but, then there is a limitation
to the store of air supplied, which can be no more than the quantity
extracted from the water. The temperature of water is maintained below
the surface, and we know how that of the air varies, since a certain
quantity of heat is necessary to the vital processes; reptiles,
depending upon air for heat, hybernate or become torpid when the
temperature falls below a certain point. The rapidity of all their vital
actions will depend upon the state of the thermometer; they digest
faster in the heat of summer than in the milder warmth of spring. Their
secretions (as the poison of the adder) are in hot weather more copious,
and in winter are not formed at all. The reptiles breathe, in all cases,
by lungs; but we must except here those called _Batrachians_, as frogs
or newts, which breathe, in the first stage, by gills, and afterward by
gills and lungs, or by lungs only. The _Batrachians_, again, are the
only exception to another great characteristic of the reptile class, the
hard, dry covering of plates or scales. The reptiles all produce their
young from eggs, or are "oviparous"--some hatch their eggs within the
body, and produce their young alive, or are "ovo-viviparous." These are
the characters belonging to all members of the reptile-class. The class
is subdivided into orders somewhat thus: 1. The _Testudinate_ (tortoises
and turtles). 2. _Enaliosaurian_ (all fossil, the _Ichthyosaurus_ and
his like). 3. _Loricate_ (crocodiles and alligators). 4. _Saurian_
(lizards). 5. _Ophidian_ (serpents); and the last order, _Batrachian_
(frogs, toads, &c.); which is, by some, parted from the reptiles, and
established as another class.

Now we have in England no tortoises or turtles, and no crocodiles: and
the fossil order is, in all places, extinct; so our reptiles can belong
only to the three last-named orders, Lizards, Serpents, and Batrachians.

Thus we come back, then, to our Lizards, of which we have among us but
two genera, a single species of each. These are the Common Lizard, well
known to us all, and the Sand Lizard, known only to some of us who
happen to live upon the southern coast. The species of lizard so
extremely common in this country, has not been found in countries
farther south, and is, in fact, peculiar to our latitude. We, therefore,
may love him as a sympathetic friend. The sand lizard (_Lacerta agilis_)
is found as far north as the country of Linnaeus, and as far south as the
north