elongated form, and are both destitute of limbs; the head in both is on a level with the body, without any contraction behind it, such as marks the neck in the higher Reptiles, and moves only by the action of the back-bone; they are singularly alike in their external features, but the young of the Serpent are hatched in a mature condition, while the young of the type to which the Caecilians belong undergo a succession of metamorphoses before attaining to a resemblance to the parent.  Or compare the Lizard and the Salamander, in which the likeness is perhaps even more striking; for any inexperienced observer would mistake one for the other.  Both are superior to the Serpents and Caecilians, for in them the head moves freely on the neck and they creep on short imperfect legs.  But the Lizard is clothed with scales, while the body of the Salamander is naked, and the young of the former is complete when hatched, while the Tadpole born from the Salamander has a life of its own to live, with certain changes to pass through before it assumes its mature condition; during the early part of its life it is even destitute of legs, and has gills like the Fishes.  Above the Lizards and Salamanders, highest in the class of Reptiles, stand two other collateral types,—­the Turtles at the head of the Scaly Reptiles, the Toads and Frogs at the Lead of the Naked Reptiles.  The external likeness between these two groups is perhaps less striking than between those mentioned above, on account of the large shield of the Turtle.  But there are Turtles with a soft covering, and there are some Toads with a hard shield over the head and neck at least, and both groups are alike distinguished by the shortness and breadth of the body and by the greater development of the limbs as compared with the lower Reptiles.  But here again there is the same essential difference in the mode of development of their young as distinguishes all the rest.  The two series may thus be contrasted:—­

Naked Reptiles.  Toads and Frogs, Salamanders, Caecilians.

Scaly Reptiles. Turtles, Lizards, Serpents.

Such corresponding groups or parallel types, united only by external resemblance, and distinguished from each other by essential elements of structure, exist among all animals, though they are less striking among Birds on account of the uniformity of that class.  Yet even there we may trace such analogies,—­as between the Palmate or Aquatic Birds, for instance, and the Birds of Prey, or between the Frigate Bird and the Kites.  Among Fishes such analogies are very common, often suggesting a comparison even with land animals, though on account of the scales and spines of the former the likeness may not be easily traced.  But the common names used by the fishermen often indicate these resemblances, —­as, for instance, Sea-Vulture, Sea-Eagle, Cat-Fish, Flying-Fish, Sea-Porcupine, Sea-Cow, Sea-Horse, and the like.  In the branch of Mollusks, also, the same superficial