Animals which compose this class have long been confounded with reptiles, from which they differ in one fundamental peculiarity in their organization. At their birth they respire by means of gills, and consequently resemble fishes. In a physiological point of view, at a certain time in their lives, these animals are fishes in form as well as in their habits and organization. As age progresses, they undergo permanent metamorphosis--they acquire lungs, and thenceforth an aërial respiration. It is, then, easy to understand that these animals hold a doubtful rank, as they have long done, amongst Reptiles, which are animals with an aërial respiration; they ought to form a separate class of Vertebrates.[8] [8] They are regarded by some naturalists as a sub-class of Fishes rather than of Reptiles, as _piscine_ forms certain of which develop to a parallelism with the ordinary _reptilian_ condition of advancement; their reproduction especially favouring this view or idea.--ED. Batrachians establish a transitional link between Fishes and Reptiles--they are, as it were, a bond of union between those two groups of animals. In the adult state Batrachians are cold-blooded animals with incomplete circulation, inactive respiration, and the skin is bare. In the introductory section to this chapter we have given the general characteristics which belong to them. The Frogs--Tree Frogs, Toads, Surinam Toads, Salamanders, and Newts--are the representatives of the principal families of Batrachians of which we propose giving the history. The Frogs, _Rana_, have been irreparably injured by their resemblance to the Toads. This circumstance has given rise to an unfavourable prejudice against these innocent little Batrachians. Had the Toad not existed, the Frog would appear to us as an animal of a curious form, and would interest us by the phenomena of transformation which it undergoes in the different epochs of its development. We should see in it a useful inoffensive animal of slender form, with delicate and supple limbs, arrayed in that green colour which is so pleasant to the eye, and which mingles so harmoniously with the carpeting of our fields. [Illustration: Fig. 4.--The Edible Frog (_Rana esculenta_).]