BY S. J. JOHNSTON. 351 



tors of present-day frogs a very long time ago, when their dis- 

 tribution was much less extensive than it is to-day. The mutual 

 relationships of these groups of trematodes support the view that 

 the Anura originated somewhere about the centre of the Palte- 

 arctic region, and migrated both westwards and south-westwards. 

 They may have reached the western portion of the Boreal land- 

 mass, existing right across from Asia to North America, in early 

 Tertiary times, or they may have made their way westward in 

 Pliocene times when a considerable migration of vertebrates west- 

 wards is known to have taken plaee(34). The Australian forms 

 must have found their way down here before the separation of 

 the Australian continent from south-eastern Asia, a separation 

 which is generally supposed to have taken place somewhere about 

 late Cretaceous or Eocene times. The greater diversity of the 

 North American frog-trematodes would seem to indicate that 

 they have been longer separated from the parent-stock than the 

 Asiatic and .Australian forms, so that the America- wards migra- 

 tion probably took place in the earlier of the two periods 

 suggested. 



In view of the probable land-connection between Avistralia and 

 South America through the Antarctic, a connexion which is sup- 

 ported by a good deal of biological evidence, it is unfortunate 

 that practically nothing seems to be known about the frog- 

 trematodes of South America. I can find only two indirect 

 references to any such trematodes, viz., in Braun(4, p. 906), and 

 Klein(38, p. 68). A pretty close similarity has, however, been 

 shown by Zschokke(106) to exist between some cestode-parasites 

 of South American and Australian marsupials. 



The close similarity existing between the respective represen- 

 tatives in the four groups of frog-trematodes in question here 

 reminds us that the trematodes, owing to the conditions under 

 which their lives are passed, have probably evolved much more 

 slowly than their hosts, for the amphibian ancestors of our 

 present-day groups, at the time of their dispersal, must have been 

 considerably dilTerent from their representatives now living. 



In conclusion, I wish to thank Professor Haswell, in whose 

 laboratories this work has been carried out, for much valuable 



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