53 6 THE POPULAR SCIENCE MONTHLY. 



the colors of certain animals, but the question was first fully 

 entered upon in Lord Walsingham's presidential address to the 

 Yorkshire Naturalists' Union in 1885.* The predominance of 

 dark varieties of insects and white varieties of birds and mam- 

 mals in northern latitudes is connected with these facts. "Birds 

 and animals living through the winter naturally require to retain 

 in their bodies a sufficient amount of heat to enable them to main- 

 tain their existence, with unreduced vitality, against the severities 

 of the climate. Insects, on the contrary, require rapidly to take 

 advantage of transient gleams of sunshine during the short sum- 

 mer season, and may be content to sink into a dormant condition 

 so soon as they have secured the reproduction of their species ; 

 only to be revived in some instances by a return of exceptionally 

 favorable conditions." 



It would be fatal for the temperature of one of the higher ver- 

 tebrates to sink a few degrees below the normal, except in the 

 case of certain species, such as the dormouse, etc., which have the 

 power of hibernating in a dormant condition ; such animals were 

 once 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 supported by 

 the fact that young dark-colored caterpillars, like those of the 

 emperor moth (Saturnia carpini), or tortoise-shell butterfly ( Va- 

 nessa urticai), 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 learned 

 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 Lepidop- 

 tera of different colors were placed on a surface of snow exposed 

 to bright sunshine ; 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 chcerophyllata) , "had de- 

 cidedly won the downward race among them." 



It is therefore certain that the absorption of radiant heat is 

 favored by the dark colors of northern insects, and it is in every 

 way probable that they are benefited by the warmth received in 



* See Entomological Transactions of the Union for 1885. 



