V A GLIMPSE INTO THE PAST 139 



The last group of animals which we may consider 

 in this connexion is that of the Marsupial Mammals. 

 These animals occur at the present day in the 

 Australian region and in Central and South 

 America. In America we find the Opossums 

 (Didelphys), closely related to the Australian 

 Dasyures, and in the Andes of Ecuador a peculiar 

 Bush-rat occurs (Coenolestes), which combines some 

 of the characters of the two great Marsupial sub- 

 orders, the Polyprotodontia and Diprotodontia, 

 and is in a sense intermediate between them. The 

 discovery of Marsupials in the Eocene beds of 

 Paris closely related to the American Opossums, 

 and the existence of still more primitive Marsupials 

 in some of the yet earlier deposits of England (for 

 instance the Stonesfield Slate of Oxford), led 

 naturalists to seek a northern origin for the Mar- 

 supials, whence it was supposed they spread 

 south-westward into America and south-eastwards 

 into Australia through Asia. There are, how- 

 ever, some difficulties in the way of this simple 

 interpretation. The primitive Diprotodont group, 

 of which Coenolestes is the only living form, is 

 represented in the Miocene deposits of Patagonia 

 by a number of genera, but it is altogether absent 

 from the northern hemisphere. The typical Dipro- 

 todonts are confined to Australia, so that unless 

 there was some land connexion between South 

 America and Australia in the past, we must sup- 

 pose that the development of the Coenolestidae in 

 South America and of the true Diprotodonts in 



