CHAP, ii THE WEB OF LIFE 19 



plosive materials in a muscle, are always breaking down 

 and being built up again from what is taken in from the 

 outer world. The fundamental fact is that the organism 

 is continuously dependent on its surroundings for up- 

 keep and renewal. It is like a whirlpool in the river, 

 but with an individuality and power of persistence all 

 its own. 



Interesting, because of its sharply defined and far- 

 reaching significance, and because the essential mass is 

 so nearly infinitesimal,* is the part played by iron in the 

 story of life. For food-supply we are dependent upon 

 animals and plants, and ultimately upon plants. But 

 these cannot produce their valuable food-stuffs without 

 the green colouring-matter in their leaves, by help of 

 which they are able to utilise the energy of sunshine and 

 the carbonic acid gas of the air. But this important 

 green pigment (though itself free from any iron) cannot 

 be formed in the plant unless there be, as there almost 

 always is, some iron in the soil. Thus our whole life is 

 based on iron. And all our supplies of energy, our powers 

 of doing work either with our own hands and brains, or 

 by the use of animals, or through the application of steam, 

 are traceable if AVC follow them far enough to the sun, 

 which is thus the source of the energy in all creatures. 



2. Inter-relations of Plants and Animals. We often hear 

 of the " balance of nature," a phrase of wide applica- 

 tion, but very generally used to describe the mutual 

 dependence of plants and animals. Every one will 

 allow that most animals are more active than most plants, 

 that the life of the former is on an average more intense 

 and rapid than that of the latter. For all typical plants 

 the materials and conditions of nutrition are found in 

 water and salts absorbed by the roots, in carbonic acid 

 gas absorbed by the leaves from the air, and in the energy 

 of the sunlight which shines on the living matter through 

 a screen of green pigment. Plants feed on very simple 

 substances, at a IOW T chemical level, and their most char- 

 acteristic transformation of energy is that by which the 

 kinetic energy of the sunlight is changed into the potential 

 energy of the complex stuffs which animals eat or which 



