ADAPTATION FOR THE SPECIES 147 
their hatching, while the warmth is supplied by the 
mother’s body. It is often a matter of surprise to 
many people that a bird should contrive to build a nest 
so exquisitely circular. The trick, after all, is not 
quite so difficult as it looks. The robin gathers up a 
few sticks and places them as the beginning of the 
platform. More and more are brought and woven 
into each other, making a framework altogether too 
big for the nest. Then mud is brought and plastered 
inside of this. With the plastering of this mud the 
careful circularity of the work begins. Every time a 
little material has been added the robin sits down in 
the nest and revolves her body, in this way shaping 
the interior much as the potter shapes a pot. In the 
case of the artisan, it is the pot that revolves. In the 
case of the robin, the bird itself revolves. The effect 
is the same in both cases—a circular vessel is pro- 
duced. A little lining added to the interior of the nest 
softens it for the reception of the eggs. In this ex- 
quisite home the robin lays her eggs, and sits upon 
them until they are developed enough to hatch, and 
then feeds the young until they are old enough to 
feed themselves. 
Far more remarkable than any of the devices thus 
far described are the wonderful developments which 
have come in the class of animals known as the mam- 
mals. Here the most wonderful protection is made 
