556 ORIGIN OF MAMMALS xix. 9 



perhaps significant that both of these very early mammals burrow and 

 make nests to assist in the protection of their young. 



The Monotremata thus show a peculiar mixture of mammalian and 

 reptilian characteristics. In their brain, hair, warm blood, heart, and 

 diaphragm they are mammalian, but in skeleton and egg-laying habit 

 they resemble reptiles. A large part of their interest is that they sug- 

 gest an intermediate stage, in many features, between the two groups. 

 Thus the pectoral girdle appears to be that of a reptile partly changed 

 to a mammal. There are certainly many things to be discovered from 

 these extraordinary creatures, which have remained with little change 

 in fundamental organization for possibly nearly 150 million years. 

 The characters they show literally provide us with a view of the past, 

 yet the facts that these two alone have survived and that they show 

 special features of their own remind us sharply that evolutionary 

 change is almost universal : new types replace the old almost if not 

 quite completely. 



