86 DR. B. A. BENSLEY ON THE EVOLUTION 



or the narrow iliac bones, we already know to have been carried over to the primitive 

 forms (Dideli:)hyidse, Creodonta) of both of the derived groups, and reduced in the more 

 specialized ones. Others, such as the double dentition and the normal pentadactyl pes, 

 were carried over to the Pla centals and lost in the Marsupials ; wliile still others, such 

 as the marsupial bones and the non-placental allantois, were carried over to the Marsupials 

 and lost in the Placentals. Secondly we have to ask, What were the features of a 

 divergent adaptation which, acting on the ancestral forms, brought about their division 

 into Marsupial and Placental series, and gave to the former the characters which now 

 distinguish them, namely, the reduced milk-dentition, the jji-eliensile pes, and premature 

 birth ? The assumption by the ancestors of the Marsupial division of arboreal habit may 

 be noted as a possible answer to this question. Given the primary differentiation of the 

 Marsupials as a group, we have finally to ask. What is the sequence of events in their 

 secondary adaptive radiation ? This question involves the history of several faunas com- 

 prising the Jurassic series of the northern hemisphere (providing its members were 

 actually related to the Marsupial division), the Oligocene Didelphyidse of the same region, 

 the Miocene series of South America, the existing didelphyid fauna of that country, and, 

 finally, the Marsupial fauna of the continent of Australia and tlie adjacent islands. 



Generally speaking, the history of a fauna can only be satisfactorily determined by 

 reference to j^alsBontology. In the Marsupial fauna of Australia we have the result of a 

 radiation which we may be sure has proceeded along much the same general Hues as 

 that wliich has given rise to the existing Placentalia. Many members of each series 

 present characters which have more or less perfect counterparts or parallels in the other. 

 One has only to compare the dental characters of the Dasyuridse, the Phalangeridae, or 

 the Phascolomyidse with those of the Creodonta, the Primates, or the E-odentia to 

 appreciate the intimate structural corresi^ondences between the members of the two 

 groups. In attempting to define the details of the Australian radiation, however, 

 we are at once confronted with the difiiculty that the fossil deposits of that country have 

 not as yet furnished ancestral or collective types. The reason of this is rather obscure. 

 There is still the possibility that older deposits may be found containing such types ; 

 and when we consider the progress of palaeontological discovery in other countries, 

 it seems difficult to believe that the finding of such forms in Australia is more than a 

 matter of industrious exploration. There is a further possibility, however, that the 

 ancestors of the Australian fauna passed the incipient phases of their evolution in another 

 country, such as Asia or even South America. Again, the researches of Huxley and 

 Dollo have shown that the ancestors of the existing Marsupials must have been arboreal 

 animals ; and this raises a suspicion that as such they may have been protected to a much 

 greater extent than terrestrial animals living in caves, or upon the open plains, or 

 frequenting the water-side, from those conditions which woidd result in the preservation 

 of their remains by fossilization. However this may be, apart from a comparatively 

 small number of arboreal forms [Pelaurus, Dromlcla) and some small terrestrial forms 

 {Mpuprijmmis, Ferameles), remains of which have been brought to light in cave- 



