20 PHILOSOPHY OF ZOOLOGY. 



ably lower than their own, the black animal will give out 

 its heat by radiation to every surrounding object colder 

 than itself, 'and speedily have its temperature reduced; 

 while the white animal will part with its heat by radiation 

 at a muck slower rate. The change of colour in the dress 

 of animals is therefore suited to regulate their temperature 

 by the radiation or absorption of caloric. 



While it is requisite that the temperature of some species 

 should be preserved as equably as possible, the cooling ef- 

 fects of winter are likewise resisted by an additional quan- 

 tity of heat being generated by the system. An increase 

 in the quantity of clothing takes place to prevent that heat 

 being dissipated by communication with the cold ob- 

 jects around, and the dress changes to a white colour, 

 to prevent its loss by radiation. In summer, the pernicious 

 increase of temperature is prevented by a diminished secre- 

 tion of heat or the secretion of cold, increased perspiration, 

 the casting of a portion of the winter covering, and by a 

 superior intensity of colour in the remainder giving it a 

 greater radiating power. This last character would, in the 

 sunshine, by absorbing heat, prove a source of great in- 

 convenience, were its effects not counterbalanced by other 

 arrangements, and by the opportunity of frequenting the 

 refreshing shade, or bathing in the stream. 



In those cases, where particular parts only of the cloth- 

 ing change their colour, there are probably local circum- 

 stances connected with the secretions, or the sexual system, 

 which render such arrangements necessary. Hair growing 

 from a part which has been wounded, is always paler colour- 

 ed than that which is produced on the sounder parts, inti- 

 mating the operation of local causes on the colouring secre- 

 tions, or local purposes to be served by the change. 



It is probably for the purpose of preventing a wasteful 

 dissipation of the heat of the system, that the dress of many 



