XVI 
INTRODUCTION. 
sources, it follows that tlie animal functions must be carried on with a greater 
or less degree of activity and power, according to the temperature of the sur¬ 
rounding atmosphere. Hence we find that, in common with all other reptiles, 
they are not only enabled to sustain long fasts, but in fact digest their food 
only in proportion to the heat of the climate. In cool weather they take food 
but seldom, digest it slowly, and will even live for many months, and even for 
years, without any nourishment. Their respiration, circulation, muscular 
motion, and every other function, are diminished in a corresponding degree. 
When the temperature sinks to a certain point, the functions of life gradually 
cease to act; the animals refuse to feed, the respiration becomes slower and 
slower, the circulation more and more languid, and a state of torpidity, almost 
resembling death itself, takes place, in which condition they remain during the 
winter, and until the return of spring, by its increasing temperature, renews 
their action, and restores them to their natural state of life and activity. 
The Testudinata are obviously composed of three distinct principal groups, 
of which the conformation bears a striking and satisfactory relation to their 
habits and mode of life. These are the terrestrial, the fresh-water, and the 
marine forms; and the characters by which they are distinguished are so con¬ 
stant and so influential on their situation, food, and other circumstances of 
their history, as to justify a separate consideration. I shall however first give 
such a general description of their structure, as may be necessary to illustrate 
their history and their zoological characters, though without entering at present 
into any detailed account of their anatomy. 
The peculiar structure of the heart, upon which the imperfect condition of 
the circulation depends, consists in the ready communication of the two cham¬ 
bers of the ventricle. The blood, as in the higher animals, enters the heart by 
two auricles, of which the right receives the blood which has been circulated 
through the body, and the left that which has been aerated in the lungs. 
But in the ventricle the two kinds of blood become more or less mixed, so that 
neither is that portion which is sent to the general system, absolutely decar¬ 
bonized, nor is that which goes to the lungs, to receive the necessary change, in 
such a state of impurity as when it arrived at the heart after having gone 
through the general circulation. 
