50 THE STORY OF BIRD-LIFE. 



and then to retire and quietly digest it by the 

 process known as "rumination," or "chewing the 

 cud." 



The hard seeds and nuts devoured by the 

 pigeon or the fowl are slowly passed on to the 

 gizzard, M'here they are gradually broken up and 

 again passed on to the intestine, to be finally 

 absorbed as food — a beautiful instance of 

 division of labour. The gizzard in these birds 

 is a large, muscular (fleshy) organ, the walls 

 of which are of great thi'^kness, and the interior 

 of which is armed with two horny pads. These 

 are made to rub one against another, after the 

 fashion of millstones ; their work is further aided 

 by small stones which the bird swallows on pur- 

 pose, indeed without these it is doubtful if diges- 

 tion could proceed. The gizzard and its stones 

 serve the bu'd in the place of teeth for masticating 

 its food. 



There is a South American bird — the hoatzin — 

 which has so far upset the order of things as to 

 .make its crop do the work of a gizzard. In the 

 transformation, it has thickened its walls, and be- 

 come hard and horny inside. Following on this 

 another change has taken place. The weight 

 of so much food, retained so long as it must 

 be in the gizzard, has brought about a 

 gradual decline in the form and size of the 

 merry-thought, between the arms of which, in 

 ordinary birds, the crop lies, and of the breast- 

 bone, so much so that the bird's power of flight 

 has become seriously hampered, why, we shall 

 see later. 



In the stomach (gizzard) of the great crested 



