6 THE VIVARIUM. 
dependent to a very great extent upon external heat for that of 
their bodies. For instance, if a cold Snake be wrapped up in 
unwarmed blankets it will remain cold, for it is able to generate 
very little or no heat of its own; but on the other hand, if a 
cold, healthy man be enveloped in unwarmed blankets he will 
soon become quite hot, which would never happen in the case of 
the Reptile. Reptiles and Batrachians, therefore, are not found 
in the Arctic or Antarctic regions, but in the tropical and sub- 
tropical countries they are in the greatest variety and abundance. 
Most Reptiles and Batrachians spend the cold months of the 
year in a state of torpor, commonly called hibernation; while a 
few of them, in very hot climates, estivate, that is, they remain 
more or less dormant during the greatest and driest heat of the 
summer. 
The late Mr. Thomas Bell, in the introduction to his “ History 
of British Reptiles,” well says that “‘the phenomena of hiberna- 
tion are amongst the most remarkable and interesting which 
occur in the history of animals.”” Both the low temperature of 
winter and the absence of suitable food tend to cause this hiber- 
nation. During hibernation the respiration is very slow, and 
apparently ceases, digestion is at a standstill, and insensibility 
is all but complete. 
In our own, and other countries, as winter approaches, most 
of the insectivorous birds are able to migrate to warmer climates 
and more abundant food; but as Reptiles and Batrachians, in 
common with the Squirrel, the Bat, the Dormouse, and some 
other animals are unable to cross the seas, they are obliged to 
retire to their various hibernacula, or winter quarters, which they 
make according to their species and needs under ground, under 
stones, under heaps of rubbish, in clefts of rocks, in crevices of 
banks, in the mud at the bottom of ponds, in nests of their own 
construction, in the hollow of trees, and similar places. The 
state of torpidity, however, of the cold-blooded Reptiles and 
Batrachians is more complete during hibernation than is that 
of the warm-blooded animals, such as those which have been 
mentioned above. 
MM. Duméril and Bibron, in their great work upon Reptiles 
and Batrachians (completed in 1854), described no less than 121 
