124 PRIMITIVE ANIMALS 



a lake or stream, one opening of the burrow being 

 above the level of the water, the other below, so that 

 it can enter its home either from the water or the 

 land. In the breeding season the female lays a small 

 yellowish egg, which it incubates in a rough nest of 

 grass or sticks at the back of the burrow ; here the 

 young hatches out and is suckled by the parent. No 

 pouch develops in the Platypus. 



The Monotremes illustrate very well the limita- 

 tions with which the term Primitive Animal must be 

 accepted. In the fact of their laying eggs and in 

 some of their anatomical features these animals 

 undoubtedly retain certain Reptilian features which 

 we know on other grounds the ancestors of the 

 Mammalia must have possessed, but we should be 

 very far astray if we figured the Mammalian ancestors 

 in anything like the shape of the two existing Mono- 

 treme animals. These two animals in many respects 

 are highly specialized forms which owe many of their 

 characteristic features, such as the loss of teeth, the 

 curious structure of the mouth and jaws, the webbed 

 feet and the limbs adapted for digging, to peculiarities 

 in their habits, which are probably modern acquisitions 

 and were certainly not possessed by the generalized 

 ancestral form which gave rise both to them and the 

 modern placental Mammals. 



Whilst the Monotremes may be certainly classed 

 as Primitive Animals, with the restrictions mentioned, 



