COLOEATION AFFECTED BY THE ENVIRONMENT. 73 



is, as in the feathers of many birds, the proximate cause 

 of the colour. The green of this beetle changes late in the 

 year to a dusky colour ; but the same effect is produced 

 by 'unusually frosty spring weather,' and is seen in insects 

 captured in high altitudes. 



Though the actual nature of the change does not appear to 

 be thoroughly understood, it is clearly to be set down to direct 

 effects of weather, and not to a regularly recurrent change 

 which has been fixed by natural selection. 



Experimentally the same change can be produced, according 

 to Dr. Kriikenberg, in the wing-cases of Cantharides by 

 boiling them in water, or soda, or hydrochloric acid. 



This cannot of course be compared with the effects produced 

 by natural temperatures, but it shows that even colours which 

 are not due to pigments can be affected by similar processes, 

 which possibly cause an alteration in the structure of the parts 

 coloured (see the reference to the " Moon Moth," on p. 60). 



Change of Colour in Arctic Animals. 



But the most remarkable changes of colour which have a 

 definite relation to environmental changes are shown by those 

 Arctic animals which become white at the approach of winter, 

 and regain their darker colour in the summer. 



Such a change, however, is not universally found among the 

 inhabitants of the polar regions. Among those in which no 

 such change occurs, some are white all the year round, while 

 others are- not white, but some other colour. The polar bear 

 is white all the year round, and so are the snowy owl, the 

 American polar hare, and the Greenland falcon. On the other 

 hand, the raven, the musk ox, and the glutton, undergo prac- 

 tically no change at the beginning of the Arctic winter. The 

 reindeer is somewhat intermediate between such extremes as 



