216 ME. p. CHALMERS MITCHELL 0]V SO-CALLED 



primitive and what are secondary. In different groups certain 

 structures are on the "wane, others are in process of development, 

 and such general changes appear to characterize whole groups ; 

 the complete assemblage of related animals appears to be moving 

 in the same direction ; the same structures appear to be in 

 process of advancement or of degeneration, so that the special 

 characters of the whole group tend to become more and more 

 accentuated. In this general progress, individual species or 

 genera may advance specially rapidly or may lag behind ; and 

 those which are most or least advanced in the direction of the 

 whole group are by no means necessarily most closely related to 

 one another, although in the structures most affected by the 

 general course of the changes, strong convergent resemblances 

 result. In the case of pigeons for instance, there are anatomical 

 considerations by which one may consider particular individuals 

 or species more or less " pigeony '' pigeons (to coin a convenient 

 inelegancy), and the species which are most " pigeony " are not 

 specially related one to the other. For some time I have been 

 engaged on the anatomy of this group with the special view of 

 tracing such progressive advances and degenerations. The work 

 involves dissection of many hundreds of individuals, and it will 

 be long before I am in a position to publish final results. There 

 are many sources of error, some of which can be eliminated only 

 by comparison of the anatomy of many individuals of the same 

 species and the same variety. I have for the present purpose 

 drawn on my notes only for such points as appear to me to be 

 unusually clear; and in this way, although I leave out many 

 features which I have no personal doubt will prove of interest, 

 I gain in immediate certainty and brevity. The general con- 

 clusion to which I come is that the eutaxic forms display a 

 number of anatomical features which show them to be well to 

 the front among pigeons generally in the progressive changes for 

 which evidence is to be found among pigeons — that, in fact, they 

 are more " pigeony " than their diastataxic allies. I do not mean 

 that every eutaxic pigeon exhibits every progressive advance or 

 degeneration more- notably than every diat^tataxic form, but that 

 on the Avhole they do exhibit such changes in an unusual degree. 

 In the argument which I am attempting to develop there is 

 the apparent flaw that advance in one direction is not necessarily 

 associated with advance in other anatomical structures. It is 

 a familiar condition to find extreme specialization in certain 

 directions associated with extremely primitive conditions in other 



