XIV INTRODUCTION. 
The phenomena of hibernation are amongst the most re- 
markable and interesting which occur in the history of ani- 
mals. That the cold-blooded creatures of which we are 
now treating, with their limited respiration and cireula- 
tion, their entire dependence upon external heat to carry 
on their functions, and the consequent diminution of all 
their powers, and of their expenditure also, during a low 
degree of external temperature, should be able to sustain 
life for an almost indefinite period, under peculiar cireum- 
stances, is not, perhaps, so surprising. But this simple 
torpidity is very different from that true hibernation to 
which many of the warm-blooded animals are liable. The 
bats, the squirrel, the dormouse, the marmots, and many 
others, may be cited as examples. It is sufficient in 
reference to the present class to observe, that in temperate 
or cool climates, the whole of them retire during the winter 
to some place of concealment ; the land Tortoises to exca- 
vations which they often dig for themselves ; the fresh- 
water Tortoises to holes in the muddy banks, or to the 
mud at the bottom, of their native lakes or rivers; the 
Lizards and Serpents to holes in trees, under stones, 
beneath dead leaves, or in similar hiding-places, where 
many species, especially of the latter order, congregate 
in large numbers, and are found closely entwined together. 
Here they pass the winter in a state of almost lifeless re- 
pose, the functions of life so nearly suspended, that none 
of the external signs of its existence are visible. The 
circulation is extremely slow, the respiration apparently 
altogether stopped, digestion absolutely suspended. The 
return of the genial warmth of spring calls them again into 
action. The circulation is restored, the blood is again 
fitted for its various offices by the return of regular respi- 
ration, the functions of the digestive organs are again 
