174 AMPHIBIA 



aquatic respiratory organs is no evidence of primitive character. 

 The reasons which lead to this conclusion are of three kinds, 

 palaeontological, anatomical, and bionomical, that is those con- 

 nected with the life-histories of certain forms. Palaeontologically 

 there is reason to believe that the Palaeozoic Stegocephalia were 

 the ancestors of all the later Amphibia, and in these the evidence 

 goes to show that gills were present in the young but not in the 

 adult condition. Secondly the peculiarities of the limbs are 

 obviously adapted to terrestrial progression and could not have 

 been evolved except in response to the needs of an animal that 

 walked on the ground, while on the other hand a terrestrial 

 animal could not breathe by gills. In fishes the pelvic girdle 

 and limb are smaller than the pectoral and the pelvic girdle is 

 not attached to the vertebral column ; in Amphibia the hind 

 limbs when well developed are the larger and the pelvic 

 girdle is attached to the vertebral column. Thirdly we know 

 from numerous observations and experiments that the larval 

 stage of Salamandridae and Anura may under unusual con- 

 ditions be prolonged, and the animal may even become sexually 

 mature in the aquatic larval state, as is normally the, case with 

 the axolotl in the lakes of Mexico. It is most probable there- 

 fore that the retention of gills in existing Amphibia is due, not 

 to the persistence of an ancestral condition, but to a retention 

 of the aquatic habits and larval characters in forms descended 

 from ancestors which were entirely gill-less and terrestrial when 

 adult. It must be borne in mind however that even in the 

 Carboniferous period there were Labyrinthodonts, namely the 

 Aistopoda, which had no limbs and which are believed by some 

 authorities to have had external gills throughout life. It has 

 been suggested that the modern Apoda (p. 167) which in struc- 

 ture of skull and the possession of dermal scales most resemble 

 the Labyrinthodonts, are directly descended from these ancient 

 limbless forms. The resemblance of the Aistopoda, however, 

 to other Labyrinthodonts in the skeleton indicates that they are 

 themselves derived from terrestrial forms provided with limbs. 

 Another important point is that some of the Labyrinthodonts 

 such as Archegosaurus, have definitely arranged grooves on the 

 bones of the head which probably contained in life sensory 

 tubes like those of fishes ; this is an indication of strongly 

 aquatic habits but not necessarily of aquatic breathing, for such 



