160 ADAPTATION OF THE BILL. 



There are other purposes to be answered by the 

 form of the bill besides the mere capture of the food, 

 and adaptation to the place where it is, and the 

 manner of getting at it. It is a general law in the 

 economy of animals, that they shall be safe while 

 they are feeding, at least from dangers connected 

 with the food itself, or the place where it is. 



Now, besides being pruners and weeders to the 

 vegetable kingdom, and a sort of general scavengers 

 for removing the waste of all nature, birds appear 

 specially appointed for keeping within proper bounds 

 the numbers of fishes, mollusca, insects, and reptiles. 

 The power of production in all of these is very great ; 

 and, with the exception of the fishes, which settle 

 matters by eating each other (often their own species), 

 this productiveness is far above the average support, 

 or even the room which there is for them in nature. 

 The tadpoles which appear in one brook would, 

 were they all to live and breed, speedily cover a 

 county with frogs ; the caterpillars on one branch 

 would, if so breeding, soon clear a forest ; and the 

 snails would speedily multiply till not a green leaf 

 were to be found. The ophidian and saurian rt>p- 

 tiles are, in many of their species, co-operators with 

 the birds ; but they frequent the places where the 

 food of birds is abundant, and they are not fitted for 

 long migrations. The motions of molluscous animals 

 are proverbially slow ; and though many of the insect 

 tribes are clever on the feet, the wing, or both, they 

 are not capable of long journeys. Locusts and some 

 other tribes do migrate ; but no insects can continue 

 long on the wing. They want the feathers, the cha- 

 racteristic organs of long flight ; and though their 

 muscles act to very considerable advantage, they 



