NO. 8 PARASITES METCALF 3 



of more recently developed and more efficient relatives in the north/ 

 He rejects all idea of east and west land connections in the Southern 

 Hemisphere between the continents. 



Let us apply evidence from host-parasite data to the two concepts 

 (i) of parallel evolution as explaining discontinuous distribution 

 and (2) of onlv northern centers of origin and radiation for all 

 Tertiary and Mesozoic forms, and let us in our illustration confine 

 ourselves to the Leptodactylidae. and some of their parasites. 



In the recta of Australian and American southern frogs occurs a 

 characteristic ciliate protozoan, Zellcriella, one of the Opalinidae, and 

 some of these Australian and South American ciliates are almost if 

 not quite specifically identical. This genus of ciliates is absent from 



^ Matthew postulates a northern source of origin for each group, which is 

 like an ebullient spring with wave after wave overflowing, each successive 

 wave pushing the previous wave outward in all directions in which conditions 

 allow dispersal. Like the " age and area " hypothesis of Willis, this is too 

 geometrical and too little biological. The recent tendency to attempt to express 

 all biological conditions in geometrical figures and in formulae seems likely 

 to prove but transient. To each of these hypotheses there is both theoretical 

 and factual objection. It would indeed be strange if, as Matthew thinks, the 

 animals [and plants?] dispersing in radiating streams should all leave behind 

 them their ability themselves to become centers, springs, of further evolution 

 on a large scale. There seems no theoretical ground of any" sort for this cor- 

 ollary implied in Matthews' hypothesis. But, and this is more important, the 

 facts do not seem to agree with Matthews' theory of one center of origin, with 

 newer and newer forms continually appearing here and pushing the more archaic 

 ones to the " ends of the earth." It seems rather to be the most vigorous, most 

 dominant forms which spread to great distances, not the most ancient. It seems 

 that these dominant animals and plants spread by their own vigor rather than 

 that the less vigorous, archaic forms are pushed out to the far places of the 

 earth by the vigorous competition of more dominant species. 



Note the conditions among the Anura. The genus Bufo arose probably in 

 late Cretaceous times in northwestern South America or more probably in 

 southeastern Asia (Metcalf, 1923, 1923a) and it has spread to all temperate 

 and tropical lands except such as have been isolated and inaccessible (Austral- 

 asia, Madagascar). The genus Hyla arose apparently in tropical South America 

 in mid-Tertiary times and its spread to North America was after the middle 

 Pliocene, when the Isthmus of Panama was formed; yet, in the comparatively 

 short time since the mid-Pliocene one species of Hyla has spread throughout 

 eastern and northeastern Asia, over Europe and on into northern Africa. The 

 true frogs, Rana, probably the most modern of the Anura, have spread to all 

 accessible portions of the world except that since they entered South Am.erica 

 in the later Pliocene they have not spread beyond the Amazon river. No, it 

 is not the more archaic forms but the more dominant forms that seem to be the 

 wide wanderers. When a decadent group like the bell toads (Discoglossidae) 

 has representatives in distant lands, it indicates, apparently, that once they were 

 vigorous and have now become decadent (Metcalf 1928, 1928a). 



