96 THE BOOK OF NATURE STUDY 



the young tortoises sleep within their egg-case through the 

 cold winter. Now some means of protecting the young or 

 eggs through the cold of winter, or the drought of summer, is 

 very common among freshwater animals. In both these re- 

 spects the freshwater forms differ from their marine allies, which 

 do not need to sleep, because the sea does not become so cold in 

 winter as fresh water, and which do not need special protection 

 for their young, because the sea does not dry up as ponds may 

 do in summer, and except in high latitudes does not freeze. Life, 

 generally speaking, is harder in fresh water than it is in the sea, 

 and we shall find in consequence that there are fewer kinds of 

 animals in the ponds and streams than in the sea. Whole groups 

 like the Echinoderms and like the Ccelenterates are either not 

 represented in fresh water or only represented by very few forms. 

 On the other hand, while quite a considerable number of the higher 

 plants, the descendants of land forms, have succeeded in going 

 back to fresh water, very few of these higher forms indeed have 

 succeeded in going back to the sea. 



Though we have no native aquatic reptiles, we have several 

 native aquatic Amphibia. Among these, first of all, for many 

 reasons, must stand the frog. Every child at some period of 

 its life should watch the development of the tadpole, and as the 

 eggs are abundant and the little creatures perfectly hardy, if 

 properly managed, there is no difficulty in his doing so. 



In the spring, at a date which varies very greatly with the 

 position of the place and its height above sea -level, the frogs 

 may be found in the ponds, the male clasping the female with 

 his fore limbs, the first finger having roughened pads which aid 

 him to keep his hold. The eggs are fertilised by the male as 

 soon as they are laid, and the females will lay eggs in captivity 

 if a pair of the frogs are taken from a pond. Frogs form charming 

 pets, and live well in captivity. In the case of the common frog 

 (Rana temporaria) only a small amount of water should be put 

 in the aquarium, and the animal should be given abundant 

 opportunity of quitting the water. The water-frog of the Con- 

 tinent (Rana esculent a), found in a few parts of the east of 

 England, is much more aquatic in its habits, never travelling 

 far from ponds or ditches, while the common frog only seeks 



