iir GEOGRAPHICAL DISTRIBUTION 73 



terranean waters of Carniola, where the whole country is nothing 

 l)ut limestone. 



Cold is another powerful limiting factor. The absolute 

 northern limit of Amphibian life coincides rather closely with 

 the somewhat erratic line of .0° Centigrade of annual mean 

 temperature, a little to the north of which line the ground 

 remains permanently frozen below the surface. The surface- 

 crust, which thaws during the summer, engenders an abundance 

 oi insects as food-supply, but its freezing down to the icy bottom 

 makes hibernation impossible. There are, of course, some ex- 

 ceptions, for instance the occurrence of Urodela in the Schilka 

 river and in the district of Lake Baikal. 



Eanges of mountains are far less effective barriers than is 

 generally supposed. In many cases the fauna is the same on either 

 slope, and they act rather as equalising or dispersing factors, 

 especially wlien they extend from north to south. Witness the 

 ^Vndes, owing to which Ecuador and Peru bear a great resem- 

 lilance to the Central American fauna, and differ from the 

 tropical parts of South America. The existence of an Amhly- 

 sforiia in Siam is another instance. 



The more specialised a family the more intimately is it con- 

 nected with the physical features of the country. Typically 

 arljoreal frogs are dependent on the presence of trees. Some have 

 midoubtedly spread into treeless countries and have changed 

 into prairie-frogs, e.g. Acris. They come out, so to speak, as 

 something different at the other end, and it is inilikely that 

 these modified descendants redevelop exactly the same features as 

 tlieir ancestors before the migration. Baldwin Spencer ^ met 

 with only six species of frogs in Central Australia, Liitmody- 

 nnstes, Chiroleptes, Hclcioporus, and Hyla. They are in the main 

 identical with certain forms found in the dry inland parts of 

 New South Wales and Queensland. They are to be regarded as 

 innnigrants from the latter regions, which have been able in the 

 majority of cases to adapt themselves to unfavourable climatic 

 conditions by means of a marked development of the burrowing 

 liabit, to which in certain cases has been added a capacity for 

 absorbing and holding w^ater. 



^ llic Horn Scientific Expedition; 1897, p. 155. 



