176 AMPHIBIA 



evolved in aquatic vertebrates, i.e. fishes, but these fishes still 

 possessed fins, and their descendants only acquired legs when 

 they became terrestrial. Thus Dr. Gadow's conclusion that in 

 the existing perennibranchiate forms the retention of the gills 

 is not an ancestral feature but is due to the persistence of 

 larval organs in adaptation to aquatic habits, is in all proba- 

 bility correct. This is another example of what we may call 

 the zig-zag course of evolution which has taken place in 

 many groups of animals. We shall see that, in fishes, forms 

 most completely adapted to life in the open sea have descended 

 from primitive fringe-finned Ganoids which lived in shallow 

 inland waters, and by the development of lungs had almost be- 

 come adapted to an air-breathing mode of life ; so in Amphibia 

 after the new type had been evolved by a more complete 

 adaptation to terrestrial conditions, not only in the respiratory 

 organs but in the limbs, some of the descendants of this type 

 have returned to the aquatic mode of life and breathe by gills 

 in the adult state. The final results of such a reversal of the 

 course of evolution are however never closely similar to the 

 types from which that course started : the marine bony fishes 

 (Teleosteans) which have a closed air-bladder, or have lost that 

 organ altogether, are not similar in structure to the cartilaginous 

 sharks (Elasmobranchs) from which we have reason to think 

 the bony fishes were originally derived, and the Amphibia 

 which live permanently in water are not fishes although the first 

 Amphibia were evolved from fishes ; the direction of evolution 

 may be reversed, but the exact steps are never retraced, and 

 the end of the journey is never the same as the starting-point. 

 In number of existing species the Amphibia are the poorest 

 of the classes of Vertebrata : the total number is about iooo, 

 of which no less than 900 belong to the Anura or tail-less 

 forms. Of Urodela there are about 100 species and of Apoda 

 about forty. The latter group resemble in their paucity and 

 distribution the lung-fishes among the fishes, and like them 

 have retained more than the other groups some of the primitive 

 characters of the ancestral forms of the Palaeozoic period. In 

 number of species fishes are about eight times as numerous, and 

 reptiles three and a half times, as the Amphibia. Even in the 

 earliest period of their history, the only period for which we 

 have more than the scantiest records, the numbers were not 



