172 THE VITAL FUNCTIONS. 



lowed merely by accident, or in consequence of 

 the stupidity of the bird, which mistakes them 

 for grain. But this opinion has been fully and 

 satisfactorily refuted both by Fordyce and by 

 Hunter, whose observations concur in establishing 

 the truth of the common opinion, that in all birds 

 possessing gizzards, the presence of these stones 

 is essential to perfect digestion. A greater or less 

 number of them is contained in every gizzard, 

 when the bird has been able to meet with the 

 requisite supply ; and they are never swallowed 

 but in order to assist digestion. Several hun- 

 dred were found in the gizzard of a turkey, and 

 two thousand in that of a goose : so great an 

 accumulation could never have been the result 

 of mere accident. If the alleged mistake could 

 ever occur, we should expect it to take place 

 to the greatest extent in those birds which are 

 starving for want of food ; but this is far from 

 being the case. It is found that even chickens, 

 which have been hatched by artificial heat, 

 and which could never have been instructed 

 by the parent, are yet guided by a natural in- 

 stinct in the choice of the proper materials for 

 food, and for assisting its digestion ; and if a 

 mixture of a large quantity of stones with a 

 small proportion of grain be set before them, 

 they will at once pick out the grain, and swallow 

 along with it only the proper proportion of stones. 

 The best proof of the utility of these substances 



