206 DR. E. A. BEXSLEY O.X THE EVOLUTION 



Didelpliyidse across the Antarctic continent witli centres of radiation in Australia and 

 South America. 



It may be mentioned in passing that if an antarctic connection existed there is 

 evidence from the Marsupials that it did not include Tasmania, because the fauna of that 

 region is made up for the most part of specialized forms having identical or prototypal 

 representatives on the mainland. In the Dasyuridae we have Phascogale minima, a 

 tyjoically Tasmanian form, and in P. Sioainsoni a closely related form inhabiting 

 Tasmania and the adjacent portion of the mainland. While both of these forms are 

 fairly primitive, they are specialized as compared with the Papuan and Australian form 

 P. faripes. Of the species of Latfuurus, two are common to East Australia and Tasmania. 

 One of them, D. macidatus, is the most specialized member of the Dasyuridse, with the 

 exception of Saroophilus and Thylacimis. The remaining form, P. vloerrinus, is only 

 slightly less specialized. The actually primitive species, D. hallucatus and D. albo- 

 punctatus, are North Australian and Papuan respectively. Nothing can be inferred from 

 the present Tasmanian distribution of SarcophUiis and Thijlacums, since both are found 

 fossil on the mainland. 



Passing to the Peramelidse we find two Tasmanian forms, P. Gunni and P. obesula. 

 The former species is easily shown to be a specialized derivative of the East-Australian 

 P. nasuta, while the latter, itself one of the most sjiecialized members of the family, 

 enjoys a wide distribution on the mainland. 



In the Phalangeridye only one of the three primitive genera, namely Dromicia, 

 possesses Tasmanian representatives. These are L. lepkla, a fairly primitive type, 

 possibly ancestral, in some respects at least, to Gumnohelideus, and D. nana, which is 

 not only highly specialized but is also not typically Tasmanian, since remains of it have 

 been found in cave-deposits of New South Wales. 



Passing to the Potoroine division of the Macropodidse, we find in the distribution 

 of the species of Totorous a clear case of migration from Australia to Tasmania. 

 Proceeding from the West- Australian P. platyops we find a progressive specialization 

 passing through the South -Australian P. Gilberti, the New South Wales form P. tridaa- 

 tylus, to the Tasmanian P. " apicalis.'" 



These facts are suggestive of a comparatively recent derivation of the Tasmanian fauna 

 from forms inhabiting the adjacent portions of the mainland. 



As to the time at which the ancestral forms of Marsupials gained access to Australia, 

 the most diverse opinions have been expressed. Owen supposed the group to have been 

 present there in Mesozoic times, while Wallace estimated the time of entry as Jurassic. 

 According to Spencer's view it was Cretaceous, while in Lydekker's opinion it was early 

 Eocene. While it is interesting to notice in these opinions an increasing appreciation of 

 the newness of the Australian fauna, it is probable that even the lowest estimate of the 

 duration of their evolution is still much too large. In the first place, on comparing the 

 Australian radiation with the general radiation of Placentals, we note the fact that the 

 former is in a backward stage of development. For example, the differentiation has not 

 proceeded beyond the prodviction of families, although, as recognized by several writers, 

 these families have the potential value of placental orders. In the existing didelphyid 



