COVERING OF ANIMALS. 271 



tible change in the rapidity of its growth. The moulting or 

 casting of their plumage in birds furnishes examples to the 

 same point. 



' Changes of color are not so common or so important. 

 But in cold climates there are many striking instances of a 

 complete change from a dark color to a white, both of fur 

 and of plumage. The summer dress of the Alpine hare is of 

 a tawny gray ; but, as winter approaches, it changes to a 

 snowy whiteness, continues so until spring, and then resumes 

 its tint of gray. The ermine, which in summer has a fur of 

 a pale reddish brown, exhibits in winter a dazzling white. 

 Among birds, a similar change takes place in a great num- 

 ber of species, as in a variety of the smaller kinds known 

 familiarly under the name of the snow-birds. 



' It is obvious, according to the known laws of the trans- 

 mission of heat, how both these provisions concur towards 

 maintaining, during the winter, a proper temperature in the 

 bodies of animals. Animal heat is maintained, not by the 

 influence of external causes, but by an internal principle. 

 The object to be attained is, then, to prevent the escape of 

 this heat to other bodies, and preserve it within the system. 

 The increased thickness of the fur effects this purpose by 

 obstructing its gradual transmission to the cold bodies around, 

 and its color, by diminishing the degree of radiation, which 

 is always less from light-colored than from dark substances. 

 Upon the same principle, under ordinary circumstances, the 

 thin covering and dark color are favorable to the comfort of 

 the animal during summer, since they serve to keep down its 

 temperature by carrying off all superabundant heat, both by 

 gradual communication and by radiation.* 



* The second method by which animals avoid the danger 

 and suffering to which they would be exposed by the extremes 



* ' The uncomfortable sensation of heat in summer arises not directly from the 

 external heat, which is seldom so high as that of our bodies, but rather from the 

 animal heat of the system itself, which is prevented by the high temperature of the 

 atmosphere from being carried off as rapidly as usual, and hence becomes, as it 

 were, accumulated. Whatever circumstances, therefore, favor either the radiation 

 or transmission of heat, will contribute most to comfort ; and consequently a dark 

 and thin covering, and one which is a good conductor, would seem to be most proper 

 both for men and for other animals, under ordinary circumstances. Where there is 

 exposure, however, to the rays of the sun, the reverse would be true ; and this 

 appears at least not to contradict experience. If these remarks are well founded, they 

 ibviously explain how the color of the negro is adapted to the regions he inhabits, 

 y tavoring the radiation of r.eat whenever the temperature of the air is below 

 "at or 'lis body. And even m the depths of Africa, it is seldom that the ther- 

 .nometv r will, for any lenU> cf time, indicate a degree of heat above that of om 

 bodies.' 



