EXTERNAL FEATURES 41 



spots, a perfectly white tree-pipit, a white and greyish- 

 white sand-martin. A white coot with normal eyes has 

 been described ; the whole plumage was snowy white 

 except a few ashy feathers at the end of each wing. 



The putting on of white plumage in the ptarmigan 

 illustrates a constitutional rhythm corresponding to an 

 external periodicity ; but it is not to be supposed that the 

 cold of winter is directly responsible for making the new 

 feathers colourless. The cold acts as a stimulus to a pre- 

 established rhythm. The difference between new white 

 feathers and their coloured predecessors is that gas vacuoles 

 take the place of pigment granules, the whiteness being due 

 to the reflection of the light from a multitude of internal 

 surfaces, as in the case of foam. According to Metchnikoff 

 (1906), a coloured feather in ptarmigan and willow grouse 

 may be changed into a white feather by the activity of 

 wandering amoeboid cells (chromophagous) which transport 

 the pigment into the skin. 



An interesting little point is that when pigment is quite 

 absent (in albinos), the structural specialisation so frequently 

 involved in the production of colour effect is also absent 

 (Chandler, 19 16). 



Physiological Meaning oJE Pigments. — Haemoglobin has 

 obviously a fundamental physiological significance as 

 an oxygen-capturer and carrier, but its colour plays no 

 part in this significance. Haemocyanin which is also a 

 common oxygen-carrier in invertebrate types is faintly blue. 



It is possible that some superficial pigments of a dark 

 colour have a protective value against the ultra-violet rays 

 of sunlight. 



It is possible that some pigments are of value in relation 

 to the regulation of temperature in the body, just as the 

 absence of pigment in the winter plumage of the ptarmigan 

 gives that bird the dress physiologically most economical. 



It is certain that dark pigment is of use in making the 

 eye a dark chamber, and the changes in the pigments of the 

 retina have some direct significance. 



Apart from such suggestions, what can we say at present 



