228 ME. p. CHALMEES MITCHELL ON SO-CALLED 



•vrhile in more specialized forms the procoracoid process becomes 

 smaller, loses its connection with the clavicle, the latter deve- 

 lopiug an epicleideal plane which meets and may overlap the end 

 of the scapula. In fig. 6 I, a comparatively primitive condition, 

 as seen in Columha, and with minor details in most diastatasie 

 pigeons, is represented. The procoracoid is large and articulates 

 prominently with the clavicle, while the latter is not expanded 

 at its end and is well separated from the scapula. In fig. 6 IV, 

 the more advanced condition, as seen in a typical Passerine, is 

 represented. The procoracoid is much reduced, and does not 

 meet the clavicle. The clavicle is expanded at its end, and 

 meets and overlaps the scapula. Among the eutaxic forms, the 

 procoracoid is generally small and never meets the clavicle by a 

 broad articulation, but is either free from it, as in B (fig. 6, II), 

 a, C, and E; or just meets it above, as in A, D, and F 

 (fig. 6, III), the junction being in a different place and due 

 rather to the growth of the epicleideal plane than to size of the 

 procoracoid. The clavicle among the eutaxic forms is either 

 quite close to the scapula, on account of growth of the epicleideal 

 plane, as in B (fig. 6, II), or actually meets it, as in the other 

 eutaxic forms (fig. 6, III), Avhile in all the diastataxic forms 

 I have seen it remains separate. 



Summary of Anatomical Argument. 



When the anatomical dift'erences between eutaxic forms and 

 their diastataxic allies are compared, it appears that the eutaxic 

 forms are on the average distinctly more specialized. If there 

 is any general progi ess along special lines among pigeons, the 

 eutaxic forms are well advanced. Their anatomical features 

 app; ar to show that, if eutaxy be a derivative of diastataxy, it is 

 at least a striking coincideuce that in other respects eutaxic 

 pigeons are more specialized than diastataxic forms. I'or my 

 own part I am personally so much impressed by the evidence for 

 the gradual modification of a whole group in the same direction, 

 that I cannot avoid regarding the eutaxy of these pigeons as 

 being simply a part of their general specialization. 



Size and Eutaxy. — The eutaxic pigeons, like eutaxic birds 

 generally, are small compared with their allies. A, a, D, and E 

 are very small pigeons, B and C are moderately small, and E is 

 the only fair-sized bird. They are certainly smaller than the 

 sand-grouse, which would appear to be the nearest diastataxic 



