the rainless season. LYELL
mentions the instance of some snails in Italy which, when they
hybernate, descend to the depth of five feet and more below the surface.
_Princip. of Geology,_ &c, p. 373.]

Dr. John Hunter[1] has advanced an opinion that hybernation, although a
result of cold, is not its immediate consequence, but is attributable to
that deprivation of food and other essentials which extreme cold
occasions, and against the recurrence of which nature makes a timely
provision by a suspension of her functions. Excessive heat in the
tropics produces an effect upon animals and vegetables analogous to that
of excessive cold in northern regions, and hence it is reasonable to
suppose that the torpor induced by the one may be but the counterpart of
the hybernation which results from the other. The frost that imprisons
the alligator in the Mississippi as effectually cuts it off from food
and action as the drought which incarcerates the crocodile in the
sun-burnt clay of a Ceylon tank. The hedgehog of Europe enters on a
period of absolute torpidity as soon as the inclemency of winter
deprives it of its ordinary supply of slugs and insects; and the
_tenrec_[2] of Madagascar, its tropical representative, exhibits the
same tendency during the period when excessive heat produces in that
climate a like result.

[Footnote 1: HUNTER'S _Observations on parts of the Animal oeconomy_, p.
88.]

[Footnote 2: _Centetes ecaudatus_, Illiger.]

The descent of the _Ampullaria_, and other fresh-water molluscs, into
the mud of the tanks, has its parallel in the conduct of the _Bulimi_
and _Helices_ on land. The European snail, in the beginning of winter,
either buries itself in the earth or withdraws to some crevice or
overarching stone to await the returning vegetation of spring. So, in
the season of intense heat, the _Helix Waltoni_ of Ceylon, and others of
the same family, before retiring under cover, close the aperture of
their shells with an impervious epiphragm, which effectually protects
their moisture and juices from evaporation during the period of their
aestivation. The Bulimi of Chili have been found alive in England in a
box packed in cotton after an interval of two years, and the animal
inhabiting a land-shell from Suez, which was attached to a tablet and
deposited in the British Museum in 1846, was found in 1850 to have
formed a fresh epiphragm, and on being immersed in tepid water, it
emerged from its shell. It became t