ADAPTATIONS 433 



the disposal of these very different food-stuffs included under 

 the general head of flesh and vegetable. More than this, we 

 are confronted here with a number of exceptions to the general 

 rule, and with apparently unnecessary complexities of structure, 

 as when, without apparent rhyme or reason, an elaborate mechan- 

 ism has been evolved to deal with a perfectly normal diet, such 

 as is disposed of with ease in all other cases without any special 

 modification. 



In many birds the oesophagus or gullet is dilated in the 

 region of the furcula or merry-thought ; but slightly in some, 

 while in others, as in the Pigeons and Game-birds, there is 

 formed a relatively enormous chamber. In the Pigeons this is 

 symmetrical and bilobed, but in the Game-birds it forms a single 

 globular cavity with a very thin wall. In birds of prey the 

 crop is only slightly developed, a temporary chamber, the lumen 

 of the gullet, contracting when its store of food has passed ; the 

 same is true also of many fish-eating birds. In the great 

 majority of birds there is no crop, and between this complete 

 absence and the huge chamber of the Pigeon there is every 

 possible gradation. Its walls contain no special digestive glands, 

 except in the case of the Hoatzin (p. 313), and its precise function 

 may well form a subject for future investigators. It would seem 

 to fulfil, in the case of the Pigeons, Game-birds and many of 

 the small seed-eating, perching birds, the same purpose as the 

 ruminating stomach of mammals. That is to say, it enables 

 a large store of food to be taken in rapidly and carried away 

 to some secluded spot for digestion. In animals which have 

 to seek their food in open situations where they are at the mercy 

 of prowling carnivores, such a device is of immense importance. 

 Thus, then, we may take it, that this highly specialised crop has 

 arisen by the action of natural selection, which has increased 

 the size of this organ by selecting for survival those birds which 

 possessed favourable variations in the direction of an increased 

 capacity in this normally enlarged area of the gullet. Those 

 in which the size of this region of the gullet tended rather to 

 decrease, or to remain stationary, became naturally more ex- 

 posed to the attacks of their enemies since they had to return 

 oftener to the danger area to take in fresh supplies of food. 



The extraordinarily thick fleshy mass which surrounds the 

 stomach or " gizzard " of fruit and grain-eating birds presents 



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