Hibernation 



brate evolution: they made the change from water life to land 

 life at the time when the earth reached the condition in which 

 land existence was possible. 



Their small numbers now — only one-eighth that of the 

 Fishes, one-fourth that of the Reptiles, and one-tenth that of 

 the Birds — and the few fossils found would seem to prove that 

 they never became a dominant group. Not remaining perfectly 

 adapted for water life, as are the Fishes, never becoming in- 

 dependent of water and fully adapted to land life, as are the Rep- 

 tiles, they have been at a disadvantage as a race. They have 

 been limited in their distribution to moist situations near water, 

 and have been wiped out of existence in large numbers when con- 

 ditions changed, because they were not able to migrate over dry 

 regions until they again found a proper habitat. Also, during 

 those ages when the earth was occupied by Fishes, Batrachians, 

 and Reptiles only, the Batrachians could not but be unequal 

 to the situation, pitted against two devouring races, the Fishes 

 in the water and the Reptiles on land. 



VI. Temperature and Hibernation 



All Batrachians, like Fishes and Reptiles, are cold-blooded, 

 i.e. they have a variable body heat, depending on the tempera- 

 ture of the environment. In this they are distinguished from 

 Birds and Mammals, which have an unvarying body temperature. 



Because of this variation of the body temperature with that 

 of the surroundings, Batrachians can endure extremes of heat 

 and cold, but are greatly influenced by them. With decreasing 

 temperature the processes of respiration and circulation gradually 

 slacken speed, and the animals become more and more lethargic 

 until they sleep. With rise of temperature to a limit varying 

 from 20° to 30° Centigrade in the different species, they gain in- 

 creased activity. 



Members of the Salientia can endure an astonishing amount 

 of cold; even freezing in the water of a pond or in the mud at 

 its bottom will not of necessity cause death. The circulation 

 and all life processses may stop, but if the blood and protoplasm 

 of the heart do not fall much below freezing-point, the frozen 

 parts will recover. Since sleep is induced in specimens at vary- 

 ing temperatures below 10° C, it is easy to understand why Ba- 

 trachians are not found in the extreme north. 



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