EVOLUTION AND GEOLOGICAL HISTORY 177 



great in comparison with those of fishes. Very soon after 

 their origin they gave rise to the reptiles which proved them- 

 selves more capable of multiplying and possessing the earth. 

 In order to become really abundant any type of animal, 

 whatever the general adaptations or peculiarities of structure 

 which distinguish it, must be able by minor adaptations to ac- 

 comodate itself to the different conditions of the various 

 regions and localities of the world. The sea, extending over 

 three-fourths of the surface of the globe, affords vast scope for 

 the multiplication of those animals which are able to live in it. 

 Fishes therefore possess an immense space from which the Am- 

 phibia are entirely excluded, for these animals in the course of 

 their evolution have never acquired the power of tolerating salt 

 water. By their larval aquatic life the majority of Amphibia are 

 restricted to the neighbourhood of stagnant fresh waters. 

 Reptiles on the other hand, by the development of a firm egg- 

 shell and the embryonic adaptations connected with it, 

 became independent of water and were able to populate the 

 dry places of the earth, while some of them became adapted to 

 arboreal life and some even adopted marine habits. Among 

 the Amphibia only the Anura have shown any great plasticity of 

 organisation, that is to say any capacity for varied adaptations, 

 especially for arboreal life, and this with their great fecundity 

 explains the fact that they are the dominant group among the 

 Amphibia. The class as a whole is to be regarded as merely 

 the survival of the transitional form by which the reptiles 

 were evolved from the fishes, and for this reason is of the 

 greatest interest to the zoologist. There can be little doubt 

 that in many cases such transitional forms have become 

 entirely extinct and not only so but have left scarcely any 

 trace of their existence in the record of the rocks : with the 

 exception of Archtzopteryx, for example, there are neither sur- 

 vivors nor fossils to show us the intermediate stages between 

 the reptile and the bird. 



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