Vol. xliii.] 146 



the arm, including the one opposite the gap, are a little 

 longer than the following ones and are inserted a little lower 

 down ; while the first five corresponding under coverts are a 

 little smaller than the following ones and inserted a little 

 higher up. Whether or not all diastataxic wings show 

 this condition or not, it is enough at present that some 

 certainly do. 



" This state of things may be explained by supposing that 

 there was, very early in the development of the wing, a dis- 

 placement of these feathers next the wrist in every row, so 

 that they were moved up a notch, as it were, into the next 

 row, as if by a twisting of the skin in which they were 

 inserted. Such a twisting, displacing five feathers from 

 the row forming the hinder edge of the wing, so that they 

 stood in the next row above and had their place supplied by 

 four from the next row below, would, when this edge-row 

 became developed into the large remiges and the adjacent 

 rows took the subordinate station of coverts, bring exactly 

 the arrangement called diastataxy. 



'^The supposed twisting of the skin might be brought 

 about by the tension due to the new kind of horizontal 

 flexion of the wrist-joint when the forward extremity began 

 to be used as a wing. 



" The explanation of diastataxy formerly put forward by 

 Pycraft likewise supposed a displacement of the feathers in 

 the rows, but in a different direction, and failed to account 

 for the slight difference in size and in position observable in 

 the five feathers of each row of coverts nearest the carpal 

 joint. But this fact as well as the fact of the gap in the 

 remiges and the extra coverts opposite to it are all neatly 

 explained by the twisting or displacement in the manner in 

 which Steiner imagines it ; and a reasonable cause for the 

 twisting, in the tension of the skin as explained, is supplied 

 also. 



" This diastataxic condition is the primitive one. Steiner 

 saw evidences of it in the wing of ArcJiceopteri/x. It was 

 established before the Avian tree had any branches. Eutaxy 

 is the later condition, and may be looked upon as a process 



