THE INNER TISSUES OF ANIMALS. 313 



macerating chamber, the gizzard becomes a chamber adapted 

 to triturate more effectually. This adaptation requires simply 

 an exaggeration of certain structures and actions which 

 characterize stomachs in general, and, in a less degree, 

 alimentary canals throughout their whole lengths. The 

 massive muscles of the gizzard are simply extreme develop- 

 ments of the muscular tunic, which is already considerably 

 developed over the stomach, and incloses also the oesophagus 

 and the intestine. The indurated lining of the gizzard, 

 thickened into horny buttons at the places of severest pres- 

 sure, is nothing more than a greatly strengthened and 

 modified epithelium. And the grinding action of the gizzard 

 is but a specialized form of that rhythmical contraction by 

 which an ordinary stomach kneads the contained food, and 

 which in the oesophagus effects the act of swallowing, while 

 in the intestine it becomes the peristaltic motion. Allied as 

 the gizzard thus clearly is in structure and action to the 

 stomach and alimentary canal in general ; and capable of 

 being gradually differentiated from a stomach where a grow- 

 ing habit of swallowing food unmasticated entails more 

 trituration to be performed before the food passes the pylorus ; 

 the question is — Does this change of structure arise by direct 

 adaptation? There is warrant for the belief that it does. 

 Besides . such collateral evidence as that mucous membrane 

 becomes horny on the toothless gums of old people, when 

 subject to continual rough usage, and that the muscular coat 

 of the intestine thickens where unusual activity is demanded 

 of it, we have the direct evidence of experiment. Hunter 

 habituated a sea-gull to feed upon grain, and found that the 

 lining of its gizzard became hardened, while the gizzard- 

 muscles doubled in thickness. A like change in the diet of 

 a kite was followed by like results. Clearly, if differentiations 

 so produced in the individuals of a race under changed habits, 

 are in any degree inheritable, a structure like a gizzard will 

 originate through the direct actions and reactions between 

 the food and the alimentary canal. 



