CHAPTER II 



EVOLUTION AND GEOLOGICAL HISTORY 



The earliest Amphibia, the extinct Labyrinthodonts. Absence of transitional 

 forms in the Secondary Formations. Number of Amphibia in comparison with 

 other classes. 



IN considering living Amphibia only, the forms which be- 

 long to the fauna of the present period of the earth's 

 history, we naturally come to the conclusion that the 

 aquatic larval stage with its gills and gill-clefts retains the char- 

 acters of the fish-like ancestor, and that the metamorphosis 

 repeats with little alteration the original changes by which the 

 fish became adapted to a terrestrial and air-breathing mode of life. 

 In accordance with this view zoologists formerly regarded those 

 Amphibia which retain the organs of aquatic respiration most 

 completely in the adult condition as the most primitive, and 

 least modified from the condition of the fish-like ancestors. 

 Thus the Proteidae and Sirenidae as well as the Amphiumidas 

 would be the least modified Amphibia showing most perfectly 

 the ancestral characters, while the Anura and the Apoda would 

 be the most recent modifications and specialisations. Before 

 accepting such views, however, we must inquire whether they 

 are confirmed or contradicted by the evidence of palaeontology 

 and ascertain whether the Amphibia which existed in earlier 

 geological periods really resembled the perennibranchiate forms 

 of the present day ; we must consider what has been the geologi- 

 cal history of the Class. 



The earliest known terrestrial vertebrates provided with 

 two pairs of five-toed, obviously terrestrial limbs (p. 158), are 

 the animals known from the structure of the teeth in some of 

 the most typical forms as Labyrinthodonts, from the bony 

 covering of the head as Stegocephali. In studying the 

 remains of the skeletons of these extinct vertebrates the 



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