10 THE WONDER OF LIFE 



Alga, Clathrocystis cerugi'msa, there may be 500 millions 

 to the square yard ; at the autumn maximum of a well- 

 known Diatom Melosira varians, which has a summer 

 maximum as well, there are about 7,000 millions to the 

 square yard, so that the waters of the lake form a veritable 

 living soup. Perhaps, outside of Bacteria, this is near the 

 climax of productivity. 



The same exuberant productivity is equally characteristic 

 of many tracts of the open sea, where a vessel may steam 

 for days through floating meadows, several feet deep, of 

 simple vegetation — mostly consisting of unicellular Algae. 

 Thus clusters of threads, called Trichodesmium, may collect 

 on the surface in calm weather, like unmelting yellowish- 

 brown snowflakes, and extend over many acres. In an 

 ordinary sample from a warm part of the Atlantic and 

 from a depth of 50 metres (which is the most densely 

 peopled zone as far as plants go), there are likely to be 

 about 5,000 plant-cells in a htre ; but there may be as 

 many as a quarter of a million, which is a prodigious 

 exuberance of life. 



Number of Species. — There might be great abundance 

 of hfe and yet no conspicuous variety, but every one 

 knows that the number of different kinds of animals and 

 plants is far beyond what we can readily conceive. Aris- 

 totle recorded about 500 animals, but a single expedition 

 nowadays may still discover more than a thousand new 

 species — most of them rather small deer we must admit. 

 We are amazed at the number of stars which we can see 

 definitely on a clear night, perhaps four thousand alto- 

 gether, but there may be more species in one family of 

 insects. 



In the small island of Britain there is a record of the 



