xn THE COLOURS OF BIRDS 261 



appearance, or it may spread out to form an apical 

 band which then gives the plumage a cross-barred 

 appearance ; of the two the first is the more primitive. 

 Beginning with these simple types of pigmentation 

 common to the downs both of the Limicolae and the 

 Turdidae, Hacker seeks to prove that the coloration 

 of the adult thrushes can be derived from this primi- 

 tive type by various processes, especially the in- 

 creasing importance of the secondary pigmentation. 

 Thus if the secondary pigmentation increase greatly 

 in importance, it may encroach upon the colourless 

 median area and, uniting with the primary apical 

 pigment, produce a uniformly coloured feather. 

 Again, the primary pigmentation may disappear, 

 and the colourless median area form a border to 

 the secondarily coloured feather, and so on. It is 

 unnecessary to carry the consideration of Hacker's 

 theories beyond this point. There is apparently no 

 doubt that the spotted appearance of the plumage in 

 young thrushes is a primitive condition, and the 

 nature of the pigmentation of the feathers in them is 

 therefore of great interest, but when the attempt is 

 made to derive more complex forms of marking 

 from these simple ones, there is great difficulty and 

 uncertainty. A point of some interest is the question 

 whether there is any relation between the structure 

 of special regions of feathers and the characteristic 

 pigmentation of these regions. It is at least certain 

 that there is much constancy in the association of 

 special types of colour with special regions of the 

 feather. The nature of the association we shall 

 consider in the next chapter in connection with the 

 colours of certain families of birds. 



