THE RISE OF THE MAMMALIA 133 



would seem to be due to the very long period spent 

 by the young as sucklings in the pouch, making it 

 unnecessary for them to develop teeth at an early 

 period before the jaws have attained to anything like 

 their definite size. The loss of the placenta would 

 also seem to be correlated with the early period at 

 which the young begin to be suckled in the pouch. 

 These two peculiarities of the Marsupials, therefore, 

 the loss of the placenta and the loss of the milk den- 

 tition, appear to be connected with the prolongation 

 of the suckling period at both ends, the young being 

 transferred to the pouch when very little developed 

 and remaining as sucklings there until well-grown. 

 One cause of this lengthening out of the suckling 

 period may perhaps be sought in the absence of any 

 very serious enemies which might be liable to pursue 

 and attack the females when cumbered with young. 

 It is well known that in kangaroo-hunts if a female 

 carrying young of any size in her pouch is hard 

 pressed, she will fling the young away, showing that 

 they are a serious impediment to her flight. Whatever 

 may be the explanation, the fact seems clear that the 

 Marsupials have specialized along the line of increasing 

 the importance of the suckling period in the pouch, 

 and have given up the advantages of a placental 

 nutrition, and an early weaning of the young pro- 

 vided with milk dentition, whilst the Eutherian Mam- 

 mals have increased the efficiency of the placental 



