PARASITES AND THE AID THEY GIVE IN PROBLEMS 



OF TAXONOMY, GEOGRAPHICAL DISTRIBUTION, 



AND PALEOGEOGRAPHY 



By MAYNARD M. METCALF 

 The Johns Hopkins University 



A new method of approach to problems of wide interest is a most 

 welcome thing if it gives assistance in reaching reliable conclusions. 

 The concomitant study of parasites and their hosts is proving of im- 

 portance in the revealing light it is throwing .upon questions of genetic 

 relationships among organisms, upon problems* of their places and 

 times of origin, of their routes of dispersal, and thus also upon prob- 

 lems of former geographical conditions, including not only land con- 

 nections for the dispersal of land animals and plants but also inter-sea 

 connections for the wandering of marine organisms. Host-parasite 

 data cast light also upon problems of former climates, whether warm 

 or cold, wet or arid ; and they assist in questions of the whole character 

 of the habitat of former animals and plants. The method is of such 

 crucial import that it seems well to review briefly such use as has thus 

 far been made of it, and especially to point out possible extensions 

 in its use. 



In only a few instances have host-parasite data been used in con- 

 nection with these broad taxonomic, geographic and paleogeographic 

 problems, and an historical review of the use of the method can be 

 brief. On the other hand, it is no simple task to illustrate adequately 

 the method and to show at all fully the extent of its application, for 

 it enters into and illuminates some very complex problems. In these 

 complex problems host-parasite evidence must be interwoven with 

 data from taxonomy, paleontology, geology, geography, biogeography 

 and paleogeography. Like most classes of data of major significance it 

 has wide interconnections. 



A single instance of the use of host-parasite data may well be given 

 in introduction, to make the subject more concrete. Frogs of the 

 genus Rana are wanting in South America and in Australia, except 

 for R. palmipes along the northern coasts of the former continent 

 and R. papua'' at the northern tip of Australia, both of these being 



* Harrison (1928) refers to four Anura, other than leptodactylids and 

 hylids, as recent immigrants into North Queensland. I have not found first- 

 hand reference to these forms. 



Smithsonian Miscellaneous Collections, Vol. 81. No. 8 



