EVOLUTION OF THE COLORS OF BIRDS. 135 



this change does not appear to be confined to the tail, 

 but the black patches of the head and throat seem to be 

 developed in the same manner. Thus figure 11 of 

 Plate I shows a feather from the throat of Icterus 

 cucuUatus nelsoni which is changing from yellow to 

 black. Figure 12 represents a feather from the back of 

 the head of I. hullocki, taken from an immature male of 

 the second year, in which the black was appearing in 

 spots upon a field of yellow. 



It is impossible to say without more complete investi- 

 gations, and perhaps some experiments upon living 

 birds just how general this change by increase or de- 

 crease of pigment is, but it is probably quite a limited 

 mode of transition. 



The fourth mode of change, i. e., by the wearing off 

 of the dull tip of the feather, leaving the bright under 

 part exposed, is a very common one, among land birds. 

 Dr. Coues has called my attention to a term in the Cen- 

 tury Dictionary covering these two classes of change — 

 aptosochromatism, which is defined as change of color 

 of the plumage without loss or gain of any feathers. 

 He very kindly suggested two new words to indicate 

 these two forms of aptosochromatism; acraptosis, mean- 

 ing the falling off of the tip of the feather, and acropto- 

 sis, implying that the tip is not cast. Plate I shows a 

 number of acraptotic feathers. Figure 6 is a frontal 

 feather of the western yellow-throat {Geothlypis trichas 

 occidentalis) in winter plumage. The basal portion of 

 the feather is much darker than in most feathers, in fact 

 almost black, the median portion a clear ash, and the 

 tip brown. This brown terminal third is inclined to be 

 worn and very brittle. It is so broad that it completely 

 conceals the under portion, but in spring it all crumbles 

 away leaving nothing but the clear ash. Along the line 

 of demarkation of black and ashy the black base is 



