208 KB. B. A. BEXSLEY OX THE EVOLUTION 



descent, the division should not be into Diprotodontia and Polyprotodontia, but rather 

 into Syndactyla and Diadactyla. 



Owen's divisions of the Polyprotodontia naturally included the Didelphyida? as well as 

 the Australian Dasyui-idse and Peramelida3. Such a ^roup as the Didelphyidce could 

 have no place in the above classification on account of the fact that its component genera 

 present both syndactylous and eleutherodactylous types of foot-structure. The Epanor- 

 thidse present similar difficulties both with reference to this and to Owen's classification. 

 In commenting on the systematic position of Coenolestes, the living representative of the 

 Epanorthidse, Thomas remarks : " As to the general classification of the Marsupials, a 

 subject already difficult in view of the puzzling possession by the Peramelidre of 

 polyprotodonty combined with syndactyly, Coenolestes apparently only adds to the difficulty, 

 being non -syndactylous like most jjolyprotodonts, wliile it has by dentition nothing 

 whatever to do witli them. If anything, however, this fact tends to confirm the tentative 

 opinion expressed in the ' Catalogue of Marsupials,' that the primary division of the 

 order should be by dentition and that syndactyly is a secondary character. Were syndactyly 

 the primary character, the EpanorthidiJe would be thrown with the Didelphyidse with 

 which they have clearly nothing to do, and separated from what appear to be their 

 nearest allies, the Phalangeridae." 



Difficulties such as these are only apparent, arising from the attempt to apply the 

 same principles of classification to isolated and independently evolved groups. In the 

 first place, how have the various marsupial faunas arisen ? As to the origin of the 

 Didelphyidae, the evidence is wholly unsatisfactory. They may have come from a spur of 

 an earlier Jurassic radiation, or they may have been themselves the original stock of the 

 marsupial division. As to the subsequent history of the family, the evidence is more 

 complete. They enjoyed a \\ide distribution in the northern hemisphere during the 

 Oligocene period, and were probably present in South America as well. The Australian 

 fauna shows indications of didelphyid origin. The existing Didelphyidte of South America 

 represent a minor radiation, proceeding from forms approximating closely, so far as may 

 be judged fi'om the dentition, to Feratherium. It is extremely probable that at least 

 some of the Miocene Microbiotheriidae of South America were in reality Didelphyidse, 

 and were the ancestors of the Epanorthidse and their allies and of the Sparassodonta. In 

 other words, the derivation of the different marsupial faunas is the history of successive 

 radiations of Didelphyidse. Now it will be apparent that it is impossible to apply the 

 same principles of classification to two geographically isolated faunas unless their 

 resemblances are of such a nature as to lead us to suppose that two or more differentiated 

 portions of one of them represent migrated portions of the other. In the case of the 

 marsupial faunas we have to ask whether the resemblances between them can be traced 

 to such migration or whether they are the result of convergent development. 



There appears to be no possibility of a connection of the existing didelphyid radiation 

 W'itli the Australian one, the former being mucli too modern. We may take advantage 

 of this to point out certain resemblances as due to convergent development. We notice 

 especially that the Didelphyidae parallel the Dasyuridse in the tendency towards reduction 

 of the posterior premolars. Certain of them parallel the Phalangeridte in the develop- 



