THE OPALINID CILIATE IXFUSORIANS. 381 



evolution and of zoogeography. Anyone interested in this method 

 should surely consult Johnston's fine papers. 



Darling (1920 and 1921) has studied the migrations of human races 

 from data of hookworm distribution, drawing some definite conclu- 

 sions from these data. 



These six publications mentioned illustrate the application of the 

 host-parasite method of study to questions of geographical distribu- 

 tion and phylogeny. The same method can be applied with illuminat- 

 ing results to many other problems. Ancient land connections and 

 separations and the dating of the establishment and loss of such 

 connections; ancient climates; the existence and the dates of former 

 deserts, swamps, forests, mountain ranges, streams, and lakes; the 

 altitude of former land areas; the presence or absence of certain 

 food plants in former times in certain areas ; genetic relationship as 

 against parallel or convergent evolution ; place and date of origin of 

 families, subfamilies, genera, subgenera and species of both hosts 

 and parasites, and many other questions, may successfully be ap- 

 proached by this method. Tables of the occurrence of parasites and 

 their hosts, similar to the table given in this paper for the Anura and 

 their Opalinidae, might to advantage be compiled for every group 

 of animals and plants, and when prepared would be a mine of in- 

 formation from which to reconstruct Cenozoic geography including 

 faunas and floras. Much light would also be cast upon Mesozoic 

 conditions, and probably much would be learned of Paleozoic con- 

 ditions by this method. 



An illustration of the dating of the origin of a family of animals 

 through the application of the host-parasite method ma}^ be given. 

 Gadow (1909) wrote of the "southern frogs," the Leptodactylidae : 

 " Their geographical distribution is suggestive of their being an old 

 family. * * * The overwhelming majority inhabit the Neotropi- 

 cal region, a few forms extending into tropical Central America and 

 into the Antilles; the rest, some 20 species only, are confined to the 

 continent of Australia and to Tasmania." *^ But the evidence w^e have 

 adduced seems to indicate that the Leptodactylidae (fig. 228) are 

 comparatively modern, having been evolved since Australia separated 

 from Asia, but before Australia and Patagonia separated from Ant- 

 arctica. That is, they evolved between the early Cretaceous and the 

 beginning of the Pliocene, probably during the midde Tertiary. 



The zoogeographical data from the Anura and their Opalinids 

 should be correlated with other zoological and geographical (includ- 

 ing geologic) data, but this broader discussion may well be in a more 

 general, separate paper, rather than in this volume, which is essen- 

 tially a study of the Opalinidae. I am, however, introducing here a 



*2 One species is now known from Papua and the New Zealand genus Liopelma is now 

 assigned to the Leptodactylidae. 



