136 MAMMARY APPARATUS OF THE MAMMALIA 



hypothetical stages in Mammalian evolution, 

 which he termed the Prototheria, Metatheria, 

 and Eutheria. He regarded these as succeeding 

 each other in time and as having given origin 

 to the Monotremata, Marsupialia, and Placentalia 

 as three great collateral branches : 



Monotremata. Marsupialia. Placentalia. 



I 

 ^Eutheria. 



^^ Metatheria. - 

 Prototheria. — 



On the other hand, Dollo and others, on the 

 basis of Hill's brilliant discovery of a placenta 

 in Perameles, tried to derive Marsupials from 

 Placental ancestors, so that in the above 

 genealogical tree the Marsupialia and 

 Placentalia would have to change places 

 with each other. A third set of authorities, 

 especially palaeontologists like Mivart, Seeley, 

 and others have supposed a diphyletic origin 

 of Mammals ; and this view is held in its most 

 extreme form at the present time by Hubrecht, 

 who on the strength of his Trophoblast theory 

 sees even in the oviparity of the Monotremes 

 a secondarily acquired feature and would derive 

 this group, together with the Sauropsida, 



