A bird is known by its feathers, a Beast by its hairs, a Fish by its fins, but there is no such obvious feature which characterises the Amphibia and the Reptiles. In fact, they are neither fish, flesh, nor fowl. This ill-defined position is indicated by the want of vernacular names for these two classes, a deficiency which applies not only to the English language. All the creatures in question are backboned, creeping animals. Those which are covered with horny scales, and which from their birth breathe by lungs only, as Crocodiles, Tortoises, Lizards, and Snakes, are the Reptiles. The rest, for instance, Newts or Efts, Frogs and Toads, are the Amphibia. Their skin is mostly smooth and clammy and devoid of scales; the young are different from the adult in so far as they breathe by gills and live in the water, before they are transformed into entirely lung-breathing, terrestrial creatures. But there are many exceptions. _Proteus_ and _Siren_ the mud-eel, always retain their gills; while not a few frogs undergo their metamorphosis within the egg, and never breathe by gills. If we add the tropical limbless, burrowing Coecilians, and last, not least, the Labyrinthodonts and other fossil forms, the proper definition of the class Amphibia,–in other words, the reasons for grouping them together into one class, separated from the other backboned animals,–requires the examination of many other characters. {4}So far as numbers of living species are concerned, the Amphibia are the least numerous of the Vertebrata. There are about 40 limbless, burrowing APODA; 100 URODELA or tailed two- or four-footed newts, and about 900 ANURA, or tailless, four-footed frogs and toads; in all some 1000 different species. Few, indeed, in comparison with the 2700 Mammals, 3500 Reptiles, nearly 8000 Fishes, and almost 10,000 Birds. But we shall see that the Amphibia have not only "had their day," having flourished in bygone ages when they divided the world, so far as Vertebrata were concerned, between themselves and the Fishes, but that they never attained a dominant position. Intermediate between the aquatic Fishes and the gradually rising terrestrial Reptiles they had to fight, so to speak, with a double front during the struggle of evolution, until by now most of them have become extinct. The rest persist literally in nooks and corners of the teeming world, and only the Frogs and Toads, the more recent branch of the Amphibian tree, have spread over the whole globe, exhibiting almost endless variations of the same narrow, much specialised plan. The greatest charm of the Anura lies in their marvellous adaptation to prevailing circumstances; and the nursing habits of some kinds read almost like fairy-tales. CHARACTERS OF THE AMPHIBIA.[1] 1. The vertebrae are (_a_) acentrous, (_b_) pseudocentrous, or (_c_) notocentrous. 2. The skull articulates with the atlas by two condyles which are formed by the lateral occipitals. For exceptions see p. 78.