412 A HISTORY OF BIRDS 



large cushion of similar structure borne by the palate. 

 This crushing mill recalls the large flat-topped teeth of the 

 extinct Placodont reptiles or the crushing apparatus of some 

 fishes. The jaws of the aberrant Opisthocomus are provided 

 with rows of cone-shaped tubercles running along a ridge just 

 inside the cutting edges of the jaws. These horny " teeth " 

 enable the bird to feed upon the fruit of an aroid, and thus, in 

 so far to escape competition in the struggle for food, since other 

 birds without developing a similar armature must leave these 

 fruits untouched. 



The same facts meet one whatever group of birds is ex- 

 amined. Where the supply of any particular kind of food is 

 practically inexhaustible, there will be found a large number of 

 species all having the same type of beak. But it would seem 

 that, sooner or later, either because the feeding area becomes 

 inadequate to support the increased numbers crowded upon it, 

 or from idiosyncrasies of taste, some individuals begin to change 

 their diet, selecting food neglected by the majority, and this 

 forms the basis for a new line of evolution. Whether this begins 

 by the selection of small variations capable of facilitating the 

 capture of the new food or of mutations, the result is the same. 

 The Plover tribe furnishes a good object-lesson on this subject. 



In the typical Plovers the beak is short and slightly swollen 

 at the tip. Therewith the birds pick up small mollusca, Crus- 

 tacea, and worms and other aquatic organisms. In some, as 

 in the Turnstones {Strepsilas), the beak acquires a great density 

 and becomes pointed. It is used for the purpose of capturing 

 small Crustacea which lurk under stones at low tide. Using the 

 beak as a lever the bird raises, and finally overturns the stone, 

 seizing whatever may have been in hiding before escape is pos- 

 sible — hence the name " Turnstone ". In the closely allied 

 and larger Oyster-catcher {Ostralegus) the beak has increased 

 enormously both in length and density, and is now used for 

 breaking open the shells of the larger molluscs, such as mussels 

 (Mytilus). 



Yet other long-billed Plovers are the Curlews and Snipes. 

 The former, like the Ibises — birds belonging to the Stork family, 

 and not even distantly related to the Plovers — have down- 

 wardly curved beaks which are used as forceps to drag out 

 small Crustacea, molluscs, and so on, from their hiding-places ; 



