Ill 



l.i:0(.KArilR AL DlSTKlliUTlON 73 



terranean waters of Carniola, where the whole country is nothing 

 but limestone. 



Cold is another powerful limiting factor. I'he absolute 

 northern limit of Amphibian lift' coincides rather closely with 

 the somewhat erratic line oi' 0° (.'entigrade 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 

 of insects as food-supply, Init 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. 



Kanoes 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 when they extend from north to south. AVitness the 

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

 blance to the Central American fauna, and differ from the 

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

 stoma in Siani is another instance. 



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

 nected with the physical features of the country. Typically 

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

 undoubtedly spread into treeless countries and have changed 

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

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

 these modified descendants redevelop exactly the same features as 

 their ancestors Ijefore the migration. Baldwin Spencer ^ met 

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

 nnstes, Ohirohptes, Heleiojhirua, and Ifyhi. They are in tlie main 

 identical with certain forms iound in the dry inland parts of 

 New South Wales and (.,)ueensland. They are to he 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 

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

 absorbing and holding water. 



' The Horn Scientijic Jixpcdit ion, LS97, i>. 1 .">.'.. 



