CHAPTER VI 

 CLASS III. AMPHIBIA 

 Present and Past Status 



The question of the origin of the Amphibia involves the whole prob- 

 lem of the beginnings of land life among the vertebrates and the radi- 

 cal evolutionary changes that have occurred as adaptations for an en- 

 tirely new mode of Ufe. While the aquatic habitat may be said to be 

 a comparatively uniform and constant one, only slightly influenced 

 by seasonal changes, the terrestrial life, especially in temperate re- 

 gions, involves a wide range of changing conditions. 



It has already been noted that the fishes had shown marked tend- 

 encies to adopt various methods of invading the air-breathing realm, 

 some for the sake of avoiding the respiration of too much CO2 and 

 other poisonous gases in stagnant waters, some to tide over periods 

 of drought, and still others for the purpose of enabUng them to climb 

 out of the water for food (the climbing-perch, etc.). It must have 

 been in association with conditions resembling these that the first true 

 land vertebrates were evolved. 



The Amphibia are undoubtedly the most primitive land verte- 

 brates, but it is coming to be believed that the first Reptilia trod 

 closely upon their heels. The Reptiha were much more truly land 

 vertebrates than were the Amphibia, for the Amphibia are tied down 

 to the aquatic medium during at least the developmental period, in 

 most groups, and during the entire life cycle, in others. Fundamen- 

 tally the Amphibia are aquatic because their developmental processes 

 are aquatic. Only a few of the most highly speciahzed modern Am- 

 phibia lay their eggs out of water, and these have adopted various 

 unique brooding habits, which are at best mere developmental make- 

 shifts as compared with the methods employed by the reptiles with 

 their amnion and allantois. 



The Amphibia have never attained the heights of success and of 

 dominance in nature that has been attained by fishes, reptiles, birds, 

 or mammals. Possibly this lack of complete success has been the re- 



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