NO. 8 PARASITES METCALF 5 



Australia and New Zealand, and also by way of the Isthmus of Suez 

 to Africa. Here again the parasites of the southern frogs furnish 

 evidence to be used in connection with other considerations. 



No southern frogs are known, either recent or fossil, from Euro- 

 Asia, Malaysia or North America, except two species from the Texas 

 coast. The opalinid ZcUcriclla is found in the southern frogs of 

 South and Central America and in Tasmania and Australia and it 

 probably will be found in Papuan representatives of this anuran 

 family. If the southern frogs were ever in Arctogea with their 

 ZellericUa parasites, both have completely disappeared. Why has not 

 Zelleriella, at least, remained even if the frogs are gone? If Zclleriella 

 ever was in southern frogs in Arctogea it should still be in some of 

 the other Anura of these lands. Other families of Anura in South and 

 Central America have adopted ZcUcriclla, for example the toads 

 (Bufo), the tree frogs (Hylidae), the Dendrobatinae, the spade-foot 

 toads, the Gastrophynidae, and even the ranids (PhyUobates, Pros- 

 therapis, and one Calif ornian Rami). The absence of Zclleriella from 

 Arctogea thus emphasizes the absence of the southern frogs as indi- 

 cating that neither ever were at home in the North. 



There remains the j^ossible hypothesis that the southern frogs were 

 once in the North but that their parasitic Zelleriellas did not evolve 

 until their hosts had spread to the Southern Hemisphere. But this 

 wholly gratuitous hypothesis does not help us, for we would still have 

 to account for the discontinuous southern dispersal of the Zcllcncllae 

 as due either to parallel evolution in South America and in Aus- 

 tralasia, a wholly improbable conception, or to a southern land 

 connection between these two now separated regions. The hypothesis 

 of northern origin and southward dispersal of the southern frogs 

 becomes grotesque in view of the evidence furnished by Zclleriella, 

 and we shall see later that the evidence from the southern frogs and 

 Zelleriella is reinforced by that from the southern crayfishes and their 

 imrasites as well as by much other evidence. 



In our illustration we have seen host-parasite data used to indicate : 

 (i) Genetic relationship between hosts; (2) places of origin and 

 routes of dispersal of both hosts and parasites; (3) ancient land 

 connections between now distinct and widely separated land masses. 



Taken in connection with generally accepted paleogeographical con- 

 ceptions, similar host-parasite data can be used to indicate times as 

 well as places of origin of host groups and parasite groups. For 

 example both the southern frogs and their Zelleriella parasites are 

 far more abundant and are more diversified in America than in 



