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 



csecilian 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 

 Distribution. te J c . _ . J ,„ . 



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 group 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. 



