INTESTINAL TEACT OE MAMMALS. 529 



for inferring systematic affinity, since there is no difficulty in supposing the same 

 organ to have been reduced or lost in independent branches of the common stock. 

 Resemblances, on the other hand, that are new acquisitions, that depend on definite 

 anatomical peculiarities, must be the most likely field for the discovery of clues to 

 affinity, simply because it appears to be less probable that the same anatomical device 

 should have been produced independently than that it should have been acquired only 

 once. The value of the clue afforded by such resemblances will be in direct relation to 

 the complexity of the anatomical structure. 



It will be convenient to set out from a conception of what may be taken to be the 

 most primitive, generalised, or archecentric condition of the mammalian intestinal 

 tract. From a general review of the characters found in the different existing groups, 

 I infer that in its most generalised, and probably ancestral, condition the mammalian 

 intestinal tract consisted of a tube not very much longer than the length of the body. 

 It was suspended in simple folds at the periphery of a dorso-ventral mesentery stretching 

 from the stomach to the cloaca. It was of fairly even calibre throughout its length 

 and was divided into two regions, probably not far behind the centre of its length, by 

 a pair of caeca of moderate size. It was drained by a longitudinal vein running a 

 nearly straight course in the mesentery from the cloaca to the liver and receiving minor 

 tributaries from the loops of the tract suspended at the edge of the mesentery. Of 

 these minor tributaries, one of conspicuous size came from the primitive yolk-sac and 

 afterwards formed the main anterior mesenteric vein which in most Mammals, in 

 association with the growth of Meckel's tract, appears to be the principal vein of the 

 portal system, the primitive longitudinal vein being reduced to the comparatively 

 unimportant posterior mesenteric vein. 



In the Monotrernata, Meckel's tract has become lengthened in the characteristic 

 mammalian fashion, whilst the hind-gut is relatively long and shows in a simple 

 fashion the typical mammalian specialisation into colic loop and rectum. The single 

 caecum is degenerate. There are no positive indications of affinity. 



In the Marsupials, when allowance has been made for the increase of length due to 

 vegetarian habit, there is no important difference between the groups. The intestinal 

 tract is more generalised than that of Monotremes, this being shown in the frequently 

 great proportional length of the hind-gut without marked differentiation of that 

 region into distinct portions, and in the frequent traces of a paired condition of the 

 caeca. 



The so-called Edentata show characters as diverse as is to be expected from the 

 heterogeneous nature of the group. The Xenarthra cannot be divided in accordance 

 with vegetable or animal diet, but display a primitive simplicity very like that in the 

 Marsupials. Meckel's tract is lengthened, but the hind-gut remains conspicuously 

 long without well-marked differentiation into colic and rectal portions. Traces of the 

 primitive paired condition of the caeca are even more conspicuous than amongst 



