132 PRIMITIVE ANIMALS 



corresponding teeth of a different form which become 

 the permanent last premolars. It was for long dis- 

 puted to which series the deciduous premolars and 

 the permanent teeth belonged, whether the permanent 

 incisors, canines and front premolars belonged to the 

 milk or to the true permanent series, and whether the 

 deciduous last premolar represented a milk dentition 

 in the process of development or of degeneration. 

 It has now been found that a complete series of teeth, 

 corresponding to milk incisors, canines and premolars, 

 are formed as rudiments, but only one pair of these in 

 each jaw, the deciduous last premolars, are actually 

 cut to the surface. The deciduous last premolar 

 represents therefore a milk dentition, the rest of 

 the members of which remain below the gums. The 

 permanent incisors, canines, premolars and molars 

 correspond therefore to the permanent series in other 

 Mammals, while the milk dentition is degenerate, with 

 the exception of the last premolars. We arrive, there- 

 fore, at the conclusion that the Marsupials are not 

 only descended from a form of Mammal with a 

 better-developed placenta, but also from a form 

 which possessed a complete milk dentition, so that 

 in these two respects the modern Marsupials are 

 degenerate and not primitive. The fostering of the 

 young in the pouch is, however, a primitive character 

 which has been lost in the higher Mammals. The 

 degeneration of the milk dentition in the Marsupials 



