2o8 Flashlights on Nature 



and sui-ffc up again in spring, though in most 

 varied fashions. 



ConsidtM-, once more, the curious set of circum- 

 stances wliich renders this singular plan feasible. 

 Water freezes at 32 degrees Fahrenheit. P\)r the 

 most part, under normal conditions, the water at 

 the top of the pond is the warmest, and that at 

 the bottom coldest ; for the hot water, being ex- 

 panded and lighter, rises to the surface, while the 

 cold water, being contracted and heavier, sinks to 

 the depths. If tliis relation remained unchanged 

 throughout, when winter came, the coldest water 

 would gradually congeal at the bottom of the 

 pool : and so in time the whole pond would 

 freeze solid. In that case, life in it would obvi- 

 ously be as impossible as in the ice of the frozen 

 pole or in the glaciers of the Alps. Hut by a 

 singular variation, ju^t before water freezes, it 

 begins to expand again, so that ice is lighter 

 than water. Thus the ice as it forms rises to 

 the surface, and leaves at the bottom a layer of 

 slightly warmer water, some four or live degrees 

 above freezing point. It is usual to point this 

 fact out as a beautiful instance of special pro- 

 vision on the part of nature for the plants and 

 animals which live in the ponds ; but to do so, 

 I think, is to go just a step beyond our evidence. 

 Nature does not lit all places alike for the develoj)- 

 ment of life ; she does not ht the desert, for ex- 

 ample, nor the interior of glaciers or frozen oceans, 

 nor, for the matter of that, the rocks of the earth's 

 mass ; nor does she try to tit living beings for such 



