DIGESTIVE SYSTEM OF BIRDS. 173 



respiratory systems must be so actively exercised to enable the 

 Pigeon to take its daily flights and in some species their annual 

 migrations — a less complicated intestinal canal may naturally be 

 supposed with such increased energy in the animal and vital 

 functions to do the business of digestion, than in the more sluggish 

 and terrestrial vegetable feeders ; and accordingly we find that 

 the requisite complexity of the intestinal canal is obtained by an 

 increased developement of the coecal processes in the Gallince, 

 while in the Columbidce the coeca remain as little developed as in 

 the Insessores, which they resemble in powers of flight. If we 

 regard the coeca as excretive organs, their differences in the above 

 orders may be in like manner explained by their relations to the 

 locomotive and respiratory functions. 



In the Cursores the developement of coeca seems to have re- 

 ference to the quantity of food, and the ease with which it may 

 be obtained, according to the geographical position qf the species. 

 In tlie CassoAvary, which is a native of fertile and tropical islands. 

 New Guinea, North of Australia, New Britain, &c., vegetable 

 food of a more easily digestible nature may be selected, and it 

 need not be detained long, where a fresh supply can be so readily 

 procured. But in the Ostrich, which dwells amidst arid sands 

 and barren deserts, every contrivance has been adopted in the 

 structure of the digestive apparatus to extract the whole of the 

 nutritious matter of the food which is swallowed. 



In the Grallatores, where no material differences of locomotive 

 powers or means of obtaining food exist, the coeca present in their 

 developement a direct relation to the nature of the food, and are 

 most developed in the Gruidce. The same holds good in the 

 Natatores. 



Why the increased extent of intestinal surface in the above 

 different cases should be chiefly obtained by the elongation of the 

 coeca, will appear from the following considerations. In conse- 

 quence of the stones and other foreign bodies Avhich birds swallow, 

 it is necessary that there should be a free passage for these 

 through the intestinal canal, Avhich is therefore generally short 

 and of pretty uniform diameter. In the omnivorous Birds of the 

 tropics, as the Hornbills, Toucans, Touracos, and Parrots, which 

 dwell among ever-bearing fruit-trees, the rapid passage of the 

 food is not inconsistent Avith the extraction of a due supply of 

 nourishment, but is compensated by the unfailing abundance of 

 the supply. But AAdiere a greater quantity of the chyle is to be 

 extracted from the food, and AA^here, from the nature of the latter, 

 a greater proportion of foreign substances is required for its 



