208 THE MEANING OF EVOLUTION 



faint peep, breaks the shell of the egg, and steps out 

 into the open air. 



I have given this somewhat lengthened description 

 of the development of the chick because of the light 

 it throws upon the method pursued by the mammals. 

 The features which have been described in the case 

 of the chicken s egg could be as fully observed in 

 the case of the turtle or any of the other reptiles. 

 IMammals are descended from the reptiles of the 

 Mesozoic, and whatever peculiarities there may be 

 in their method of producing their young must be 

 derived from the reptiles. If we wish to know how 

 the earliest mammals produced their young, we can 

 only judge by the lowliest members of the group 

 that live upon the earth to-day. The most primitive 

 of these is the so-called Duckmole, of Australia. This 

 little creature has habits not unlike those of the musk- 

 rat. It burrows in the bank of a stream, and makes 

 a nest at the end of the burrows where it lays its 

 eggs. This is one of the very few warm-blooded, hair- 

 covered animals which still lays eggs. A little higher 

 in the scale stand the kangaroo and the opossum. 

 These creatures keep the egg inside of the body until 

 it is hatched. But this happens in so short a tim^ 

 that the young animal is exceedingly immature and 

 as yet unable to stand the outside air. Accordingly 

 there is a double fold of skin on the abdomen of the 



