222 ZOOLOGY. [PART II. 



cessive heat in the tropics produces an effect upon ani- 

 mals 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 coun- 

 terpart 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 incle- 

 mency of winter deprives it of its ordinary supply of 

 slugs and insects ; and the Tenrec 1 of Madagascar, its 

 tropical representative, exhibits the same tendency during 

 the period when excessive heat produces in that climate a 

 like result. 



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 over- 

 arching 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 re- 

 tiring under cover, close the aperture of their shells 

 with an impervious epiphragm, which effectually pro- 

 tects 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. 2 But the exceptions serve to prove 



1 Centetcs ecaudatus, Illiger. | See Dr. BAIRD'S Account of Helix 



2 Annals of Natural History, 1850. j desertonim; Excelsior, $c. } ch. i. p. 345. 



