II.] ORIGIN OF AUSTRALIAN FAUNA. 55 



and the Australian lands have thenceforward gone on developing 

 the marsupial and monotremate types into the various living and 

 extinct races we now find there." 



Since this passage was written, the case has been somewhat 

 materially altered by the discovery of the dasyuroid marsupials of 

 the Patagonian Tertiaries ; while recent researches have tended to 

 show that the alliance between the Dasyuridce and the Didelphyida 

 is much more intimate than was formerly supposed to be the 

 case 1 . This being so, it is a fairly safe assumption that both 

 families are descended from a single common ancestral stock 

 which, apart from any question of a connection between Australia 

 and South America, can hardly have originated anywhere than in 

 the northern hemisphere, seeing that the Didelphyidce are totally 

 unknown in Notogaea. There is, however, as already stated, no 

 evidence of the existence of opossums before the Oligocene ; and 

 it is in the highest degree improbable that the two families were 

 differentiated as far back as the Jurassic, or even the Cretaceous 

 epoch. Not improbably polyprotodont marsupials survived in 

 south-eastern Asia till the early portion of the Eocene division of 

 the Tertiary epoch, and in this region both Dasyuridce and 

 Didelphyidce. were differentiated. Representatives of the former 

 family soon afterwards found their way into Australia and New 

 Guinea, while the opossums would appear to have dispersed 

 in one direction into Europe and in the other into North America, 

 eventually making their way from the latter country at a late 

 epoch in the Tertiary period into South America. 



Assuming that the Patagonian dasyurids are more or less 

 closely allied to the Australian forms (and this certainly appears 

 to be the case), it may be taken for granted that they have not 

 originated independently. From considerations advanced in the 

 next chapter, it is almost impossible to believe that they travelled 

 by way of North America ; and if this be so, their only mode of 

 migration would be by means of a land-bridge between South 

 America and Australia by way of the Antarctic continent, or 



1 In the British Museum "Catalogue of Marsupials and Monotremes," 

 p. 315 (1888), Mr O. Thomas writes that the family Didelphyidce "is, on the 

 whole, very closely allied to the Dasyurida, from which, were it not for its 

 isolated geographical position, it would be very doubtfully separable." 



