144 MOUNTAIN AND MOORLAND 



carbon-compound like starch or sugar which may be 

 utilised, if need be, by the animal partner. This is a 

 simple instance of the interlacing or interlinking of 

 lives, which takes many forms in animate nature. 



There are many small water animals in the tarn, but 

 few large ones. It is easier for small animals just 

 because they are small to survive the winter, when 

 the surface of the tarn is often covered for a long time 

 with a sheet of ice. But even the small animals lying 

 low in the mud would be apt to perish if it were not 

 for a very interesting property of freshwater. Why 

 does not the tarn freeze solid, which would put an end 

 to most of its tenants ? Freshwater is peculiar, though 

 not unique, inasmuch as it expands, instead of con- 

 tracting, when its temperature is lowered to near the 

 freezing-point. Now this expansion brings the 

 coldest water to the top, thus tending to lessen the 

 loss of heat from the deeper water. The coat of ice, 

 once formed, remains on the surface in virtue of its 

 buoyancy, and if the frost continues it becomes up to 

 a certain limit thicker. But in proportion to its 

 thickening it serves as a shield lessening the loss of 

 heat from the water below. For this reason, then, if 

 the tarn is a true tarn that is, of considerable depth 

 the warmer water below is kept fluid, and this makes 

 it easier for the life of water plants and water animals 

 to continue, especially in a resting phase. On the 

 high tarns there is often a mantle of snow on the ice, 

 and this also helps to lessen the further cooling of 

 the water, for dry snow makes a non-conducting 

 garment. Along with this must be taken the fact that 

 many of the small aquatic animals e.g., Water Fleas 

 and Wheel Animalcules form specially protected, 



