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The birds are another class of backboned animals which exhibit identical principles of relationship.  A heron has long legs and wide-spreading toes, which keep its body out of the water as it stalks about the marshes where it seeks its food; its bill is a long slender pincers.  Compare it with an eagle; the latter has a short and heavily hooked beak to tear flesh, while its stout legs bear strongly curved talons to hold its struggling prey.  Swimming birds like the swan and duck and loon possess feet which are constructed in general like those of the former examples, but they are webbed and shortened to serve as paddles.  In the penguin we find a counterpart of the seal among mammals; its feathers are much reduced and its fore limbs are no longer wings enabling the animal to fly, but they are paddles which it uses when it swims in pursuit of fish.  Finally the ostrich and wingless bird of New Zealand—­the Apteryx—­have wings that are useless vestiges, which, in the latter case, are hidden under the brushlike feathers covering the body.  It is unnecessary to add more examples, for even these few illustrations establish exactly the same principles of relationship and evidences of evolution that are to be found in the series of mammalia.

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.