INTESTINAL TEACT OF MAMMALS. 523 



to lose the caeca, and direct adaptation to diet appears only to accelerate or to retard 

 these inherited tendencies. So also in Mammals, no more than a general relation 

 between diet and eaecal development can be shown to exist. First, there is no 

 relationship between diet and the complete or partial presence of a pair of caeca. The 

 occurrence of such a pair appears to be rather an " accident " of inheritance than in any 

 relation to function, an accident more common in less specialised forms. 



The large size of a single caecum, in Mammals, is rather more definitely correlated 

 with a herbivorous as opposed to a frugivorous, carnivorous, piscivorous, or omnivorous 

 diet than is the case with Birds. Thus it is small in the Monotremes ; large in all the 

 Marsupials except the Carnivorous Dasyuridae. In the Ungulates the caecum varies in 

 size considerably, and Maurice Schiff (24) endeavoured to establish the existence of a 

 correlation between the complexity of the stomach and the size of the caecum. In the 

 Ruminant Ungulates, for instance, where the stomach is large, the caecum is relatively- 

 small ; in the Horse and Tapir, where the stomach is simple, the caecum is relatively 

 large; in the Pigs, which have a simple stomach, the caecum is again capacious; whilst 

 in the Hippopotamus, where the stomach is complex, the caecum is absent. I find it 

 difficult, however, to follow this comparison far. Many of the Antelopes and Gazelles 

 have relatively large caeca, although in them the stomach is complex. The Rhinoceros, 

 which has a simple stomach, has a caecum not relatively very large. The Tragulidae, 

 with a complex stomach, have a capacious caecum. In the llodents, the diet ranges from 

 being herbivorous to practically omnivorous, and the caecum is always relatively large ; 

 1 find it impossible, however, to see any correlation between the comparative size and 

 the extent to which the diet is strictly herbivorous. Outside the limits I have mentioned 

 the correlation between size of the caecum and diet breaks down completely. There 

 is no distinction between the plant-eating and insect-eating Edentates in this matter. 

 The Sirenia arc all strictly herbivorous, but amongst them Ehytina alone seems to have 

 possessed a really capacious caecum. In the small Carnivora, many of which are 

 practically vegetable-feeders, there is no relation between the diet and the caecum. 

 Amongst the Primates, the caecum is always large in Lemurs, whether these be 

 omnivorous, frugivorous, or insectivorous; and in the Monkeys and Apes there is 

 always a caecum, varying from group to group in capacity and without direct relation 

 to the nature of the food. 



Where the caecum is normally small, there is no direct relation between diet and 

 complete absence. It is absent in the piscivorous Dolphins, present in the equally 

 piscivorous Seals. So far as I can find out, tho^c Insectivora and Chiroptera in which 

 the caecum is present do not differ, so far as diet is concerned, from the members of these 

 Orders in which the caecum is normally absent. 



In the case of reduced caeca, it is necessary to remember, as Birmingham (2) lias 

 suggested, that the formation of a vermiform appendix may be not degeneration, but 

 the elaboration of a new highly glandular organ. An elaborate comparative study of 



