204 ORIGIN OF LIFE IN AMERICA 



Arizona, while a third, the rubber boa or silver snake actually 

 passes into Washington State and central Nevada. 



The recent increase of dryness in the south-west has no 

 doubt affected the original fauna considerably. Semi-aquatic 

 forms and aquatic ones have either been destroyed or forced 

 to take refuge in the neighbouring States where climatic con- 

 ditions were more favourable. To this cause may be attri- 

 buted the present scarcity of amphibians in most of the south- 

 western districts, where many of them, we may presume, 

 originally had their headquarters. The only American mem- 

 ber of certain toads, which on account of the peculiar shape 

 of their tongues have been called Discoglossidae, occurs in 

 Washington State in western North America. This family 

 has always been looked upon with particular interest, because 

 to it belongs the solitary amphibian known from New Zealand. 

 This toad (Liopelma) must have reached New Zealand, ac- 

 cording to Dr. Stejneger,* before Cretaceous times by means 

 of a very ancient land connection with the north. The same 

 author expresses the opinion that the Himalayan Mountains, 

 or rather the region to the south-west of them, was the ori- 

 ginal home of these discoglossoid toads, and that they spread 

 from there to New Zealand and North America. On the latter 

 continent we still find, as I remarked, a single genus of the 

 Discoglossidae (Ascaphus). The present centre of dispersal 

 of these discoglossoid toads is southern Europe, since three 

 genera are found there, viz., Discoglossus, Alytes and Bom- 

 binator. The first two are strictly European, whereas a single 

 species of Bombinator also inhabits northern China and Korea. 

 Considering the fact that these toads do not occur in south- 

 western Asia, and that both Discoglossus and Bombinator 

 have been found in European Miocene beds, the Mediter- 

 ranean Eegion seems more likely to have been the original 

 centre of dispersal than south-western Asia. At any rate, 

 that event leads us, no doubt, to the dim and distant past of 

 the early part of the Mesozoic Era. 



The allied family Pelobatidae is likewise of great faunistic 

 interest, as the two genera Scaphiopus of North America and 

 Pelobates of Europe are only distinguished by slight differ- 



* Stejneger, L., "Distribution of Discoglossoid Toads," pp. 91 93. 



