INTESTINAL TRACT OF BIEDS. 181 



of a large bird, or a graminiferous or piscivorous bird, or lengthened in the case of a 

 frugivorous bird. In more general terms : when we are satisfied that an apocentricity 

 is multiradial, as is certainly the case when it is homoplastic, \ve must neglect it when 

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



Fig. 2. 



V-< 



MfTACENTRE .; r * 



" \ 

 MULTIRADML ' 



APOCENTRIC 



PSEUOOCENTRIC 



rUNIHADIAl 

 ' APOCENTRIC 



I ARCHECENTR/C I 



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 around 

 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 



