262 THE AMERICAN NATURALIST. [VoL. XXXV. 
sharply as families. In fact, they show such a composition as 
must have existed at some stage of the placental radiation. 
Now the Macropodidz show no signs of being persistent 
primitive types, and there seems to be no valid reason for 
assigning to the marsupials a different potential of evolution. 
The only explanation which may be given for the above facts 
is, therefore, that the Australian radiation has been of com- 
paratively short duration. And if the placental radiation 
began in the late Cretaceous or in the early Eocene, as now 
appears probable, it is unlikely that the marsupial radiation 
can have begun until well on into the Tertiary. An ancestral 
association with the Oligocene opossums of the North or even 
with the Lower Miocene forms of South America is thus not 
improbable. 
As to whether or not the ancestral forms gained access 
to the Australian region by means of a land connection would 
appear to be a matter of little consequence if the fauna is of 
didelphyid origin. Such minute animals as they must have 
been, judging from the Oligocene forms of the northern 
hemisphere and the diminutive opossums of South America, 
might have gained access to the region by transportation in 
various ways. 
In conclusion, the writer wishes to acknowledge his indebt- 
edness to Professor H. F. Osborn of Columbia University for 
many kind suggestions during the preparation of this paper, 
and to Professor Allen of the American Museum of Natural 
History, New York, Professor W. B. Scott of Princeton Uni- 
versity, and Professor R. Ramsay Wright of the University of 
Toronto, for the privilege of examining the marsupial collec- 
tions of the above-named places. 
COLUMBIA UNIVERSITY, NEW YORK. 
! This suggests Huxley's statement of twenty years ago: “ And I suspect that 
most, if not all, of the Australian forms are of comparatively late origin." (On 
the Application of the Laws of Evolution, etc., P. Z. .S. (1880), p. 656). 
