THE USES OF COLOUR 17 



colours do so with difficulty. For this reason black 

 clothes are most trying, and white most comfortable, 

 in the hottest weather. Conversely, a dark surface 

 readily parts with heat by radiation, while a white 

 surface retains heat far more effectually. 



A few writers had suggested that these principles 

 may explain the colours of certain animals, but the 

 question was first fully entered upon in Lord Wal- 

 singham's presidential address to the Yorkshire 

 Naturalists' Union in 1885. 1 The predominance of 

 dark varieties of insects and white varieties of birds 

 and mammals 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 maintain 

 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 summer 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 favourable conditions.' 



It would be fatal for the temperature of one of the 

 higher vertebrates " to sink a few degrees below the 

 normal, except in the case of certain species, such as the 

 dormouse, &c., which have the power of hybernating 

 in a dormant condition; such animals were once 



1 See Entomological Transactions of the Union for 1885. 



