234 NATURAL THEOLOGY. 



pointed prickles. These, though they should be 

 called teeth, are not for the purpose of mastica- 

 tion, like the teeth of quadrupeds ; nor yet, as in 

 fish, for the seizing and retaining of their prey ; 

 but for a quite different use. They form a filter. 

 The duck by means of them discusses the mud ; 

 examining with great accuracy the puddle, the 

 brake, every mixture which is likely to contain 

 her food. The operation is thus carried on : — 

 The liquid or semi-liquid substances in which the 

 animal has plunged her bill, she draws, by the 

 action of her lungs, through the narrow interstices 

 which lie between these teeth, catching, as the 

 stream passes across her beak, whatever it may 

 happen to bring along with it that proves agree- 

 able to her choice, and easily dismissing all the 

 rest. Now, suppose the purpose to have been, 

 out of a mass of confused and heterogeneous sub- 

 stances, to separate for the use of the animal, or 

 rather to enable the animal to separate for its own, 

 those few particles which suited its taste and di- 

 gestion, what more artificial or more commodious 

 instrument of selection could have been given to 

 it than this natural filter ? It has been observed 

 also, (what must enable the bird to choose and 

 distinguish with greater acuteness, as well, proba- 

 bly, as what greatly increases its luxury,) that the 

 bills of this species are furnished with large nerves, 

 that they are covered with a skin, and that the 

 nerves run down to the very extremity. In the 

 curlew, woodcock, and snipe, there are three pairs 



