228 NATURAL THEOLOGY. 



But, under the present division of our subject, 

 our business with feathers is as they are the cover- 

 ing of the bird. And herein a singular circum- 

 stance occurs. In the small order of birds which 

 winter with us, from a snipe downwards, let the 

 external colour of the feathers be what it will, 

 their Creator has universally given them a bed of 

 black down next their bodies. Black, we know, 

 is the warmest colour ; and the purpose here is, 

 to keep in the heat arising from the heart and cir- 

 culation of the blood.^" It is further likewise re- 

 markable, that this is not found in larger birds ; 

 for which there is also a reason. Small birds are 



^ When we attempt to apply the Hghts of experimental philo- 

 sophy to this subject, the inquiiy is not a little embarrassing. A 

 loose woolly texture, or down, as it implies the presence of air in 

 its interstices, air being a bad conductor of heat, is therefore a 

 warm covering ; it prevents the expenditure of animal heat. 

 When we consider the colour of the coverings of birds, we must 

 take new elements into our process of reasoning — we must re- 

 flect on the effects of the conduction and radiation of heat. The 

 conduction is the conveyance of heat ; and the radiation is the 

 parting with it into the atmosphere or into space. We have 

 already explained why the interior covering of the arctic bird 

 should be loose ; as to the colour, its effect must result from radia- 

 tion. It appears (to use the vulgar language) that the influence 

 of cold both on quadrupeds and birds is to increase their woolly 

 or downy covering, and, in many instances, to change the exterior 

 colour to white ; in other and more correct words, a provision is 

 made for changing their clothing so as to suit their altered cir- 

 cumstances. This change of colour corresponds with philoso- 

 phical experiments — a white surface absorbing the least, and 

 radiating the least, it should therefore tend to confine the vital 

 heat within the animal, and carry it off slowly to the atmosphere. 



