WINTER-SLEEPERS. Ill 



identity of the influence exerted by a reduction of temperature, 

 both in high latitudes and equatorial regions, is not to be dis- 

 puted. This consists, as we have seen, in the fact that every 

 reduction of temperature below the optimum, whether that 

 optimum be high or low, so diminishes the vital energy of 

 many, though not of all animals, that they gradually fall asleep, 

 and remain in a condition resembling sleep as long as the low 

 temperature, which induces that condition, lasts. 



In general, warm-blooded animals seem to be protected 

 against the effects of a reduced temperature by their power of 

 keeping up, by combustion, the warmth indispensable to their 

 existence. But they do not all escape its influence nevertheless. 

 Everyone who has lived for any length of time between the 

 tropics, and has observed the people there, knows that the natives 

 become excessively sleepy under a sudden and great diminution 

 of warmth ; and that the most inveterate winter- sleepers belong 

 to the warm-blooded mammals is equally well-known. It is in 

 no way surprising to learn that a cold-blooded snail, mollusc, or 

 frog becomes lethargic during the winter, because the warmth of 

 their bodies always exactly or very nearly corresponds with that 

 of the surrounding medium, air or water, and because they are 

 not fitted by their internal structure to produce such warmth as 

 is indispensable for the energetic vital action of their organs ; 

 hence they must gradually subside into sleep, i.e. a torpid 

 condition of life, as soon as the temperature of the air or water 

 falls below the point where their powers of assimilation begin. 

 But we may well feel surprise at seeing that a warm-blooded 

 animal, whose body is constantly maintained by internal 

 processes at the high temperature of from 36 to 38, 35 is 

 nevertheless incapable of resisting the lowering and soporific 

 influence of cold. With regard to this point a fact of the 

 greatest importance has lately been discovered. We have learned 

 from tolerably numerous observations that in several hybernat- 

 ing mammals, which have been examined with this view, the 

 temperature of the body is very considerably lowered during 

 their winter-sleep. The lowest temperature hitherto detected 

 in such a creature is 2 centigrade in the Zizel, Spermophilus 

 ci!il!ns, according to Horvath's researches. To this naturalist 36 



