CHAP. II The Web of Life 19 



varied forms of life crystallise out of their amorphous 

 beginnings in a manner that we conceive to be analogous to 

 the growth of a crystal within its solution. Further, we do 

 not believe in a "vital force." The movements of living 

 things are, like the movements of all matter, the expression 

 of the world's energy, and illustrate the same laws. But 

 to these matters we shall return in another chapter. 



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 perhaps 

 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 ot animals, or through the 

 application of steam, are traceable — if we 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 appli- 

 cation, 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 sub- 

 stances, at a low 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 



