130 COMPARATIVE ANATOMV. 



be called teeth, are not for the purpose of mastication, lik^ 

 the teetl] 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 in- 

 terstices which lie between these teeth ; catching, as the 

 stream passes across her beak, whatever it may happen to 

 bring along with it, that proves agreeable to her choice, 

 and easily dismissing all the rest. Now suppose the pur- 

 pose to have been, out of a mass of confused and hetero- 

 geneous substances, 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 digestion ; what 

 more artificial, or more commodious instrument of selec- 

 tion, 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, 

 probably, as what increases its gratification and 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 of nerves, equal almost to the 

 optic nerve in thickness, which pass first along the roof of 

 the mouth, and then along the upper chap, down to the 

 point of the bill, long as the bill is. 



But to return to the train of our observations. The sim- 

 ilitude between the bills of birds and the mouths of quad- 

 rupeds, is exactly such, as, for the sake of the argument, 

 might be v/ished for. It is near enough to show the con- 

 tinuation of the same plan; it is remote enough to exclude 

 the supposition of the difference being produced by action 

 or use. A more prominent contour, or a wider gape, might 

 be resolved into the effect of continued efibrts on the part 

 of the species, to thurst out the mouth, or open it to the 

 stretch. But by what course of action, or exercise, or en- 



^ There is a remarkable contrivance of this kind in the genus bal^na, 

 or proper whale. Numerous parallel plates of the substance called 

 whalebone, cover the palatine surface of the upper jaw. and descend 

 vertically into Ihc inouth ; the lov/er edges are iringed by long fibres, 

 which serve the animal, when taking in the water, to retain the mol- 

 luscaj, with which the water abounds, -and which constitute its food. 



Faxton- 



