116 BIOLOGY. [Book ii. 



Let us notice, however, that it is jfossible, under the condi- 

 tions of artificial culture, to rear, without silica, plants which 

 habitually contain much of it, maize for example. 



Lime seems to be indirectly useful to plants, by serving as a 

 vehicle to the sulphuric and phosphoric acids, and by neutralizing 

 the oxalic acid, hurtful to the plant in which it is formed ; but 

 any other base could do as much. 



3. Influences of Light, Heat and Electricity. 



We have already spoken at length upon the influence of solar 

 light upon vegetal nutrition. It is such, that, without thig 

 influence, nearly the whole of the ' vegetal world would cease to 

 exist. In consequence, tlie animal kingdom, which, in the 

 continental parts of the globe, lives, directly or not, at the 

 expense of the vegetal kingdom, would also become extinct, at 

 least in its higher branches, on the surface of these terrestrial 

 regions. In effect, without solar light, mushrooms alone, of all 

 plants, could still live upon the continents. 



But animal life would probably find refuge in the sea. A 

 number of the lower marine animals are nourished and live 

 without the help of plants. Now these rudimentary organisms, 

 to which we must grant the faculty of synthetising complex 

 organic substances, after the manner of plants, could themselves 

 furnish alimentation for higher aquatic animals, as often actually 

 happens. All this aquatic fauna can thus live without light. 

 This is even the case now, as the dredgings and soundings made, 

 during the last few years, at the bottom of the ocean and of 

 large lakes has proved. 



Solar light only penetrates the sea to a very small depth,"' 

 Below 50 fathoms (1 fathom =- 1™ "82,) a very sensitive photo- 

 graphic paper is no longer impressed. At this depth also, the 

 vegetal kingdom is only represented by rare specimens, and 

 below 200 fathoms it absolutely disappears. Now the bottom of 

 the Atlantic ocean, which is as much as 1400 and 1500 metres 



