548 
GEOGRAPHICAL ZOOLOGY. 
[part IV. 
means of crossing the ocean, as they are not unfrequently 
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 Reptiles 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 Rakearctic 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. Remembering 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. 
