HIBERNATION 57 



are many creatures which not only grow new parts, 

 but can grow new individuals out of each old part ; 

 because their nerve-matter is so arranged that no 

 serious separations are made by the cutting. In more 

 advanced creatures — eyen in the high fishes — ^the con- 

 centration of the great nerve-centers is too complete 

 for the best repair work. A leg of an axolotl will be 

 reproduced in a month, and a tail, replacing one that 

 is lost, will soon grow out again with new vertebrae or 

 bones forming in it. It is said that the bones do not 

 again form in the regrovm tails of lizards — only gristle. 

 Fishes often eat off the gills of water-newts, and these 

 are readily regrown. But in all cases of legs and taUs, 

 though they may be regrown repeatedly, the new ones 

 are rarely so perfect as the old ones. 



HiBEENATION 



All forms of amphibians hibernate in winter. 



Some dive into the mud at the bottoms of pools, 

 some dig burrows, some crawl into crevices. Methods 

 differ in species close akin. Some terrestrial tailed 

 forms hibernate on land. On the other hand, some 

 bury themselves in mud in summer and sleep away 

 months in the tropics — awaking again with the rainy 

 season, as is the case with some fishes (the lung-fishes) 

 which are in many respects quite like amphibians. 



The common toad, the csecilians, the spadefoot 

 toad, the obstetric frogs, and some salamanders bur- 

 row and spend much time at any season in holes. 

 Some of the tailed forms are known to revive a^ter 

 being frozen solid. 



