ADAPTATIONS 437 



the point where the small intestine passes into the rectum. 

 Their presence, or absence, from a systematic point of view, is 

 of some importance. A heritage from reptilian ancestors, of 

 moderate length and thin-walled in the more primitive forms, 

 they may become reduced to mere vestiges ; or one, or both, 

 may even disappear completely, while in some cases they have 

 become enormously developed, having their cavities divided by 

 spirally arranged septa, as in Chauna and some of the Rails and 

 Cranes, or produced into large bulbous sacs having the external 

 wall produced into papillate hollow outgrowths, as in the 

 Martineta Tinamou (Calodromas elegans) (p. 23). Originally 

 they were probably digestive in function, and in many birds 

 they still remain so, serving for the dissolution of cellulose. 



Where the diet is largely insectivorous or vegetivorous the 

 caeca are large ; while a carnivorous or piscivorous, and especi- 

 ally a frugivorous diet, leads to their reduction or complete 

 disappearance. The Passerine birds, which exhibit great 

 variety in their choice of food, have small caeca which have 

 become transformed into glandular organs. 



In spite of this generalisation, however, the study of the 

 colic caeca of birds is one of great complexity, and bristling 

 with contradictions, as Dr. Mitchell in his brilliant memoir has 

 shown. Thus the Passerine birds affect all diets, yet display 

 but one type of caeca. The Owls and Accipitrine birds have an 

 almost identical diet, the larger forms being carnivorous the 

 smaller insectivorous, yet the Owls have very large, flask-shaped 

 caeca, while in the Hawks they are vestigial or absent. The 

 Game-birds and Pigeons have a similar diet, yet the former have 

 large caeca, partly digestive and partly secretory or excretory in 

 function ; while in the latter they are vestigial, or absent, or re- 

 duced, but transfused into glandular organs; the Gulls feed 

 on fish and garbage and have the caeca vestigial or absent, 

 while in the Skuas — which are also Gulls — they are large, thin- 

 walled and digestive in function. 



Thus, then, the evolution of the alimentary canal by no 

 means follows that of the organism as a whole. That is to say, 

 it runs along lines of its own governed by factors by no means 

 yet understood. 



While the various structural peculiarities which have been 

 described with regard to the alimentary system are commonly 



