126 THE BOOK OF NATURE STUDY 



animals. The warm-blooded animals are the birds and the 

 mammals ; they have an automatically working nervous arrange- 

 ment by which the temperature of the body is regulated and 

 kept constant. If we allow for certain intelligible exceptions 

 (e.g. animals in fever, hibernating mammals, fledgling birds), 

 warm-blooded animals are those which keep the temperature 

 of their body constant, no matter what the external temperature 

 may be. But cold-blooded animals, such as reptiles, amphibians, 

 and fishes have not got the regulating mechanism referred to> 

 and their body temperature varies directly with the temperature 

 of their surroundings. Though not necessarily the same as that 

 outside, the internal temperature varies directly with it, so that 

 the animal becomes colder when the surroundings are cold, 

 warmer when they are warm. " Cold-blooded " does not mean 

 that the body temperature is necessarily very low ; it means 

 " of changeable body temperature." 



THE YEAR'S LIFE OF THE FROG 



It is profitable to try to induce the habit of picturing the 

 life of an animal throughout the year. The frog is a good type 

 to work with, for most of the chapters are familiar. There is 

 the winter's rest under moss or in the mud of the pond-side, 

 where the frogs lie inert, mouth shut, nostrils shut, with their 

 heart beating slowly. There is the pairing and egg-laying in 

 spring, and the juvenile life lasting for about three months 

 altogether in the pond. The small fully formed frogs about 

 the size of the nail of our little finger leave the pond in mid- 

 summer and make for the meadows and fields, sometimes 

 migrating in crowds. They grow to about three-quarters of an inch 

 in length by October, when those that survive take to winter 

 quarters and hibernate. 



Frogs have numerous enemies, so that, in spite of their 

 enormous families, their numbers are kept down. They fall 

 victims to birds, such as stork and buzzard ; to mammals, such as 

 fox and stoat ; to the grass-snake ; and, not least, to man both as 

 epicure and as biologist. They are often stoned by country- 



