INTESTINAL TRACT OF BIRDS. 271 



various correlations with other organs and with external conditions established during 

 embryonic, youthful, and adult life. In the case of the alimentary canal we know just 

 enougii, from the transforming experiments of Hunter and others, to feel certain that 

 a large part of the final structure is determined by the conditions in which the hereditary 

 material is grown. The phylogenetic tree of an organ such as the alimentary tract may 

 be little more than the exhibition of a long series of experiments in growing similar or 

 identical " germs " in different culture media. And, finally, we know nothing as to how 

 far these hereditary masses, as they were marshalled into the actual branches of the 

 actual phylogenetic tree of the organisms that contained them, carried within themselves 

 historical limitations or determinations towards further development in specialized 

 directions. 



Such questions, however they may seem remote from an actual study in comparative 

 organogeny, in reality lie at the root of every attempt to use characters in classification : 

 if by classification there is implied more than the convenient disposal of particular 

 animals in particular pigeon-holes according to their possession of obvious anatomical 

 characters. The study of characters by themselves and for themselves must precede 

 the attempt to use these characters in genealogical classification. 



In the study of characters, the first proposition is whether they are primitive or 

 modified, archecentric or apocentric. So far as the subject-matter of this paper is 

 concerned, I have tried to show that for Aves the presence of a specialized duodenal 

 loop, of a Meckel's tract consisting of a nearly circular expanse of mesentery with a 

 simply convoluted gut suspended at its periphery and symmetrical about a median vein 

 running towards a vestige of Meckel's diverticulum, of a pair of colic Cfeca with free 

 lumen of moderate length and with walls partly glandular and partly absorbing, and of 

 a rectum relatively long, are archecentric characters. When birds possess many or even 

 all of these characters, we are tempted to say, but we cannot say definitely, that they are 

 closely akin. The retention by some descendants of a common stock of an ancestral 

 heritage in the ancestral form does not imply that such members are more closely akin 

 than other members that have dissipated or improved the common heritage. I have 

 shown that in all the great accepted groups of birds there are some members with the 

 archecentric condition. 



The second proposition that may have to be made regarding characters is that they 

 are modified from the archecentric condition, or apocentric,- and in this modification 

 degrees of apocentricity may be exhibited. I have shown that the lengthening and 

 twisting of the duodenum, the appearance of asymmetry in Meckel's tract and its produc- 

 tion into variously-formed and variously-placed loops, the special formation of the loops 

 described as supra-csecal kinks and supra-duodenal loops, the great lengthening of the 

 colic cseca or their reduction to vestiges or to short compact glands, and the shortening 

 and straightening of the rectum *, are apocentric characters. That two birds possess 



* Since writing this memoir, I have been interested by noticing that Professor Metschnikoff attributes many of the 

 digestive troubles of man to inheritance of a long rectum. The higher birds of every group apparently have passed 

 through a similar condition, but have succeeded in very greatly reducing the length of the rectum. P. C. M. 



40* 



