272 DE. P. CHALMERS MITCHELL ON THE 



apocentricity in an organ is no evidence of their affinity. In the first place, the apocen- 

 tricity may be the mere result of growing the same inherited " germs " in similar 

 culture media. I call such apocentricities multiradial, implying that they are the result 

 of similar conditions on the same plastic material. The lengthening of the whole gut 

 and the spiral twisting of portions of it, and in particular the lengthening of the cseca, 

 are plastic or multiradial effects and can have no direct bearing on affinity. The 

 extreme shortening and widening of the whole gut and the disappearance of the cseca, 

 or even their transformation to nipple-like excretory organs, are multiradial. The 

 production of a supra-duodenal loop and its retention after shortening of the long caeca 

 in connection with which it arose are multiradial apocentricities. These again give us 

 no clue to affinity. There is no reason to suppose that even in the actual phylogenetio 

 tree of birds, a branch the members of which now possess an organ with archecentric 

 character, may not have come from a branch the members of whicli now possess an 

 apocentric character in the same organ. For, in the first place, the apocentricity may 

 have arisen after the branching; and, in the second place, if the apocentricity be truly 

 plastic, the transmitted germs in another environment may grow only into the ancestral 

 form. It is probable, however, that apocentricities, even if multiradial, leave some 

 legacy of complexity in their simplified descendants, and such conditions of character I 

 have called pseudoceutric. The conclusion to which consideration of multiradial apocen- 

 tricity leads is that even if the phylogenetic trees based on the study of the intestinal 

 tract be absolutely correct, and if they be compared with equally correct trees based on 

 the examination and valuation of other characters, these trees may not coincide. 



Apocentricities, however, may be so definite;, so precise, or anatomically so complicated, 

 that they appear to imply a phylogenetic contraction of the range of variability in 

 respect to the structure in question. Such a demarcation of modification along a single 

 and definite radius I have called uniradial apocentricity. When further divergent 

 modifications occur on a single radius, there is formed what I have termed a metacentre, 

 and what seems to imply that the range of variability has been limited or defined, with 

 the result that future modifications all retain an indication of their more limited or 

 defined starting-point. I take it that the archecentre of the alimentary canal of birds 

 is a metacentre of the alimentary canal of the Sauropsida, and that, again, is a metacentre 

 of the alimentary canal of the whole vertebrate stock. I have tried to show that the 

 uniradial apocentricities of the intestinal tract of birds relate to the number and position 

 of the minor loops into which Meckel's tract is produced. 



The obvious use of the study of characters as regards classification is, then, the 

 valuation of characters as archecentric and apocentric, and the discovery among the 

 apocentricities of the uniradial modifications with their metacentres. When these have 

 been determined and valued, the characters have yielded all the material that they afford 

 for genealogical enquiry. When one set has been exhausted, recourse must be made to 

 another set. In other words, the work of the anatomist whose goal is the achievement 

 of natural classification, is the study of the definition and limitation of variation within 

 groups. There is, however, another and most important side to this valuation of 



