98 THE COLOUES OF ANIMALS 



a state of efficiency. This consideration is better 

 deferred until after the probable nature of the 

 mechanism has been discussed. 



The physical cause of the change of colour 



It is now necessary to inquire into the actual 

 physical cause of the change in appearance. It has 

 already been explained that the dark colour depends 

 upon absorption, while the whiteness depends upon 

 scattering of light. The former is occasioned by 

 pigment granules, the latter by included gas bubbles. 

 When the latter are sufficiently abundant, the hair 

 becomes white in spite of the pigment ; if then the 

 gas were absorbed the dark colour would be restored.^ 

 It appears to be well authenticated that in certain 

 cases patches of human hair have become white 

 during some nervous attack, again becoming dark at 

 its cessation. Such changes can be explained by the 

 evolution of gas (probably carbon di-oxide) at the base 

 of the hair, and its subsequent absorption (probably 

 by some alkaline fluid). It is therefore probable that 

 the nervous system can so modify the processes taking 

 place in the cells at the base of the hair as to cause 



^ This explanation only applies to the existing dark hairs "which 

 become white. It is very improbable that any pigment exists in the 

 new hairs which make up the great part of the winter coat. Hence, 

 in testing the explanation offered above, the hairs must be selected 

 with the greatest care, and the investigation should be conducted in 

 connection with an experiment like that of Sir J. Koss (see pp. 94-96). 



