dogs have a great horror
of heat. They fear the torrid heat of the south as much as in our
climate they like to lie warmed by gentle rays; there is no shadow too
deep for their siesta. Therefore, on these Egyptian hills every dog
hollows out a lair on both slopes. One of these dwellings is thus
turned towards the east, the other towards the west. In the morning,
when he returns from his nocturnal expeditions, the animal takes
refuge in the second, and remains there until mid-day, sunk in
refreshing sleep. At that hour the sun begins to reach him, and to
escape it he passes over to the opposite slope; it is a curious sight
to see them all, with pendent heads and sleepy air, advance with
trailing steps to their eastern retreat, settle down in it, and
continue their dream and their digestion till evening, when they again
set forth to prowl. We never grow tired of admiring the intelligence
of t