26 o 
COLOUR IN NATURE 
CHAP. 
paper on the subject, adopts the view that longi¬ 
tudinal striping is the most primitive condition, that 
this tends to develop into a spotted condition by 
the suppression of portions of the stripe, and that 
the fusion of spots gives rise to cross-striping. 
Kerschener, on the other hand, regards cross-striping 
as the primitive condition from which spots are 
derived. In point of fact, the distinction is perhaps 
less important than it seems, for Hacker s conception 
of waves of pigmentation passing down the shaft 
might equally be regarded as resulting in longi¬ 
tudinal striping, or in a very primitive form of cross¬ 
barring. Hacker’s observations were made chiefly 
upon nestlings of thrushes and chats (Tuididae and 
Saxicolinse), and also upon certain of the Limicoline 
birds. His researches lead him to regard the most 
primitive form of colouring as that seen in some of 
the downs of the Limicolae, such as Podiceps rubri- 
collis, where there is merely a little pigment collected 
at the tip of an otherwise colourless down. This is 
his primary pigmentation. Most downs, however, 
show, on the other hand, in addition to this terminal 
patch of pigment, a basal pigmented area due to the 
process of secondary pigmentation. In this way is 
produced the characteristic appearance of the feathers 
of young thrushes, where there is a pigmented downy 
area, and then a clear colourless area defined by a 
terminal pigmented band. Besides occurring in the 
young of the thrushes and their allies, this type of 
coloration is found in the adults in the simplest 
feathers, such as those of the cheeks, the chin, etc. 
The primary pigment may form a dark spot at the 
apex of the feather, giving the plumage a spotted 
