EXTERNAL FEATURES 43 



include a median crown patch, and five paired patches — 

 corresponding to the ear, neck, shoulder, side, and rump. 

 These are physiologically independent of each other, and 

 may have different colours. He claims that in some wild 

 species the formation of these white markings may be seen 

 in gradual progress, complexifying the pattern. The 

 converse of this centrifugal pattern-making is also to be 

 seen, as in the special pigmentation of extremities. One 

 cannot go far at present with these theories, but that is no 

 reason for not thinking over the physiological conditions that 

 may underlie the heterogeneous pigmentation of different 

 parts of the body. 



The general point we wish to make is this : there is 

 reason to believe that the famiUar cross-barring of the 

 feathers of some hawks is primarily related to a diurnal 

 fluctuation of blood-pressure. When the feather is a- 

 making and pigment is being deposited in the cells, the 

 fluctuations in the blood-pressure result in the bars, which 

 are the ripple-marks of growth. But this is not contradictory 

 to the idea that there might be an adaptive advantage in the 

 cross-barring, such as some would find in the cuckoo's 

 breast feathers being rather hawk-like. 



The interpretation of distinctive markings affords 

 attractive opportunity for biological ingenuity, but it is not 

 easy to win conviction except in cases where the field 

 observations of competent and critical naturalists are unani- 

 mous. A good instance of uncertainty may be found in 

 the case of white markings, (a) Sometimes in relation to 

 darker upper parts a white area may make a bird very in- 

 conspicuous, and we say " concealing coloration " or 

 other words to that effect, (b) Sometimes the white patch 

 is flaunted, exposed in flight, and we say " a sight clue for 

 kin." (c) Sometimes, as in the wheatear, the white patch is 

 very much in the rear, so that, as we say, the hawk's thrust 

 grips only feathers, {d) Sometimes the whiteness is all 

 underneath, and is not much seen except when the male 

 makes a banner of his wings within the range of his desired 

 mate's vision, and then we say " preferential mating " or 



