Evolution 



inefficient for respiration as soon as they become dry. An entirely 

 new type of organ had therefore to be evolved, and this occurred 

 on the same lines as in the Dipnoi, by the development of a pair 

 of sacs from the upper part of the digestive canal, in which the 

 blood is made to circulate, and which are kept filled with air 

 taken direct from the atmosphere. It is of course very well 

 known that an ordinary amphibian is not a lung breather through- 

 out its whole life. The metamorphosis of a gill-breathing tadpole 

 into a lung-breathing frog, illustrated in Fig. 85, is a pheno- 

 menon with which everyone is familiar. And this condition, in 

 which a change in the mode of life is made by each individual 

 in the course of its development, is the typical one. But the 

 modern amphibians include types ranging from completely water 

 to perfect land forms. Some, like the Austrian 01m (Fig. 86), are 

 gill breathers throughout their whole life. One which is normally 

 of this type, the Axolotyl, illustrated in Fig. Sy, can be made 

 to acquire lungs and assume a land mode of life. Others, which 

 normally make the metamorphosis, can be prevented from doing 

 so by being confined to the water, and complete their life-histories 

 in the condition of gill breathers. In still other forms {e.g. the 

 Coecilians, Fig. 89) the change is made before the young creature 

 leaves the egg, and the independent life is commenced in the 

 condition of a land animal. 



Correlated with the development of the lungs is a change in 

 the structure of the nostrils, from the condition of blind sacs, as 

 they occur in the fishes, to that of air passages, communicating 

 with the upper part of the alimentary canal, and thence with the 

 lungs. 



The living forms of the Amphibia differ considerably from the 



types which constituted the group in those long-distant ages when 



it was in the heyday of its prosperity. The latter forms were 



characterised especially by a system of armour -plating over the 



head, which frequently extended under the breast and even covered 



the greater part of the lower surface, and which appears to have 



formed a protection against the multitude of sharks which 



populated the waters in which the amphibians partly lived. The 



ic8 



