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, 1 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." 



4. 



