PROCEEDINGS OP SECTION D. 277 



become altered very considerably, the parasites have probably altered 

 only a little, but still have altered ; so that we find the old types of 

 Pneumonoeces, that affected to live in the lungs, represented by a dozen 

 or so species scattered over various parts of the earth. And so on with 

 the others, e.g., Gorgoderinae, Brachycoelinse, &c. 



It is a remarkable fact that, of the six species of flukes known 

 to-day as parasites of frogs inhabiting Southern Asia, four of them 

 appear to find their nearest relatives in flukes from Australian frogs. 

 Mescoelium sociale Liihe is certainly more closely related to the Aus- 

 tralian species of Mesocoelium than to Brachycoelium crassicolle R. its 

 European, or B. hospitale Staff, its American representative. Pneu- 

 monoeces capyristes Klein is more nearly related to P. australis than to 

 fi,ny of the European or American species of this genus, while the 

 Asiatic Pleurogenes gastroporus and sphericus ; and the Australian 

 P. freycineti and solus are more nearly related to one another than any 

 of them are to the European or American pleurogenetines. 



I think the Asiatic frog flukes, so far as they are known, are more 

 nearly related to the European than are the Australian, standing as 

 it were intermediate between the two latter groups. The American 

 frog flukes, on the other hand, many of which have evolved into distinct 

 genera, are not so nearly related in their structure to the European as 

 are the Asiatic. And, in addition to this, the American genera, generally 

 speaking, contain more species than the same genera in Asia and 

 Australia, and this may be taken to indicate that the American frogs, 

 with their flukes, have beeji longer separated from the parent stock. 



The great similarity of the four groups of flukes of frogs found in 

 the four regions mentioned points to the fact, I think, that the flukes, 

 which are a very old group of animals, existed in the ancestors of 

 present-day frogs a very long time ago, when their distribution 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 Palaearctic region, and migrated 

 both westwards and south-eastwards. They may have reached the 

 western portion of the Boreal land mass existing right across from Asia 

 to North America in Tertiary times, or they may have made their way 

 westward in Pliocene times, when a considerable migration of verte- 

 brates westward is known to have taken place. 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 Cretaceous 

 or Eocene times. Since 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 American forms, 

 the America-wards migration probably took place in the earlier of 

 the two periods suggested. 



