128 THE ANTARCTIC CONTINENT CHAP. 
has perhaps not been laid upon this matter—the brain among the 
Polyprotodonts is less convoluted than among the genera of the 
other division, This statement is of course made with due regard 
to parallelism in size (see p. 77). It is well known that the 
complexity of a brain bears a distinct relation to the size of its 
possessor within the group. Now the most ancient Marsupials 
are decidedly more Polyprotodont-like. No European form from 
the earlier periods is distinctly to be referred to the Diprotodonts. 
But both divisions now exist in America and Australia. 
We must assume, therefore, one of three hypotheses. Either 
the differentiation into the two great divisions occurred in Jurassic 
or Cretaceous times before the migration of the order southwards ; or 
the Diprotodont type is only a type, and not a natural group, i.e. it, 
has been separately evolved in America and Australia; or, finally, 
there was formerly a land-connexion in the Antarctic hemisphere, 
along which the Diprotodonts of Australia wandered into South 
America. The middle hypothesis has this to commend it, that 
syndactylism occurs in both divisions, and that in some Dipro- 
todonts the pouch opens backwards as it does in the Polyprotodonts. 
So great are the resemblances that but little difference is really left 
—of great importance that is to say. Hence it is not difficult to 
imagine the reduction of the incisors having taken place twice. 
In favour of the first hypothesis there are no positive facts. 
Finally, in favour of the last, which is so strongly supported by 
the facts of distribution derived from the study of other groups 
of animals,’ there is at least this striking fact or rather series of 
facts: that some of the South American fossil Polyprotodonts 
have a “ strictly Dasyurine relationship.”* If there has not been 
a direct migration, then the Dasyurine type has been twice evolved, 
an improbability that few will attempt to explain away. In any 
case we shall adopt here the usual division of the Marsupials into 
Diprotodontia and Polyprotodontia. 
“ 
SuB-ORDER 1. DIPROTODONTIA. 
This group includes the herbivorous Marsupials. The incisors 
are as a rule three above, but one only in the Wombats. Below 
1 See for a further discussion of this subject the zoogeographical handbooks of 
Mr. Lydekker and myself, quoted on p. 78 (footnote). 
2 To this may be added Mr. Thomas’ observation that the family of American 
Opossums is ‘‘ very closely allied to the Dasyuridae, from which, were it not for its 
isolated geographical position, it would be very doubtfully separable.” 
