112 ORIGIN, ETC., OF THE FAUNA. 
southward to the Andean region of South America, and is doubtless a recent 
immigrant. Gadow suggests that on reaching South America it gave rise to the 
endemic neotropical genera referred to the Ranide, but it is possible that these are 
derived from an ancient neotropical stock, for at least one recent genus of the Ranide 
dates back to the Eocene, and the family itself may have a greater antiquity. 
Of the Arcifera, the American Cystignathide are almost exclusively neotropical ; 
there are several endemic South American genera, and of those that range northwards 
into Central America only Syrrhophus, Hylodes, Leptodactylus, and Borborocetes are 
represented on the Mexican plateau by a few species ; none range further north. 
The Hylide are mainly neotropical and arboreal, and except for one or two species 
of Hyla are absent from the Mexican plateau. Only Hyla ranges outside this region 
through North America to temperate Eurasia and southwards through China to 
Burma, then comes a gap, for the Australian Region is rich in species, only one of 
which crosses Wallace’s Line into the Sunda Islands. The presence of Hyla arborea 
in Madeira and the Canaries, the range of H. dolichopsis from Sumatra to the 
Solomons, suggest that these tree-frogs may be carried across the sea, and that their 
wide distribution may be partly due to this; their absence from Africa and India is 
remarkable, but there are other equally curious cases of discontinuous distribution 
in genera of no great antiquity, and it is unnecessary to infer some special connection 
between Australia and South America to explain this one; nor does the distribution 
of the Cystignathide support this view, for the Australian members of the family are 
by no means closely related to the American ones, and their common ancestor must 
date back well into mesozoic times. 
Of the American genera referred to the Bufonide two are strictly neotropical, 
Engystomops ranging from South America to Tehuantepec, and the monotypic 
Rhinophryne occurring on the Atlantic slope from Vera Cruz to Guatemala. The 
third genus, Bufo, is cosmopolitan except for the Australian Region; there are a 
number of species in Central America and southern Mexico, whilst others on the 
Mexican Plateau range into the southern United States. Except for two species in 
Celebes, Bufo is absent from the Australian Region, although there are many species 
in the Malay Peninsula and Archipelago; this is exactly the reverse of what occurs in 
Hyla, and Rana differs from both, as it is well represented in both Indo-Malayasia 
and Austro-Malayasia eastwards to the Caroline Islands. 
Scaphiopus, the American representative of the family Pelobatide, is related to the 
European Pelobates; there are about eight species from the United States and the 
Mexican Plateau ; these are diggers in the sand, and do not range southwards beyond 
the mountains of Southern Mexico. 
Dispersal of Batrachians,—The Anura and Urodela are both known to date 
back to the Jurassic, and this, as well as the presence in New Zealand of a Toad 
