MATTHEW, CLIMATE AND EVOLUTION 267 



is not a true diprotodont, but in fact belongs to the polyprotodont divi- 

 sion of the Marsupialia, and with this genus must be associated all of the 

 Epanorthids and probably all of the so-called Paucituberculata of the 

 South American Tertiaries. If then the Diprotodonta, so dominant and 

 so widely varied in Australia, were wholly absent from South America, 

 while parallel adaptations were developed there from the Polyprotodonta, 

 the distribution of these marsupials affords a valid argument against 

 instead of for any Antarctic connection during the Tertiary. 



In view of the great amount of adaptive divergence seen in the various 

 Pleistocene and modern genera of Australian Diprotodonta, the origin 

 of the suborder in Australasia or its earliest invasion of that zoological 

 region, must be dated far back in the Tertiary. On our present evidence 

 it may well be regarded as wholly autochthonic, derived from early Ter- 

 tiary or possibly from late Mesozoic polyprotodonts. Nevertheless, in 

 view of the defectiveness of the Mesozoic record, where we should chiefly 

 expect to find this group, if anywhere in the North, and the presumable 

 rarity of Tertiary survivors, there is nothing unlikely in the view that 

 they originated primarily in the North like their polyprotodont and 

 allotherian relatives and were driven southward with the former group 

 and somewhat more thoroughly extinguished in the north, while in Aus- 

 tralia they blossomed out into a great adaptive expansion paralleling the 

 absent ungulate mammals. 



It is probable that the opossums survived in North America throughout 

 the Tertiary, although there is no clear record of them in our Miocene 

 and Pliocene.®^ But we know only a small part of our Pliocene fauna 

 as yet, and the Miocene, although better known, represents chiefly the 

 animals of the open plains, the forest fauna being very incompletely rep- 

 resented. On the other hand, it seems probable that the apparent dis- 

 appearance of marsupials from Western Europe after the Lower Miocene 

 was real, and it is probable that they had disappeared even earlier from 

 Asia. They have not been found in the later Tertiaries of India or 

 China, so that they must have been rare if not absent at that time. The 

 Eocene Tertiary of Asia, where they might be expected to be common, 

 is altogether unknown.®^ 



81 A very badly preserved skull from the Colorado Miocene and a Jaw fragment from 

 the South Dakota Miocene in the American Museum collections are perhaps marsupials ; 

 but I have never been able to see in either specimen satisfactory proof that they were 

 so. and have consequently never recorded them. 



*-The earliest Asiatic Tertiary fauna is that of the Bugti beds of India, lower Bur- 

 digalian or upper Aquitanian according to Pilgrim, Rec. Geol. Sur. India, vol. xliii, pt. 4, 

 pp. 264-326. It is therefore either late Oligocene or early Miocene. 



