it
can be lighted again before any serious change in the system takes
place, all may still go on precisely as of old.

Many animals, however, find it convenient to assume a state of less
complete suspended animation during certain special periods of the year,
according to the circumstances of their peculiar climate and mode of
life. Among the very highest animals, the most familiar example of this
sort of semi-torpidity is to be found among the bears and the dormice.
The common European brown bear is a carnivore by descent, who has become
a vegetarian in practice, though whether from conscientious scruples or
mere practical considerations of expediency, does not appear. He feeds
chiefly on roots, berries, fruits, vegetables, and honey, all of which
he finds it comparatively difficult to procure during winter weather.
Accordingly, as everyone knows, he eats immoderately in the summer
season, till he has grown fat enough to supply bear's grease to all
Christendom. Then he hunts himself out a hollow tree or rock-shelter,
curls himself up quietly to sleep, and snores away the whole livelong
winter. During this period of hibernation, the action of the heart is
reduced to a minimum, and the bear breathes but very slowly. Still, he
does breathe, and his heart does beat; and in performing those
indispensable functions, all his store of accumulated fat is gradually
used up, so that he wakes in spring as thin as a lath and as hungry as a
hunter. The machine has been working at very low pressure all the
winter: but it _has_ been working for all that, and the continuity of
its action has never once for a moment been interrupted. This is the
central principle of all hibernation; it consists essentially of a very
long and profound sleep, during which all muscular motion, except that
of the heart and lungs, is completely suspended, while even these last
are reduced to the very smallest amount compatible with the final
restoration of full animal activity.

Thus, even among warm-blooded animals like the bears and dormice,
hibernation actually occurs to a very considerable degree; but it is far
more common and more complete among cold-blooded creatures, whose bodies
do not need to be kept heated to the same degree, and with whom,
accordingly, hibernation becomes almost a complete torpor, the breathing
and the action of the heart being still further reduced to very nearly
zero. Mollusks in particular, like oysters and mussels, lead very