172 THE LAWS OF ORGANIC 



There is no place to which these animals retreat 

 that is exempt from the great vicissitudes of sea- 

 sons. If they burrow in the ground, the seve- 

 rity of the cold is softened by their numbers ; but 

 if the degree of cold becomes less intense by 10 

 or 20, the warm atmosphere which they have 

 created will become warmer by the circulation 

 of more temperate air. 



CXCI. As the medium to which they are sub- 

 jected gradually improves in temperature, the 

 blood that was determined to the internal organs 

 is, by the most imperceptible steps, recalled to 

 the surface. The lungs are then enabled to pro- 

 duce a greater effect upon the diminished quantity 

 of the blood ; and as this is more stimulating to 

 the heart, it is propelled with greater force, and, 

 meeting with less resistance, on account of the 

 additional warmth from without, it is distributed 

 more equally throughout the system. While these 

 minute changes are going on, the generation of 

 animal heat is increased ; so that the situation to 

 which the animal is confined has its temperature 

 augmented. The external heat continues to act 

 upon the surface of the body and on the lungs, 

 respiration becomes more frequent and com^ 

 plete, the blood circulates with greater energy, 

 muscular exertion is excited, and these causes, 

 calculated to promote the same object, are progres- 

 sive in their influence ; and the various powers act 



