OF THE AUSTRALIAN MAHSUPIALIA. 205 



now Bohring; Straits, and tlie occurrence of fossil opossums in tlie Oligocene rocks of 

 Eiiro])e and North America. 



The difficulty as to the absence of Didelphyidse in Australia presents itself whether we 

 assume a northern or a South-Araericau origin. Spencer explaias the condition by 

 assuming that the land-connection between South America and Australia was broken at a 

 time soon after the ancestral forms of the Australian Marsupials had passed across, and 

 while the Didelphyidae were being developed in the more northerly portion of South 

 America. If this explanation be the correct one, we may ask why the form Thijlacinus, 

 whose affinities are decidedly with the South-Amei'ican Sparassodonta, gained access 

 to the Australian region, while the MicrobiotheriidiB, some of which at least present 

 the characters of the Didelphyidse and are found fossil in the same formations, were 

 excluded. The same question might be asked of the Edentate fauna of those formations. 

 There is also the difficulty of recognizing a definite period of rupture of the antarctic 

 connection and that of accounting for the origin of the Didelphyidfie of the northern 

 hemisphere. 



In a former paper the wi'iter referred to the difficulty presented by the absence of the 

 Didelphyidge in Australia as an apparent one, due to the recognition of the family as a 

 modern derived group. We are accustomerl to look upon the Didelphyida^ not only as 

 contemporaries of the Australian Marsupials, but also as forms possessing fixed family 

 characters. When we consider, first, that the modern family represents an exceedingly 

 plastic group, the members of which are at the present time undergoing an incipient 

 radiation ; secondly, that the structural differences separating the family from the 

 Dasyuridae and the presumed common ancestors of the Peramelidae and Phalangeridae are 

 extremely sHglit; and, thirdly, that the ancestors of the Australian fauna on entering 

 that region must have found themselves under conditions most favourable for differential 

 development in a place offering diverse and unoccupied sources of food-supply — it is not 

 difficult to conceive that the Didelphyid* in establishing the foundation of an extensive 

 radiation may have thrown aside their original didelphyid characters. It seems 

 preferable to believe that the Didelphyidse were formerly present in Australia, as well as 

 Europe and North and South America. 



The evidence in favour of a South- American origin of the Australian fauna is bised 

 partly on the general faunal evidence of an antarctic land-connection, concerning Avhich 

 there seems to be little doubt, and partly on the presumed special rcseuiblances between 

 the Australian fauna and that of the Patagonian Miocene. Puller reference to these 

 resemblances will be made in the next section, but it may be mentioned at this point that 

 those occurring between the diprotodont forms of both series are of too general a kind to 

 be interpreted as indicating more than a parallel development from common ancestral 

 types, and that those; between ThylaciuKs and the Sparassodonta do not indicate a South- 

 American origin of the Australian fauna, since there are no definite reasons for believing 

 Thylacinm to have a special affinity with the Dasyurida), and it has not the slightest 

 resemblance to a prototypal form. The evidence of a connection of the Patagonian Anth 

 the Australian forms at present limits itself to the possibility that Thylacinm may have 

 migrated from South America, and a further possibility of a former distribution of 



28* 



