526 DR. P. CHALMERS MITCHELL ON THE 



impressed by the probability of there being some physiological explanation of this 

 elaborate arrangement. 



Rodentia. In this group two sets of interconnections exist. The colic region is divided in 

 most cases into two loops. Of these, that nearer to the caecum is involved in the 

 spiral twisting of the caecum, and a certain amount of fusion of mesentery and blood- 

 vessels takes place, the amount varying with the length and degree of twisting of the 

 csecum. The second colic loop is the representative of the region that forms the inter- 

 connection in the Pecora. In different Rodents almost every degree of fusion between 

 this loop and the Meckelian mesentery occurs, reaching, in Lepus, a stage which is 

 extremely like that found in the Pecora, and forming a striking case of convergent 

 resemblance. 



Insectivora. Practically absent. 



Chiroptera. Absent. 



Carnivora. Practically absent. 



PROsiMiiE. In the Lemurs there is the beginning, and not much more, of a connection between 

 the usually large colic loop and the Meckelian region. 



SimijE. In this group, whilst it never becomes so elaborate as in the Pecora and higher Rodentia, 

 there is almost invariably an interconnection between the colic loop and the Meckelian 

 region, the mesenterial fusion varying considerably, but the interconnection of the 

 blood-vessels by special loops being sometimes considerable. 



Homoplastic Modifications. 

 Before attempting to decide to what extent the characters of the gut-patterns in 

 Mammalia throw light on the affinities of the mammalian groups, it is necessary to 

 consider briefly how far the characters of these patterns are modified in accordance with 

 function. The mammalian gut is, of course, the same organ throughout the group, 

 and its general function and mode of action are alike ; it is to be expected that where 

 Mammals belonging to different Orders have become adapted to similar food, the gut 

 will have become modified in similar directions. Obviously such similarities can be no 

 guide to affinity ; they are what I have termed " multi-radial apocentricities," peculi- 

 arities of structure that may have been independently repeated in different cases ; and, 

 before drawing conclusions as to affinities, it will be necessary, so to speak, to correct 

 the gut-patterns in these respects, to eliminate from them the convergent resemblances. 

 It is important to notice, however, that in the mammalian gut, however great may be 

 adaptive resemblances, the inherited element dominates the structure. Take the broadest 

 example of this dominance of the inherited factor, of the nature of the material on 

 which the adaptations have to be impressed. The intestinal tract of Birds must be 

 closely similar in its origin to that of Mammals, and different groups of Birds have 

 become adapted to very much the same different sets of food as in the case of 

 Mammals. None the less, as I have already pointed out, underlying these adaptations 



