Reptiles also are grouped, like the mammals and birds, as variations about
a central theme. An ordinary lizard is perhaps the nearest in form to the
remote ancestor from which all have sprung. Some lizards are long and very
slender, with all four limbs of greatly reduced size. Others, which are
still true lizards, have lost the hind limbs, or even all the legs, as in
the "blind worms" of England. One step more, and an animal which has
progressed further along a similar line of descent would be a snake. Just
as whales as a group are derivable from forms which resemble types
belonging to another order, so snakes as an order are to be regarded as
more radically altered derivatives of some four-footed lizardlike
creature. Alligators are very much like lizards in general form, and their
order is a diverging branch from the same limb. Finally the evolution of
turtles from the same ancestors is intelligible if we begin with a short
stout animal like the so-called "horned toad" of Arizona, and proceed to
the soft-shelled tortoise of the Mississippi River system; the
establishment of a bony armor completes the evolution of the familiar and
more characteristic turtle.

Frogs and salamanders constitute another lower class, called the amphibia,
whose members are gilled during the earlier stages of development. An
adult frog is essentially a salamander without a tail and with highly
developed hinder limbs. The salamanders differ as regards the number of
fishlike gill clefts that they all possess in their young stages, but
which disappear entirely or in part during later life. In comparison with
the lizard as a typical reptile, a salamander is more primitive in all of
its inner organic systems, while in its nearly continuous body, with head
and tail gradually merging into the trunk, it also displays a somewhat
simpler form of body.