188 THE ANIMALS OF LAKES AND RIVERS 



danger. High temperatures, with resultant desicca- 

 tion, are just as dangerous, and must also be provided 

 against. The amoebae in a waterspout can be dried to 

 dust, and will he unharmed within their protective 

 cysts until the rain comes, and the spout once again 

 becomes suitable for their active life. In the swamps 

 of the Paraguay or of the Central African rivers, the 

 lung-fishes lie in their mud cases, waiting till the wet 

 season again permits them to become active. Drought 

 and freezing, high temperatures and low, are risks 

 which every fresh-water organism must face, and the 

 lessons learnt in the conflict with them are stamped 

 deep on all the inhabitants of stream and lake. In 

 these, as on dry land, life in the temperate and frigid 

 zones is markedly seasonal, and it is interesting to note 

 that where the problem of seasonal adaptation cannot 

 be directly faced, it may be avoided. For example, 

 those shallow bodies of water which are favoured by 

 frogs as spawning-places are very apt to dry up in full 

 summer, but the tadpole avoids the risk of drought 

 by adjusting its time of metamorphosis to the time of 

 drought, so that it is ready to leave the water at the 

 time when the pond would naturally dry up. Every 

 summer one may find cases where the adjustment has 

 not been sufficiently delicate, so that thousands of tad- 

 poles die because the drought comes before they are 

 ready for their land life. In autumn the ponds fill again, 

 and the little frogs return again to them to pass the 

 winter in their mud. 



One other unfavourable feature of rivers and lakes 

 is found in the strength of their currents. The waters 

 of the ocean are swayed also by currents, but within 

 considerable limits, and with the exceptions already 

 given (p. 161), such currents cause little harm to 



