In one respect in which Reptiles agree among themselves, they agree also with the Classes of Birds and Fishes; the reproduction of their species is effected by means of eggs. For the most part these are encased in a calcareous covering, either hard and shelly as in the Tortoises, Crocodiles and some Saurians, or tough and leathery as in most of the Serpents: in the Amphibia, however, they are destitute of any covering. In a few instances, the young are brought forth alive; but these are rather apparent than real exceptions to the general rule, the eggs in such cases being hatched in the body of the parent, or ruptured in the act of deposition. Our native smaller Lizard, Viper, and Slow-worm, afford familiar examples of this peculiarity. Reptiles do not in general incubate their eggs; but there is reason to believe that the Boas and Pythons are exceptions to this rule.
The heart in this Class is so constructed that at every pulsation only a part of the blood from the system is thrown into the lungs, the remaining portion returning into the circulation without being aërated. As animal heat is derived from the communication of oxygen to the blood, in its exposure to the air when passing through the lungs, it follows that the imperfect aëration of the blood in these animals is attended with a diminution of vital heat, and that they are what is called cold-blooded; in other words, the heat produced by respiration is so small, that the difference
