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. 
Mammals 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 burrow, 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 time 
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 
