566 



SCIENCE. 



[N. S. Vol. XI. No. 276. 



IV.) These animals first appear in the 

 oligocene of Germany. It is also, of course, 

 possible that they may have taken a north- 

 ern route as indicated by the I'emains of 

 Rhytina in the North Pacific. 



Dr. Louis Dollo, of Brussels, has recently 

 endeavored to demonstrate that all Marsup- 

 ials have been evolved from arboreal forms 

 like the Opossum.* If we can draw a par- 

 allel with the adaptive radiation of the 



Chart III. — Restoration of Antarctica by elevation to the 3040 sounding line, showing old continental 

 lines and greater depth between Africa and Antarctica. 



Before confining our attention to Arcto- 

 gcea, let us further consider the mesozoic re- 

 lations of the three realms. (Chart I. and 

 Chart II.) 



In the Jurassic period stem forms of 

 Insectivores, Marsupials and possibly of 

 Monotremes* are found in Arctogtea and 

 seem to establish the theory of northward 

 origin of the mammalia as a class. 



*The writer's view (1888) that the Jurassic Mam- 

 mals of England and Wyoming embrace primitive 

 placentals or Insectivores as well as Marsupials and 

 Multitnberculates (? Monotremes) is now generally 

 ticcepted. 



placentals during the 3,000,000 years, more 

 or less, of the Tertiary, we maj^ safely con- 

 clude that such a primitive family, entering 

 the Australian region during the Cretaceous 

 period either by way of Antarctica (Spencer) 

 or by way of the Oriental region (Wallace 

 and Lydekker), might have peopled Aus- 

 tralia with all its wonderfully diversified 

 forms of Marsupial life. The Didelphyidae 

 are to the Marsupials what the Creodonts 

 are to the placentals in point of potential 



* Les Ancetres des Marsupiaus etaient-ils arbori- 

 coles? Sliscellanees Biologiques, Paris, 1899. 



