§ 252. The _Vertebrata_ illustrate afresh the truths which we have already traced among the _Annulosa_. Flying through the air, swimming through the water, and running over the earth as vertebrate animals do, in common with annulose animals, they are, in common with annulose animals, different at their anterior and posterior ends, different at their dorsal and ventral surfaces, but alike along their two sides. This single bilateral symmetry remains constant under the extremest modifications of form. Among fish we see it alike in the horizontally-flattened Skate, in the vertically-flattened Bream, in the almost-spherical _Diodon_, and in the greatly-elongated _Syngnathus_. Among reptiles the Turtle, the Snake, and the Crocodile all display it. And under the countless modifications of structure displayed by birds and mammals, it remains conspicuous. [Illustration: Figs. 273–280.] A less obvious fact which it concerns us to note among the _Vertebrata_, parallel to one which we noted among the _Annulosa_, is that whereas the lower vertebrate forms deviate but little from triple bilateral symmetry, the deviation becomes great as we ascend. Figs. 273 and 274 show how, besides being divisible into similar halves by a vertical plane passing through its axis, a Fish is divisible into halves that are not very dissimilar by a horizontal plane passing through its axis, and also into other not very dissimilar halves by a plane cutting it transversely. If, as shown in Figs. 275 and 276, analogous sections be made of a superior Reptile, the divided parts differ more decidedly. When a Mammal and a Bird are treated in the same way, as shown in Figs. 277, 278, and Figs. 279, 280, the parts marked off by the dividing planes are unlike in far greater degrees. On considering the mechanical converse between organisms of these several types and their environments--on remembering that the fish habitually moves through a homogeneous medium of nearly the same specific gravity as itself, that the terrestrial reptile either crawls on the surface or raises itself very incompletely above it, that the more active mammal, having its supporting parts more fully developed, thereby has the under half of its body made more different from the upper half, and that the bird is subject by its mode of life to yet another set of actions and reactions; we shall see that these facts are quite congruous with the general doctrine, and furnish further support to it.