528 DE. P. CHALMERS MITCHELL ON THE 



Lengthening of the Gut and Thickening of its Walls in Piscivorous Forms. 



This homoplastic modification is as well marked in Mammals as in Birds. The gut 

 of fish-eating forms is very long, with very thick wall and relatively small calibre. The 

 explanation, no doubt, is similar to that of the herbivorous modification. Fish, as it is 

 eaten by wild animals, contains a large bulk of indigestible matter, and so requires an 

 extended absorbing surface ; moreover, no doubt as a protection against wounding by 

 bones, the wall is specially strong and the calibre narrow. The modification applies 

 to Meckel's tract. Where it is possible to distinguish the limits of that region and the 

 hind-gut, it is seen that the hind-gut of piscivorous Mammals is never specially long 

 or capacious. 



Shortening of the Gut in Frugivorous Forms. 



Curiously enough, whilst in frugivorous Birds the gut is always strikingly short, 

 wide, and simple, a similar change has not taken place in Mammals. Frugivorous 

 Bats have the gut rather longer than insectivorous forms, and amongst Lemurs and 

 Monkeys there is no evidence for the existence of such a modification. 



Increase of Length in Large Forms. 



In Birds the relation of the length and calibre of the gut to the size of the whole 

 creature is striking. If two birds of similar habit and of the same group be compared 

 in this respect, it will be found that the gut of the larger bird is relatively longer rather 

 than relatively wider. The same general rule applies to Meckel's tract in Mammals ; 

 when a smaller Mammal is compared with an allied Mammal larger in size but of the 

 same general habit, it will be found that the larger animal has Meckel's tract relatively 

 longer but not relatively wider. In the case of the hind-gut, however, on the average, 

 increase of capacity is given by increased calibre rather than by increased length. 



Summary op Systematic Inferences. 



When an assemblage of anatomical facts has been simplified, so to speak, by 

 eliminating the element due to convergent modification, i. e. those resemblances that 

 they share because of adaptation to similar functions, a further scrutiny is necessary 

 before drawing, from the likenesses and differences displayed, inferences as to systematic 

 alliance. Likenesses which are due to the common possession of primitive features 

 cannot be regarded as evidence of near relationship ; that certain members of a group 

 have retained what was once the property of all the members of that group can be no 

 reason for placing such creatures close together in a system, if the system is to be 

 based on blood-relationship. Resemblances that are due to the loss or reduction of 

 parts that once were the property of the ancestral stock cannot afford a clear ground 



