SYMBIOSIS AND THE BIOLOGY OF FOOD 265 



pared both the ocean and the earth for the further evolution 

 of plants and animals. 



What is still more important, however, though generally 

 overlooked, is this : both lichen and symbiotic bacteria are 

 cross-feeders, i.e. they draw on the inorganic world for food. 

 Contrary to the widely prevailing prejudice that habitual 

 pain and habitual carnage were indispensable and compulsory 

 at the earlier stages of evolution, before the principle of " live 

 and let live " was introduced by man, as it is thought, we find 

 that the bacteria already knew better methods, had indeed 

 found a highly satisfactory method of solving the economic 

 and sociological problem of existence. Many of them had no 

 need whatever to resort to depredation, as they derived their 

 food from inorganic compounds. Nor would a habit of depre- 

 dation have allowed them to carry on their indispensable 

 pioneer work as efficiently and as satisfactorily as it has been 

 performed. Numbers of these bacteria do not live on organic 

 food at all — such modes of feeding at their stage of life not being 

 compatible with genuine Symbiosis. It has been shown that 

 the smallest trace of organic carbon or nitrogen compounds 

 is actually injurious to them. There are the Schizomycetes, 

 or " sulphur-bacteria," numbers of which have been found to 

 thrive best in a mineral solution containing ammonium sul- 

 phate as a source of nitrogen and with chalk as a neutralising 

 agent. According to Mr. Skene's investigation, as contributed 

 to the New Phytologist, 191 4, all the organic sources of nitrogen 

 and carbon which were investigated proved to be without 

 favourable influence on the growth of the bacteria ; indeed, 

 as a rule, they tended to inhibit development. These bacteria, 

 then, are typical cross-feeders, and as such thrive well. Asso- 

 ciated with a proper habit of feeding we find the power 

 of photosynthesis, and — depending on both — the capacity of 

 carrying on genuine Symbiosis with higher (green) plants. 

 The oxygen required for their own industrial purposes these 

 bacteria are able to purchase from the symbiotic plants, and, 

 having got it, they use it to oxidise sulphur or one of its unoxi- 

 dised compounds, producing in some cases sulphur and in 

 others sulphuric acid. They are thus of agricultural use in a 

 similar way to the nitrogen-fixing bacteria in the soil and in 

 Symbiosis with the plant, and this in so far as they are mainly 

 cross-feeders. Ideal food and ideal work, therefore, go together. 

 The more, on the other hand, a green cell or plant tends 

 in the direction of parasitism, the more it is apt to lose 

 its chlorophyll. In the case of the lichen, the symbiotic 

 union between alga and fungus depends in great part, of course, 

 upon the photosynthetic activity of the green alga, which 

 could on no account afford to lose its chlorophyll lest the union 



