XII 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. 
