THE OPALINID CILIATE INFUSORIANS. 365 



ancestral home of the Hylidae, and the time of their evolution as 

 Cretaceous or later (fig. 234). 



But how did they reach Australia and Papua? They could 

 hardly have taken the Antarctic route without being in Argentina- 

 Patagonia to-day. It loolis as if the land bridge across the southern 

 Pacific Ocean from central South America to Australia, which 

 Arldt and Scharff have suggested, must be called in to account for the 

 the presence of the Hylidae in these two regions, and the map of 

 the late Cretaceous has been drawn to express this view, though there 

 is little evidence for placing the trans-Pacific bridge at this time 

 rather than in the early Tertiary (compare Arldt, my figure 23G, .4). 

 The fact that the Australasian forms have not diverged into other 

 genera than Hyla is a slight argument in favor of the later connec- 

 tion. 



We have already noted that the South American genus Zelleriella 

 is the characteristic Opalinid of the Leptodactylidae, whose origin 

 in the Argentina-Patagonia-Chile region will soon be discussed. 

 The fact that no Australian and only three American Hylids harbor 

 Zelleriella seems at first thought a further indication that Hylids 

 did not pass from America to Australia by way of Patagonia, the 

 early home of Zelleriella; but the force of this suggestion is much 

 lessened when it is noted that since middle Pliocene times Leptodac- 

 tylids with their Zelleriellas have been in contact with Hylids 

 throughout all tropical America, and of course in Australia also. 

 The Hylids are resistent to infection by Zelleriella and doubtless 

 always have been so. In their migration to Australia the Hylids 

 carried archaic Protoopalinas but not Zelleriellas. 



The passage of Hylids from Australia to Papua could have oc- 

 curred in the early Tertiary, according to Arldt's charts (fig. 236, A). 

 Absence of Hylids from New Zealand suggests New Zealand's isola- 

 tion at the time of the Australia-Brazil connection. 



The Hylidae now occupy all tropical South America and Central 

 America, the Greater Antilles, the Bahamas, and the Bermudas, and 

 have spread over all of North America where climatic conditions 

 allow. Since the middle Pliocene, or possibly a little earlier, the 

 Isthmus of Panama was available for their northward migration 

 (Vaughan, 1919), and about the middle of the Pliocene the two 

 ridges, from Yucatan to Cuba and from Honduras to Jamaica, were 

 open to them. At this time the Bahamas were united to the Greater 

 Antilles. The fauna and flora of the Bermudas present difficult 

 problems, and the presence there of a Hyla with a Zelleriella is one 

 of the difficult things to explain. 



Entrance to Central America from northern South America was 

 apparently possible during the early Tertiary by way of the eastern 

 Pacific land strip (fig. 236). Scharff (1911) has presented consider- 



