42 THE BIOLOGY OF BIRDS 



save that pigmented substances may be waste-products, or 

 reserve-products, or by-products of the bird's metaboUsm ? 



The External Uses of Colour, — Without admitting that 

 it is incumbent on the naturalist to show the utility of 

 all the details of coloration, we are free to recognise that 

 colour may often be of use. Let us first of all collect 

 the utilitarian suggestions in one paragraph, (i) Coloration 

 may give the bird a garment of invisibility, of special value 

 during incubation or when feeding on the ground. It may 

 break up the bird's body so that it becomes a difficult 

 target for the hawk. (2) Coloration may assist in the 

 recognition of kin by kin, of the parents by the young, 

 or of one sex by the other. (3) Rather different is Mr. 

 Pycraft's suggestion that the bright colours sometimes 

 seen around or in the mouths of nesthngs may serve as a 

 guide to the parents when rapidly feeding the young. 

 (4) Coloration may be a factor in the ensemble of attractive- 

 ness that stimulates sex-interest and sex-excitement, and 

 makes pairing more effective. 



It is on general grounds probable that there are primary 

 physiological reasons for certain types of pattern in plumage. 

 Just as the lie of the hair in particular parts of different 

 mammals is probably dependent to begin with on the 

 conditions of muscular strain, blood supply, and innervation 

 in different areas of the skin, so it may be with the lines 

 along which feathers develop in birds. But this is not incon- 

 sistent with adaptive variations within the prescribed limits 

 being established by selection, whether natural or sexual. 



Similarly, in regard to the deposition of pigment, there 

 may be physiological conditions which determine centres 

 of pigment formation or deposition in the skin, while 

 adaptive variations within the prescribed limits are estab- 

 lished by selection. Colour patterns in birds have been 

 studied by Glover M. Allen (i9i4),who lays stress on the 

 idea of centres from which pigment formation begins and 

 spreads. A reduction in the area covered by any of these 

 primary patches results in a white mark at the line of junction 

 with another, constituting a " primary break." The patches 



