36 SYMBIOSIS 



Again it is thus illustrated that all important pioneer-work 

 is done on " cross-feeding," as we found already in the case of 

 the lichen and in that of the clover. The primal plant nutrients 

 are inorganic, although eventually their further elaboration 

 is facilitated by division of labour and exchanges of surpluses 

 in Symbiosis. Strenuous work and Symbiosis provide the 

 original and permanent capital for the purposes of organic 

 civilisation, which can thenceforward proceed to more extended 

 forms of division and specialisation of labour with resultant 

 exaltation of type. The evidence shows that pioneer plants leave 

 the soil permanently the richer for their presence give more 

 than they take and. thus, and with a subsequent expansion of 

 Symbiosis, provide the economic and physiological basis of 

 progressive evolution. Such then is ttye explanation of the arrival 

 of " a new race of plants " typical of the way in which evolu- 

 tion is achieved by Symbiogenesis. I would only add that the 

 case of animal and plant Symbiosis is quite similar, for the better 

 the animal is supplied by plant surpluses, the more vigorous 

 it gets, the more it can in turn supply the plant with Carbon 

 dioxide and the better it can adapt itself to the needs of pro- 

 gressive evolution generally. As regards the process of nitrate 

 formation by bio-chemical decomposition, Dr. Russell further 

 tells us that the initial products are of little value to the crop 

 or the soil. The final (i.e., thoroughly converted) products are 

 invaluable for the plant nutrition. It is of the highest import- 

 ance that the reaction shall be carried rapidly and smoothly 

 to the nitrate terminus. Where for any reason it is not so 

 completed, the plants cannot be properly fed and " the soil 

 becomes unproductive." When Dr. Russell states that the 

 second broad principle of crop production is " that the bio- 

 chemical decompositions in the soil must proceed smoothly and 

 rapidly," I would say that it is therefore essential that scope 

 must be provided for Symbiosis and cross-feeding to proceed 

 completely and unhindered. 



It has long been found that surfeit of any factor otherwise 

 essential to plant growth is harmful. Extra supplies may do 

 harm, either by direct injury or by cutting out another indis- 

 pensable substance. 



We are thus introduced to a " Law of Minimum " as the 

 third principle of Crop production. 



Here again I would point out that Symbiosis-ff/w-cross- 



