INTESTINAL TEACT OE MAMMALS. 527 



there is an essential difference between the general mode of modification of the Avian 

 intestinal tract and that of Mammals. In Birds the region known as Meckel's tract 

 tends to be so modified as to display individual loops, the number, position, and 

 character of which differ in the different groups. In Mammals such a modification or 

 series of modifications of Meckel's tract never takes place. In Birds, when the hind- 

 gut becomes long it forms irregular minor loops like those of Meckel's tract in 

 Mammals ; in Mammals, when the hind gut becomes long it tends to form minor 

 loops, the number, position, and character of which differ in different groups, just as 

 is the case with the fore-part of the gut in Birds. In Birds great increase of size of 

 the cseca is associated, on the whole, with particular diets ; in Mammals it is similarly 

 associated with particular diets ; but whilst in Birds both cseca elongate, in Mammals 

 it is only one caecum that elongates. 



I may turn now to examples within the group of Mammals. There is a general 

 similarity in the nature of the food of the herbivorous Marsupials and the herbivorous 

 Eutherian Mammals. A Kangaroo and a Sheep in the state of nature live on food of 

 a very similar kind, 1 and in captivity thrive on practically identical food; and yet, 

 notwithstanding the adaptations to a strictly herbivorous diet in the two cases, there is 

 a profound difference in the gut-patterns. Similarly, Dolphins and Seals live on very 

 much the same kind of food, and yet the character of the intestinal tracts in the two is 

 profoundly different ; the limits of the homoplastic modification are definitely set by the 

 inherited material. Finally, to take an example of an opposite nature. The terrestrial 

 Carnivora, from the Lion to the smallest Suricate, display a pattern of intestinal tract 

 essentially similar. And yet, although we give the name Carnivora to the group, 

 almost every possible kind of diet is found amongst them — purely carnivorous, 

 piscivorous, omnivorous, frugivorous. 



The most important of the homoplastic modifications that have to be allowed for are 

 as follows : — 







Lengthening of the Gut in Herbivorous Forms. 



Precisely as happens in the case of Birds, so in Mammals there is a direct association 

 between the herbivorous habit and a great length of gut. The explanation of this, no 

 doubt, is simply that the vegetable matter which such creatures devour is in a form 

 which requires not only prolonged digestive action, but from the intimate admixture of 

 indigestible material a very large absorbing surface. In the case of Meckel's tract, the 

 lengthening results simply in a greater number and closer grouping of the minor 

 loops. With regard to the hind-gut, every herbivorous Mammal has a relatively long 

 and capacious hind-gut, but this relative lengthening is less conspicuous in the 

 Marsupials, and is not associated with a specialisation into regions. 



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