380 BULLETIN 120, UNITED STATES NATIONAL MUSEUM. 



fore trust to the full the data as to former land connections which the 

 Anura and their Opalinidae give us. 



Similar twofold or multifold evidence from other groups is a great 

 desideratum. It is easy to reinforce the evidence from Anura and 

 their parasites by extending the study to other parasites than the 

 Opalinidae. Other Ciliates, Balantidium and Nyctotherus, are quite 

 abundant in the Anura, so also are Nematodes and Trematodes. 

 Many Flagellates are found, as also are Sporozoa and intestinal 

 Amoebae, though the latter are not well preserved in ordinary mu- 

 seum specimens of Anura. Discophrya is another Anuran parasite. 

 A comprehensive review of the parasites of the Anura would defi- 

 nitely settle some questions of former continental connections, and 

 probably all such questions of Cenozoic and Mesozoic land connec- 

 tions will be settled, perhaps, beyond peradventure, when we have 

 similar comprehensive studies of numerous groups of terrestrial 

 animals and their parasites. The data recorded in this paper are but 

 a beginning, but they illustrate a method of approach to these ques- 

 tions which is of supreme value in their solution. I am not delaying 

 publication of the data from the Opalinidae until I can study the. 

 other groups of parasites in my material. I hope to be able to turn 

 this material over to special students of these other groups. 



The parallel study of hosts and their parasites with reference to 

 geographical distribution and other general problems has been but 

 little used. Von Ihering (1900) in discussing the former separation 

 of South America into distinct northern and southern portions, 

 writes : " Archiplata [the portion of America originally south of the 

 trans-South American sea] contains numerous genera of Mollusca, 

 Crustacea, etc., that are common to Chile and the La Plata district, 

 such as Unio, Chilina, Parastacus, Aeglea, etc., including many spe- 

 cies and even their parasites '^^ (Temnocephala), which are identical 

 on both sides of the Andes." Also Kellogg (1905), discussing bird 

 lice, writes : " From this fact of near relationship of hosts in all the 

 cases of parasite species common to several host species it seems 

 almost certain that this common occurrence under circumstances not 

 admitting of migration of parasites from host to host, is due to the 

 persistence of the parasite species unchanged from the time of the 

 common ancestor of the two or more now distinct but closely allied 

 bird species." 



Johnston (1912 and especially 1914)" makes extensive use of the 

 host-parasite method of approach to several important problems of 



" Italics mine. 



*' These papers were brought to my attention by Dr. Cert after this volume was ready 

 for the press. I regret that in a former paper (Metcalf, 1920, b) I did not refer to 

 Johnston's remarkably interesting use of the host-parasite method of attacli upon prob- 

 lems of genetic relationships and paleogeography. 



