34 A BOOK OF BIRDS 



the ** furcula," or *' merry-thought," disappeared altogether, leaving, 

 at most, but slight traces of its former existence in the Emeus. But 

 with the flying birds the blade-bone is joined to the coracoid by elastic 

 ligaments, and the furcula, or ** merry-thought," is well developed. 



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 merry-thought 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. What are the distinctive characters of these have 

 been already described in the introductory chapter. 



Birds, like all other living things, 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 starve or seek fresh feeding-grounds, 

 or contrive to find other kinds of food in the congested area. And 

 it is this struggle to live which has brought about the marvellous 

 variety in the forms of birds which this book illustrates. For as 

 new kinds of food and new methods of feeding were adopted, so slow 

 changes of shape in body, beak, wings, and feet came into being. 

 These changes of shape came about through the process known as 

 selection. That is to say, birds which, by some peculiarity in their 

 structure, found themselves able to capture and thrive upon the 

 flesh of other animals, including also other birds, went on competing 

 among themselves for this particular kind of food, and those which 

 were best endowed — ^that is to say, those which by their longer wings 

 or sharper beak and claws, or by their ability to swim and occasionally 

 even to dive — gradually crowded out their less fortunate fellows. 

 As time passed, the peculiar characters and qualities which originally 

 gained for them the superiority over their neighbours went on in- 

 creasing, because the competition still continued, being rendered, 

 indeed, the more severe because carried on between fewer rivals — 

 but rivals almost equally well fitted for the struggle. And in this 

 way, then, in course of time, by slow degrees, the various forms of 

 birds have come into being. 



