68 AMPHIBIA CHAP. 



variable; 0/409, equable) are based upon a sounder principle, 

 but are likewise liable to exceptions. Those creatures which, 

 like Birds and Mammals, possess a specific temperature of their 

 own under normal conditions, that of hibernation being excepted, 

 are homothermous. Cold-blooded creatures have no specific 

 temperature ; they more or less assume that of their surround- 

 ings. Frogs and newts, for instance, when living in the water, 

 naturally assume its temperature, which is, of course, many 

 degrees lower in a cold spring than in a shallow pond warmed by 

 the sun on a hot summer's day. The same applies to the 

 changes from day to night. Dark-coloured tortoises basking in 

 the sun are sometimes so hot that they are disagreeable to 

 touch, since they possess but little mechanism for regulating 

 their heat. The same individual cools down during a chilly 

 night by perhaps 40 C. Anura are, however, very susceptible 

 to heat ; most of them die when their temperature rises to about 

 40 C. Under such conditions they die quickly when in the 

 water, but in the air their moist skin counteracts the heat, 

 lowering it by evaporation ; otherwise it would be impossible for 

 a tree-frog to sit in the glaring sun in a temperature of 120 F. 

 Toads and others with drier skins seek the shade, hide under 

 stones, or bury themselves in the coolest spots available, and many 

 Amphibia and Reptiles aestivate in a torpid condition during the 

 dry and hot season. Many of them can endure a surprising 

 amount of cold, and during hibernation their temperature may 

 sink to freezing-point. This power of endurance does not apply 

 to all alike ; tropical species can stand less than those which live 

 in temperate and cold regions. In spite of many assertions to 

 the contrary, it may safely be stated that none of our European 

 frogs, toads, and newts survive being frozen hard. They may 

 be cooled down to nearly 1 C., and they may be partially frozen 

 into the ice. Circulation of the blood is suspended in such cooled- 

 down frogs ; their limbs may become so hard that they break like 

 a piece of wood, but the citadel of life, the heart, must not sink 

 much below freezing-point, and must itself not be frozen, if the 

 animal is to have a chance of recovering. The protoplasm resists 

 a long time, and so long as some of it is left unfrozen the rest 

 will recover. Hibernating frogs are lost if they are reached by 

 prolonged frost during exceptionally severe winters. Every frog 

 will be killed in an artificial pond with a clean concrete bottom, 



