200 MUSTELADiE. 



change takes place, by the actual change of colour in the 

 existing fur, is the true one. 



But what is the final cause of this curiovis phenome- 

 non ? What object connected with the well-being of 

 the subjects of it does it effect in their favour? One 

 object, undoubtedly, is the safety they obtain by the 

 concealment afforded them, by an approximation to the 

 colour of tlie earth's winter covering. The Ptarmigan, 

 the Alpine Hare, and many other mammalia and birds, 

 are all more or less liable to become the prey of rapacious 

 birds or quadrupeds, which are directed in the chase by 

 their sight. The mottled browns which form the prin- 

 cipal summer colours of these creatures, are well adapted 

 for their concealment amongst the brown heaths and fern 

 of the summer and autumn ; but such colours would 

 render them conspicuous by contrast amongst the snows 

 of winter. 



But this, though perhaps the most obvious, is not the 

 most important advantage gained by the assumption of 

 the white clothing in the winter season. It is too well 

 known to require more than an allusion, that although 

 the darker colours absorb heat to a greater degree than 

 lighter ones, so that dark-coloured clothing is much 

 warmer than light-coloured, when the wearer is exposed 

 to the sun's rays — the radiation of heat is also much 

 greater from dark than from light-coloured surfaces, and 

 consequently the animal heat from within is more com- 

 pletely retained by a white than by a dark covering ; 

 the temperature, therefore, of an animal having white fur, 

 would continue more equable than that of one clothed 

 in darker colours, although the latter would enjoy a 

 greater degree of warmth whilst exposed to the sun's 

 influence. Thus the mere presence of a degree of cold, 

 sufficient to prove hurtful, if not fatal, to the animal, is 



