C^CA 69 



silas, Limosa, Scolopax, Parra, Rhinochetus, many Columbse, Acci- 

 pitres, and Passeres ; or (b) they are entirely absent, as in many 

 Columbie, Psittaci, Musophaga, Corythaix, Pici, Alcedinidae, Bucero- 

 tidee, Upupidie, Colius, Cypselidte, and Trochilidfe. 



4. Sometimes one caecum remains in a rudimentary condition 

 and the other one has disappeared ; this is the rule in almost all 

 Herodii and in Procellaria, but occasionally met "with in Steganopodes, 

 Podicipes, Strepsilas, and in Atrichia. 



The greatest development of the c^eca occurs in Struthio, Rhea, 

 Tinamus, and Meleagris, their aggregate volume ec|ualling or even 

 surpassing that of the rest of the intestinal canal, the cseca in these 

 cases, especially in Ratitre, shewing numerous transverse constric- 

 tions and sacculations, which increase the absorbing surface. 



A certain correlation exists between the caeca and the length 

 and width of the rectum. 



The examples enumerated above seem to shew that caeca are 

 not required for the digestion of meat, fruit, and grain. Fish-eating 

 Ducks have considerably shorter caeca than their strictly vegetarian 

 relations ; the same remark applies to those Waders which live upon 

 mollusks and other soft-bodied invertebrates. On the other hand, 

 the well-developed cjeca of Coracias, Caprimulgus, Merops, Cuculus, 

 and those of the likcAvise insectivorous Todies and Bee-eaters, make 

 it not improbable that in the caeca not only cellulosis (as in Mam- 

 malia) but also chitine is digested. 



Lastly, the presence or absence of the ca^ca being thus explained 

 by the food, a clew -will occasionally be afforded to the systematic 

 position of birds in which they appear against reasonable expectation. 

 It is clear that change of diet may be accomplished in a much 

 shorter time than it takes to modify the various digestive organs. For 

 instance, the exclusive meat-diet of the Birds-of-Prey has reduced 

 their caeca to mere rudiments, and it is more than improbable that 

 the insectivorous habits of many of the smaller Falconidae will ever 

 redevelop these organs, especially since these birds throw out the 

 indigestible parts in pellets. Owls now cannot be distinguished 

 from Diurnal Birds-of-Prey by their diet ; they possess large caeca, 

 and cannot therefore be derived from the Accipitres, which have lost 

 them, nor is it probable that Owls and Accipitres came from one 

 common stock and are collateral branches, because in this case both 

 would be of equal age, and we should have to assume that the meat- 

 diet had in one branch suppressed and in the other branch preserved 

 or even increased the caeca. We can only conclude that the Owls 

 are descendants of a stock of birds Avhich, like the Nightjars, lived 

 on chitinous insects (Beetles, ]\Ioths), and that they, like Podargus, 

 as shown by its predilection for mice, comparatively recently took 

 to the flesh of vertebrates. 



As might be expected, the members of any large and much 



