336 THE BIOLOGY OF THE SEASONS 



temperature ceases to be constant, and follows the fluc- 

 tuations in the external world. Meanwhile the creature 

 sleeps. Should the external temperature fall too far, the 

 sleeper may never awaken, or it may waken suddenly 

 and make violent efforts to get warm before it dies. 



Horvath made the interesting and important observa- 

 tion that a zizel, Spermophilus citillus, lying in its winter 

 sleep, always has nearly the same temperature as the 

 surrounding air. " In one case the temperature of the 

 room was 2 C. above zero, and a thermometer inserted 

 in the rectum marked exactly the same degree ; in another 

 experiment the animal was sleeping in a room at about 

 9 to 10 for several days, and its body (in the rectum) was 

 at 8-4. This shows that during their Winter sleep warm- 

 blooded animals become truly cold-blooded ; at any rate 

 this is true of the zizel, since its temperature corresponds 

 with that of the surrounding atmosphere/' l 



4. In his remarkable work on The Natural Conditions of 

 Existence as they affect Animal Life (1881), Professor Karl 

 Semper called attention to the value of Horvath's studies 

 on hibernating mammals, referring to two in particular: 

 "It is usually supposed that the awakening of winter- 

 sleepers is occasioned by a rising temperature ; but in 

 Horvath's investigations this was never the case ; during 

 two hours and forty-five minutes, which, in the one 

 experiment communicated, were needed for complete 

 awakening, the temperature of the room remained exactly 

 the same 10 C. as during the three previous days when 

 the animal was still asleep. This proves that the waking 

 up must be caused by some internal cause which we do not 

 yet know." 



"But his other observation is far more remarkable; 

 namely, that during the awakening, the body-temperature 

 1 Semper 's Animal Life, p. 112. 



