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 lie 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 
