112 THE INFLUENCE OF INANIMATE SURROUNDINGS. 



we owe, too, tte far more interesting observation, that the Zizel, 

 when lying in its winter sleep, always has the same, or nearly 

 the same, degree of warmth as the surrounding air. In one case 

 the temperature of the room was 2° above zero, and a thermo- 

 meter 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. Other facts proved by 

 Horvath's elegant experiments, tM# of which the interest is too 

 specially physiological for mention here, I have quoted in 

 Note 36, for they appear to me to deserve to be more widely 

 known than they have hitherto been. 



A second point of more general interest is the great power 

 of resistance to extreme cold manifested by many animals, and 

 their power of enduring even to be frozen up without injury to 

 their vital functions. In this respect the cold-blooded have 

 certainly the advantage over the warm-blooded races. Man is 

 compelled to supply his deficiency of natural protection against 

 severe cold in the most various ways, or to borrow them of 

 furred quadrupeds ; a rabbit will infallibly die if the heat of 

 its body is reduced to 15° centigrade. Thus this rodent, whose 

 normal body-temperature is about 31° to 32°, needs only the 

 comparatively insignificant reduction of sixteen degrees to put 

 an end to its life ; and it is to be assumed that, with the 

 exception of the winter-sleepers, the same would be the case 

 with all mammals. But it is difierent with the cold-blooded 

 animals. Their body-temperature, as we have seen, is always 

 exactly or nearly the same as that of the surrounding medium, 

 and rises and falls with its variations.^^ But their powers of 

 resisting a considerable amount of cold are nevertheless very 

 various, and even the same individual, at different stages of 

 its development, differs greatly in this respect. Erogs and 

 toads can bear to be frozen up, or even endure a degree of cold 

 approaching the freezing point, only when fully grown ; many 

 fishes, as the salmon, can do so, both as embryos in the egg 



