THE LAW OF CONCORD IN NATURE 35 



is that the same process which ensures the utmost public health 

 by the purification of sewage also provides the most ideal 

 food, i.e., " cross-food " (nitrates) for the strenuous and 

 symbiotic plant, and further that the " biological " part 

 of the process is mainly made up of the " work " of symbiotic 

 organisms, which are able to perform their indispensable services 

 and thus to form important links in the great bio-economic chain 

 of life precisely in virtue of the symbiotic character of their 

 relations with the strenuous plant. 



In other words, the " health " of the soil, the well-being of 

 the soil-" population," and the efficiency of labour on the part 

 of the higher plant, and all these entail, depend upon the 

 maintenance of a definite symbiotic nexus with symbiotic " cross- 

 feeding " as a condition fundamentally indispensable to the 

 maintenance of this nexus. Instead of saying that the important 

 work of converting the ammonia into nitrate is being achieved by 

 a process which is neither chemical nor physical, but biological, 

 I should say that we have here an instance of an essential connec- 

 tion between bio-economic and bio-chemical efficiency of work 

 such as is usually set up by Symbiosis ; for the organisms 

 concerned achieve the result by work chiefly chemical and 

 they owe their success as much to Symbiosis, as they in turn tend 

 by their work to further Symbiosis. In acquainting us with 

 the history of the soil-" population," Dr. Russell states the 

 following important facts : 



A very cursory examination shows that the soil forms only a thin layer, 

 underneath it lies the subsoil, which is wholly different in colour, texture, 

 and especially in its behaviour towards the plants. Yet there was not 

 always this difference. When the soil was first laid down it was all like 

 the subsoil, and whenever a new surface becomes exposed, either by land- 

 slips, cliff-falls, etc., it is always the subsoil type that appears. The first 

 vegetation has no great supply of plant nutrients, but plants suited to 

 the conditions nevertheless spring up. They take what they can from the 

 crude soil, they take carbon dioxide from the air, they synthesise sugars, 

 starches, cellulose, proteins, etc., deriving the necessary energy from 

 sunlight. When the plants die they fall back on the soil and return to it 

 all that they took, and a good deal more of new material besides. That 

 introduces a fundamental change. The new material thus added contains 

 stores of energy and food substances suitable for the bacterial population, 

 which forthwith flourishes. Decomposition goes on, nitrates and other 

 substances are produced, and the conditions are made more favourable 

 for the growth of a new race of plants. One of the most obvious changes 

 is the formation of nitrates, but other products are formed as well. (Italics 

 mine.) 



