THE RISE OF THE MAMMALIA 125 



there would appear to be rather more doubt whether 

 the Marsupials have a right to the same title or 

 whether their comparative simplicity in the matter 

 of reproduction is due to degeneration. There can 

 be little doubt that the Marsupials have degenerated 

 to some extent from a more advanced Mammalian 

 stock, but the balance of evidence would seem to 

 indicate that at the period when degeneration set in 

 they had by no means attained to the high standard 

 of development possessed by the modern placental 

 Mammals. 



The great majority of the Marsupials at the 

 present time inhabit the Australian region, with the 

 exception of two forms, the American Opossum and 

 the American Bush-rat ; fossil Marsupials are, how- 

 ever, fairly abundant in America and even in Europe, 

 and among these fossil forms some are of very great 

 interest owing to their great antiquity. The fossil jaws 

 from the Stonesfield Slate (Jurassic Period) are known 

 to have belonged to animals closely allied to modern 

 Marsupials, and they are the earliest known remains 

 of an undoubted Mammalian nature, since the forma- 

 tion in which they occur is of Secondary age, whereas 

 all other undoubted Mammalian fossils are Tertiary. 

 These fossil jaws belonged to animals of an insigni- 

 ficant size, but in Australia the Marsupial stock has 

 diverged into a great variety of stems and includes 

 forms as small as mice, as large as the rhinoceros, and 



