The fishes are the lowest among the common vertebrates, and they offer an
abundance of independent testimony as to the truth of the principles of
comparative anatomy. The common shark is perhaps the most fundamental
form, with a hull-like body undivided into head, trunk, and tail, and from
it have originated such peculiar variations as the hammerhead and skate.
Among fishes with true bones, a cod or trout is the most typical in
general features. Without ceasing to be true bony fishes, the trunk-fish
and cow-fish are adapted by their peculiar characters of spine and armor
plate to repel many enemies. The puff fish can take in a great amount of
water, when disturbed, so as to become too large to be swallowed by some
of its foes, illustrating another adaptive modification for self-defense.
The wonderful colors and color patterns of the tropical fish of the reef,
or of the open water forms like the mouse-fish of the Sargossa Sea, often
render them more or less completely hidden from the foraging enemy. A
flounder looks like a fish which was originally symmetrical, but which had
come to lie flat on its side upon the bottom, whereupon the eye underneath
had left its original place to appear on the upper surface. The difficult
and unusual conditions of deep-sea existence have been met by fishes in
two ways; some forms possess luminous frilled and weedlike fins, which
lure their prey to within easy reach of their jaws, while others have
enormous eyes, so as to make use of all possible rays of light in their
pursuit of food organisms. But all of these diverse forms are true
_fishes_, possessing a common heritage of structure which demonstrates
their unity of origin.

The brief review of backboned animals has shown how comprehensive are the
principles of relationship. The families and tribes of each order, such as
the carnivora, are like branches arising from a single limb; the orders in
their turn exhibit common qualities of structure which mean that they have
grown from the same antecedents, while even the larger divisions or