84 DWIGHT 



the old feather is pushed out by the new, so to speak, is a 

 matter for microscopic study and a subject by itself, but it usually 

 falls when the follicle of the new is barely visible to the naked 

 eye as a bluish spot beneath the skin. 



The systematic replacement of areas of feathers shows most 

 obviously in the wings where not only do the remiges fall out 

 one after another in definite sequence and almost synchronously 

 from each wing, but the greater coverts are regularly replaced 

 before the fall of the secondaries beneath them, the lesser coverts 

 before the median and even in the rows of the lesser coverts 

 alternation seems to be attempted. Furthermore the under wing 

 coverts are usually replaced after the moult of the upper surface 

 of the wings is completed (regularly so in young birds) the row 

 nearest the quills of the remiges following the more distant. On 

 the body the protective sequence is less obvious, but the moult 

 regularly begins at fairly definite points in the feather tracts 

 radiating from them in such manner that the outer rows of 

 feathers where the tracts are widest and the feathers of their 

 extremities are normally the last to be replaced. The tail 

 coverts, too, precede the rectrices which fall on either side in 

 pairs, the outer protecting in a measure the inner ones. If this 

 sequence is borne in mind many supposed discrepancies will 

 nicely adjust themselves, and exceptions will be individual and 

 in no wise mar an evident and far reaching plan of moult. 



The important part that the blood-supply plays in this plan 

 appears to have been quite overlooked, nor have I had oppor- 

 tunity to fully investigate it. I may say, however, that the 

 radiation of the moult from given points corresponds very closely 

 to the distribution of the superficial arteries, beginning where 

 the main trunks come to the surface and ending with their ulti- 

 mate ramifications. 



Advance of Moult in the Feather Tracts. 



Reference to plate III, will give some idea of how the pterylce 

 or feather tracts are distributed in a Passerine bird. The subject 

 photographed is a young American Robin (J^lerula migratoria) 



