DISTRIBUTION. 263 



of the remainder of the yolk. The vertical fin-like expansions of the tail rapidly 

 increase, and the body becomes relatively smaller and more slender; while the 

 limbs begin to make their appearance as buds, although the date of development 

 of the front and hind-pair varies in different groups. In the newts, the front 

 pair of limbs are the first to appear, in the frogs the reverse is the case. In 

 the latter the hind-limbs appear some considerable time before the front pair, 

 the fish-like tail persisting till the sprouting of these, when the change from 

 a herbivorous fish-like animal to one carnivorous and reptiliform begins. The 

 jaws are at first invested with horny teeth, and subsequently with horny 

 sheaths, which eventually disappear; while the tail gradually diminishes in size, 

 and finally is lost. It may be observed that no vertebrae are developed in the 

 frog's tail ; and that the long spine in which the backbone of the adult terminates 

 is an outgrowth from the hindmost vertebra. Not less remarkable is the 

 shortening of the intestinal canal, as the creature changes its herbivorous for 

 carnivorous habits. To trace in detail the development of the soft parts would 

 greatly exceed our limits of space. We may mention, however, that in one group 

 of Tailed Amphibians the external gills of some individuals may be retained 

 permanently, while in others of the same species they are cast at an early period. 

 Then, again, the number of these gills is by no means constant, for in the Cingalese 

 cascilian and the salamander there are three pairs of these organs, in the tadpoles 

 of some frogs there are two, and in others, as well as in one genus of csecilians, 

 there are only a single pair. 



. . Geologically the Amphibians are a very ancient group, their 



oldest representatives occurring in the Carboniferous and Permian 

 rocks of Europe and North America. All these ancient representatives of the 

 class belong, however, to the grouj) of Labyrinthodonts, which survived till the 

 period of the Trias, and are structurally very different from the modern forms, 

 approximating in certain respects to fishes. Indeed, since no Amphibians have 

 hitherto been discovered between the Trias and the Wealden, or lower Cretaceous, 

 rocks of Belgium, we are quite unable to assert that the modern representatives of 

 the class are the direct descendants of the Labyrinthodonts. Commencing in the 

 Belgian Wealden, the newts and salamanders occur throughout the greater part of 

 the Tertiary rocks ; but the frogs and toads are first known in North America 

 from Eocene beds, while in Europe they are not met with before the Oligocene. 



At the present time Amphibians are distributed over all parts of the world 

 except the polar regions ; although they are more dependent upon the presence 

 of water and warmth than any of the preceding classes of Vertebrates. They are, 

 accordingly, most abundant in the tropical and subtropical regions ; and as none 

 of them are marine in their habits, even a narrow arm of the sea is generally 

 sufficient to limit their habitat. When they occur on islands, it is probable either 

 that their eggs have been carried by birds, or that there has been a comparatively 

 recent separation from the mainland. In absolutely desert districts Amphibians 

 are unknown; while in countries where there is a long dry season, followed 

 by a period of rains, they are in the habit of becoming torpid during the 

 former; the length of the sleep in one Javan species being upwards of five months. 

 In cold climates all the members of the class become torpid during the winter. 



