CARINATE BIRDS 11 



girdle" bones, as they are called, degenerated. As a 

 consequence, the blade-bone became immovably fixed 

 to its supporting pillar, the "coracoid"; and the fur- 

 cula, or "merry-thought," disappeared altogether, or 

 left, at most, but slight traces of its former existence 

 in the Emus. But with the flying birds the blade- 

 bone is joined to the coracoid by elastic ligaments, 

 and the furcula, or "merry-thought," is well devel- 

 oped. 



There are certain exceptions to this rule, however; 

 and these occur in the case of some species which have, 

 like the Ostriches, lost the power of flight. Herein the 

 keel of the breast-bone has become greatly reduced, 

 while some birds which yet retain the power of flight 

 have either a very imperfect merrythought or none 

 at all. 



Flying-birds further differ from the Ostrich tribe 

 in that their feathers are more perfectly developed — 

 they only, in short, possess typical feathers. The dis- 

 tinctive characters of these have already been de- 

 scribed in the introductory chapter. 



Birds, like all other living creatures, compete one 

 with another for food; and hence a given area of 

 ground can support only a limited number of birds. 

 All above the number which can find support in this 

 area must either stai've or seek fresh feeding-grounds, 

 or contrive to find other kinds of food in the con- 

 gested area. And it is this struggle to live which has 

 brought about the marvellous variety in the forms of 

 birds. For as new kinds of food and new methods 

 of feeding were adopted, slow changes of shape in 

 body, beak, wings, and feet ceme about through the 



