18 THE COLOURS OF ANIMALS 



called 'warm-blooded,' but are now more correctly 

 termed ' homothermic,' because it is the constancy of 

 the temperature which is so important, and which 

 must be maintained whether the surrounding medium 

 be colder or warmer than themselves. Other animals 

 with an inconstant temperature are now correctly 

 called ' poikilothermic ' rather than ' cold-blooded.' 



Lord Walsingham's conclusions appear to be sup- 

 ported by the fact that young dark-coloured cater- 

 pillars, like those of the Emperor Moth (Saturnia 

 carpini), or Tortoiseshell Butterfly (Vanessa urticce), 

 seek the light side of a glass cylinder, and always 

 change their position when the cylinder is turned 

 round. The question needs further investigation, 

 and much might be learnt by interposing various 

 screens between such larvae and the light, thus cutting 

 off different sets of light-waves. 



The most important support to the hypothesis is 

 found in an experiment made by Lord Walsingham, 

 in which several Lepidoptera of different colours were 

 placed on a surface of snow exposed to bright sun- 

 shine ; in half an hour the snow beneath the darker 

 insects showed distinct signs of melting, but no effects 

 were seen beneath the others. The differences were 

 further brought out in the course of two hours, when 

 the darkest insect of the lot, a black Geometer, the 

 Chimney-Sweeper (Odezia charophyllata) , ' had de- 

 cidedly won the downward race among them.' 



It is therefore certain that the absorption of 



