INTESTINAL TEACT OF BIRDS, 181 



of a large bird, or a graminifcrous or piscivorous bird, or lengthened in tlie case of a 

 frugivorous bird. In more general terms : when we Jire satisfied that an apocontricity 

 is multiradial, as is certain!}^ the case when it is homoplastic, we must neglect it when 

 we are dealing with the one character as a guide to affinity (fig. 2). 



Pig. 2. 



v-/ 



MfTACEMrfif 



MULTIRADIAI 

 APOCENTRIC 



PSnJDOCENTRiC 



\ 



ARCHECENTR/C ; 



Diagram to explain Nomenclature of Characters. 



A complex npocentric modification of a kind that we cannot well expect to be repeated 

 independently, and that may be designated as uniradial, must be the most certain guide 

 to affinity. It happens frequently that such a modification forms a new centre aroimd 

 which new diverging modifications are produced. Such a centre I propose to call a 

 " Metacentre," borrowing a convenient term from physics. It is obvious that the 

 condition of a character, archecentric so far as the whole group of birds is concerned, is 

 metacentric with regard to the common stock of birds and reptiles, and that the trans- 

 formation of an apocentric character into a metacentre is simply an event in the general 

 process of divergent evolution. I justify the nomenclature which I am proposing 

 largely because it brings the valuation and classification of characters into line with our 

 conception of the general process of evolution. 



Finally, there remains to distinguish a form of apocentricity extremely common and 

 often perplexing. Such conditions are marked by an apparent simplicity that, however, 

 reveals its secondary nature by some small and apparently meaningless complexity. 

 Such a condition that mimics the archecentric condition but which can be distinguished 

 from it, I propose to call '" Pseudocentric." 



I trust that the ideas underlying this attempt at the valuation and nomenclature of 

 characters, so far from being novel, are merely a codification of criteria in common 



SECOND SERIES. — ZOOLOGY, VOL. VIII. 29 



