CHAPTER XXV 



ADAPTATIONS {continue^)— THE ALIMENTARY CANAL AND VAS- 

 CULAR SYSTEMS 



The form of the beak in relation to the food. Modifications of the tongue. 

 The alimentary canal. Modifications of the crop and gizzard. The convolu- 

 tions of the intestines. Peculiarities of the vascular system. 



THOUGH it may seem like drawing attention to a truism 

 to point out the fact that there is an intimate relation 

 between the form of the beak and the nature of the 

 food, yet, in a work like the present, nothing should be taken 

 for granted. Moreover, although this correlation is true in 

 general principle it is not absolutely true, since the shape of the 

 beak in many cases by no means proclaims the nature of the 

 food, beaks of the same shape having been independently 

 acquired to fulfil very different purposes ; while, on the other 

 hand, birds not even remotely related have evolved similar 

 beaks to attain similar ends. 



Some groups, as in the Gallinaceous birds and the birds of 

 prey, for example, present a great uniformity in this matter, 

 others as wide a range of variety; closely allied species of the 

 same genus may even present a very striking range of form, as, 

 for example, in the case cited by Darwin in his Voyage of the 

 Beagle, where he describes and figures the bills of the Genus 

 Geospiza, a small group of Finches inhabiting the Galapagos 

 (111. 45). The Larks supply a similar instance. Here the 

 smaller and weaker beaks are used for picking up small seeds 

 which are either easily crushed or are swallowed whole, while 

 the more powerful jaws have been evolved for the purpose of 

 crushing large and hard-shelled seeds and fruit stones ; thus no 

 source of food of this kind is wasted. The Hawfinches appear 

 to be unique in that the beak-sheath has developed, in the 

 region of the gape, tumid swellings with striated surfaces. A 

 pair of these occur on the lower jaw, which work against a 



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