PHYSIOLOGICAL 



417 



there is bound to be a zoned structure. The cross-bars on a hawk's 

 feathers express diurnal fluctuations in the blood-pressure during 

 the period of development, and the fish keeps its annual diary in 

 its scales. We cannot at present account for all the architectural 

 peculiarities that give rise to structural coloration, but in many 

 cases they are the ripple-marks of growth. We can thus under- 

 stand why there should be what may be called raw material avail- 

 able for secondary utilisation as adornment or disguise. We have 

 thus cleared the way for a study of the uses of colour. 



In certain cases a dark colour may protect the animal from the 

 glare of the sun, and this utility would not be in the least incon- 



FiG. 51. 



One of the Commonest Leaf -insects (Phyllium). From a specimen. The 

 front wings or tegmina (E) are extraordinarily leaf-like, and the legs (L) 

 have flat leaflet-like expansions which increase the protective resemblance. 

 The colour of the adult insect is green. 



sistent with the discovery of a physiological reason for the deposition 

 of an extra quantity of melanin in the skin. 



For a warm-blooded creature in very cold surroundings the 

 dress that conserves most of the precious animal heat is a coat 

 of white fur or white feathers, as in ermine and ptarmigan, but 

 this is not inconsistent with the discovery of a physiological reason 

 for the development of gas-vacuoles and the non-formation of 

 pigment in the suit of fur or feathers that is put on when winter 

 comes. Nor is it inconsistent with proving that the whiteness is 

 of protective value in making the ermine or the ptarmigan incon- 

 spicuous against a background of snow. 



Protective or cryptic coloration is certainly very common, but 

 its life-saving value should be proved, not simply assumed. Not 

 every case is so satisfactorily documented as that of the green 



VOL. I EE 



