648 GEOGRAPHICAL ZOOLOGY. [PART iv. 



means of crossing the ocean, as they are not ^infrequently 

 found in remote oceanic islands. These various causes have 

 modified their distribution. The Western Hemisphere is much 

 richer in lizards than it is in snakes ; and it is also very distinct 

 from the Eastern Hemisphere. The lines of migration of lizards 

 appear to have been along the mountains and deserts of tropical 

 countries, and, under special conditions, across tropical seas from 

 island to island. 



Crocodiles are a declining group. They were once more gene- 

 rally distributed, all the three families being found in British 

 Eocene deposits. Being aquatic and capable of living in the 

 sea, they can readily pass along all the coasts and islands of the 

 warmer parts of the globe. Tortoises are equally ancient, and 

 the restriction of certain groups to definite areas seems to be also 

 a recent phenomenon. 



Amphibia. 



The Amphibia differ widely from Eeptiles in their power of. 

 enduring cold; one of their chief divisions, the Urodela or 

 Tailed-Batrachia, being confined to the temperate parts of the 

 Northern Hemisphere. To this class of animals the northern and 

 southern routes of migration were open; and we accordingly find . 

 a considerable amount of resemblance between South America and 

 Australia, and a still stronger affinity between North America 

 and the Palsearctic continent. The other tropical regions are 

 more distinct from each other; clearly indicating that, in this 

 group, it is tropical deserts and tropical oceans which are the 

 barriers to migration. The class however is very fragmentary, 

 and probably very ancient ; so that descendants of once wide- 

 spread types are now found isolated in various parts of the 

 globe, between which we may feel sure there has been no direct 

 transmission of Batrachia. Eemembering that their chief lines 

 of migration have been by northern and southern land-routes, 

 by floating ice, by fresh- water channels, and perhaps at rare 

 intervals by ova being carried by aquatic birds or by violent 

 storms, we shall be able to comprehend most of the features 

 of their actual distribution. 



