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 



