No. 412.] THE AUSTRALIAN MARSUPIALIA. 259 
By far the most complete discussion of the question, from 
all points of view, is that given by Lydekker (96) in his Geo- 
graphical History of Mammals.  Lydekker, while agreeing 
with Wallace as to the northern origin of the Australian 
fauna, fixes its time of entry at a much later period, namely, 
at the beginning of the Eocene, on the basis of a possible con- 
nection of the ancestors of the Australian Dasyuridz with 
those of the Oligocene Didelphyidz of the northern hemis- 
phere. He remarks as follows (p. 55) : * Recent researches 
have tended to show that the alliance between the Dasyuridz 
and the Didelphyidz is much more intimate than was formerly 
supposed to be the case. This being so, it is a fairly safe 
assumption that both families are descended from a single 
common ancestral stock." . . . “ Not improbably polyproto- 
dont marsupials survived in southeastern Asia till the early 
portion of the Eocene division of the Tertiary epoch, and 
in this region both Dasyurida and Didelphyide were differ- 
entiated. Representatives of the former family soon after- 
wards 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." 
Objection has been taken to this view by Spencer (96) on 
account of the difficulty of explaining by it the non-appearance 
of Didelphyida in Australia and the paucity of polyprotodont 
types in New Guinea, and also on account of the lack of 
evidence of the former presence of marsupials in Asia. 
For a northern origin Spencer substitutes a South-American 
one taking place in the late Cretaceous through the medium of 
an Antarctic continent. This idea is also favored by P. L. and 
W. L. Sclater (99). Viewing the question from a phyloge- 
netic standpoint alone, Lydekker has recently (99) suggested 
the Prothylacinidze (Sparassodonta of Ameghino) of the South- 
American Miocene as possibly ancestral to the Dasyuridz, and 
Ameghino (93) regards the family called by him Microbiotherii- 
dæ as ancestral to both the Dasyuridze and the Didelphyide. 
Spencer, however, in attempting to derive the Australian 
fauna, encounters the same difficulty which confronted Lydek- 
ker, namely, how to explain the non-appearance of Didelphyidze 
