Dr. John Hunter[1] has advanced the 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 which imprisons the alligator in the Mississippi as effectually cuts him 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 tank, 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 torpid again on the 15th November, 1851, and was found dead and dried up in March, 1852.[1] But the exceptions serve to prove the accuracy of Hunter’s opinion almost as strikingly as accordances, since the same genera of animals which hybernate in Europe, where extreme cold disarranges their oeconomy, evince no symptoms of lethargy in the tropics, provided their food be not diminished by the heat.  Ants, which are torpid in Europe during winter, work all the year round in India, where sustenance is uniform.[2] The Shrews of Ceylon (Sorex montanus and S. ferrugineus of Kelaart) which, like those at home, subsist upon insects, inhabit a region where the equable temperature admits of the pursuit of their prey at all seasons of the year; and hence, unlike those of Europe, they never hybernate.  A similar observation applies to the bats, which are dormant during a northern winter when insects are rare, but never become torpid in any part of the tropics.