198 Transactions of the Royal Society of South Africa. 
the European frog-flukes, the Asiatic, as it were, standing midway between 
the Australian and European. The American frog-flukes, 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, Australia, and South Africa, and this may be taken to indicate that 
the American frogs, with their flukes, have been longer separated from the 
parent-stock. 
The great similarity of the five groups of flukes from frogs found in the 
five regions mentioned, points to the fact that the flukes are a very old 
group of animals, and 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 westwards, southwards, and south-west- 
wards. 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 westwards in Pliocene times, 
when a considerable migration of vertebrates westwards is known to have 
taken place. The Australian forms must have found their way there 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 South African forms must 
have found their way down here during late Pliocene 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, Australian, and South African forms, so that the America-wards 
migration probably took place in the earlier of the two periods mentioned. 
In view of the probable land connection between Australia and South 
America through the Antarctic, a connection which is supported by a good 
deal of biological evidence, it is unfortunate that practically nothing seems 
to be known about the frog-trematodes of South America. There are only 
two indirect references to such trematodes, viz. in Braun (2, p. 906) and 
Klein. A pretty close similarity has, however, been shown by Zschokke 
to exist between some cestode-parasites of South America and the Australian 
Marsupials. 
The close similarity existing between the respective representatives in 
the five 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 own present-day groups, at the time of 
