178 DR. P. CHALMERS MITCHELL ON THE 



VALUATION AND NOMENCLATURE OP CHARACTERS. 



In the description of the structure of an organ or anatomical part as it occurs in a 

 large series of different forms, it becomes necessary from simple convenience to attempt 

 some kind of valuation according to which the series of facts shall fall into definitely- 

 named groups. When there is attempted the difficult passage from descriptive anatomy 

 to morphology, it is necessary that the valuation and nomenclature should be in relation 

 to the theory of descent with modification. I assume that birds were monophyletic in 

 origin, and that the existing forms have branched out in diverging directions from the 

 ancestral group. The members of this ancestral group, at the stage when they first 

 might have been called birds, possessed an heritage of characters and tendencies, and these 

 characters and tendencies have undergone modifications different in amount and nature 

 in the different groups. The first business is to come to a decision as precise as possible 

 as to the ground-plan, or archetype, the most ancestral condition of the structures under 

 consideration. In the present case, I find that the condition of the gut in Palamedea 

 (fig. 1) may be taken (after allowance for its length, as it is a large bird and vegetarian) 

 as representing closely the ancestral type. The form of the gut is extremely simple ; 

 it is distinguished from the intestinal tract of reptiles chiefly by the fact that the three 

 divisions the duodenum, Meckel's tract, and the rectum are sharply marked off one 

 from the other. Meckel's diverticulum, the morphological median point of Meckel's tract, 

 occurs nearly at the actual median point. The pair of ca?ca are of moderate length and 

 are functional. The arrangement of the veins is also extremely simple. It will be seen 

 in the systematic part of this memoir that it is not difficult to refer the more complicated 

 types of arrangement of the tract to the condition in Palamedea. I propose to call such 

 a condition " archecentric," implying that it represents a primitive, ancestral, or central 

 condition, from which the conditions to be found in the other cases have diverged. It 

 is obvious that the possession by two or more groups of birds of a character in its 

 archecentric form cannot be an indication in itself that these groups are more closely 

 related to one another than they are to groups possessing the character in another form ; 

 for if the diagnosis of archecentricity be correct, the condition has been present in all 

 birds, and may be retained by any. For example, I have recently (28, 29) endeavoured 

 to show that the condition in the wing known as diastataxy is archecentric ; that is to 

 say, that in the ancestral wing there was a gap in the series of quills proximad of the 

 fourth secondary quill. If that be correct, the fact that two groups of birds possess 

 diastataxic wings is no reason for uniting the groups. 



When the ancestral condition is modified, it may be regarded as having moved out- 

 wards along some radius from the archecentric position. Such modified conditions I 

 propose to call " ap^centric." Again, it must be obvious that the mere apocentrioity of 

 a character can be no guide to the affinities of its possessor. For instance, in the work 

 on the wing of birds, to which I have already referred, I tried to show that the condition 

 of the wing known as eutaxic is apocentric; that is to say, that it is a modification of 

 the archecentric condition, which in this matter is the condition termed diastataxic. 

 Before deciding as to the value of eutaxy in a natural classification, it would be necessary 



