3 1 6 Why Stones are picked up by Fowls. [Book IX. 



ing down glafs, and which is always found to contain 

 fmall ftones of the hardeft materials the bird can pro- 

 cure. By the help of thefe ftones, and by means of 

 the hard internal coat of the gizzard, and the force of 

 its mufcular coat, the food is effectually ground down, 

 and fitted for entering the inteftines. 



c Spalanzani, and others, have denied (fays Dr. For- 

 dyce) that they were of this ufe, and have affirmed 

 that the ftones were picked tip by mere accident, the 

 animals miffoking them for feeds. But I have ex- 

 amined this particularly in experiments I made in 

 hatching eggs with artificial heat j I have hatched van: 

 numbers, and frequently have given the chickens fmall 

 feeds whole, taking care that they mould have no 

 ftones. In this cafe the feed was hardly digefted, and 

 many of the chickens died. With the fame treat- 

 ment in every refpecl:, others -who had their feeds 

 ground, or have been allowed to pick up ftones, have 

 none of them been loft. With tolerable care, when 

 common chickens are once hatched by artificial heat, 

 they are eafily brought up without a hen, as by inftincl 

 they will keep in that part of the furnace where there 

 is the proper degree of heat, and the proper expofure 

 to air. Inftinct alib teaches them what lubftances 

 they mould choofe for food, 1 and what quantirv of 

 ftones it is neceffary to intermix with it. For if a v; ry 

 large quantity of fmall ftones is mixed with a fmall 

 proportion of grain, they will pick out the grain, 

 fo that the proportion of ftones which they fwallow 

 {hall be very little, if at all greater than when only a 

 few were intermixed. In thofe i examined the pro- 

 portion of ftones were not at all greater when there 

 was a large quantity of them mixed with the grain, 

 than when there was a fmall proportion; and I have 

 cften obferved them choofing one -piece of ftone, and 



rejecting 



