Limiting factors of distribution.–Common salt is poison to the Amphibia; even a solution of 1 per cent prevents the development of their larvae. Consequently seas, salt lakes, and plains encrusted with saline deposits act as most efficient boundaries to normal "spreading." But undoubtedly many individuals have made long and successful voyages across the seas on floating trees. Solutions of lime are likewise detrimental to many species, and it is a general fact that limestone-terrain is poor in Amphibian life, unless, of course, sufficient accumulation of humus counteracts or prevents the calcareous impregnation of the springs and pools in meadows. Salamandra maculosa is, for instance, absent in Central Germany on the Muschelkalk, but it occurs in abundance in neighbouring districts of red sandstone or granite; nor can the larvae be reared successfully in very "hard" water. On the other hand, Proteus lives in the subterranean waters of Carniola, where the whole country is nothing but 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 of insects as food-supply, but its freezing down to the icy bottom makes hibernation impossible. There are, of course, some exceptions, for instance the occurrence of Urodela in the Schilka river and in the district of Lake Baikal.
Ranges 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. Witness the Andes, owing to which Ecuador and Peru bear a great resemblance to the Central American fauna, and differ from the tropical parts of South America. The existence of an Amblystoma in Siam is another instance.
The more specialised a family the more intimately is it connected 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. Acris. 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 before the migration. Baldwin Spencer[[39]] met with only six species of frogs in Central Australia, Limnodynastes, Chiroleptes, Heleioporus, 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 immigrants 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.
Faunistic divisions of the Amphibia.
NOTOGÆA.–South World.
* indicates Amphibia which are peculiar to the respective regions or sub-regions.
Characterised by the Cystignathidae* and by the predominance of Arcifera, which form nearly 90 per cent of the Anurous population.
I. Australian region.–Absence of Apoda and Urodela. All the Anura are arciferous, with the exception of one species of Rana in the Cape York peninsula. The fauna of the Australian continent and of Tasmania consists chiefly of Cystignathidae and Hylidae (Hyla and Hylella) and several small genera of Bufonidae (Pseudophryne,* Notaden,* and Myobatrachus*).
It is customary, and from the study of other Vertebrata quite justifiable, to divide the Australian region into several sub-regions, but the Amphibia lend no support to this. The only Amphibian in the Sandwich Islands is a Bufo, closely related to North American species. The only Amphibian in New Zealand is Liopelma,* one of the Discoglossidae which are otherwise confined to Europe, North-east Asia, and North-west America, and, to judge from their low organisation, had formerly a much wider distribution. New Caledonia possesses no Amphibia. The Fiji Islands are inhabited by one or two species of Cornufer, a genus of Ranidae. The same genus is typical of the Austro-Malayan and Papuasian islands, the fauna of which consists of Rana and Cornufer, Ceratobatrachus, several genera of Engystomatinae, Hylidae, and Pelobatidae.