MATTHEW, CLIMATE AND EVOLUTION 295 



origin of these discontinuous distributions they are interpreted l)y many 

 authors as affording evidence for various transoceanic bridges. But they 

 are not essentially different from various instances of discontinuous dis- 

 tribution among Mammalia, except that they are probably in some cases 

 of more ancient origin, and are less restricted by ocean barriers. 



The urodele Amphibia are Holarctic, save for one family, Plethodon- 

 tidffi, which has spread into northern South America and has also reached 

 Hayti. Although thus limited in dispersal, they would seem to be an 

 ancient group represented as far back as the Wealden by Hylceobatrachus, 

 said to be related to the modern Cryptohranclius}'^^ Their distribution 

 within Holarctica is more or less of a relict type, broken up by the mifa- 

 vorable environment of so large a part of this region, especially of the 

 central portion. The csecilians are tropical but have not reached Aus- 

 tralasia. 



The frogs and toads have a wide dispersal, and so far as a superficial 

 view may show, the most primitive or archaic families are limited to the 

 peripheral continents and oceanic islands, while the more progressive 

 groups are more cosmopolitan, but have not yet reached all of the outly- 

 ing regions. Some of the families, at least, would appear to be of ancient 

 origin; Palceohatrachus, allied according to Gadow^^'^ to the Aglossa of 

 the Ethiopian and Neotropical regions, is recorded from the Jurassic of 

 Spain, and is said to be common in the older Tertiary of Europe. Among 

 the modern families the Cystignathidse are chiefly Australasian and Neo- 

 tropical, but a few are still found in North America. This distribution 

 parallels that of the polyprotodont marsupials, except that the latter have 

 not reached New Zealand or the Antilles, or entirely disappeared from 

 the East Indian islands. The Discoglossidge inhabit the East Indies and 

 North America but have disappeared from the intervening portion of 

 Holarctica ; Discoglossus and other genera are found in the Middle Ter- 

 tiary of Germany. The Pelobatidee stretch across Europe and Asia and 

 northwestern North America. These three families represent evidently 

 three successive dispersals. 



The other families are more cosmopolitan. The genus Bufo has failed 

 to reach Australasia, Madagascar or New Zealand, but is replaced in 

 Australia by a (more primitive?) member of the family. The Hylidffi 

 are to-day chiefly South American and Australian, but a few members 

 still inhabit North America, They are not found in Africa or the Orien- 

 tal region, where it seems reasonable to suppose that they have been dis- 

 placed by the true frogs (Ranidae), peculiarly varied and abundant in 



1^8 F. Broili, In Zittel's Grundziige der Palseont., Yertebrata, s. 176. 1911. 

 ""H. Gadow : I.e., p. 145. 1901. 



