114 SYMBIOGENESIS 



thrive well. Associated 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 

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

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

 unoxidised 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. 



The same inferences may be drawn from examples 

 taken again from the genus Chlamydomonas, showing us at 

 the same time how " variations " arise according to differences 

 of feeding habits, and how symbiogenesis depends on appro- 

 priate nutritive conditions. 



Thus Prof. Bretland Farmer in his little book on Plant 

 Life, page 33, reports that in the Alga, Chlamydomonas, the 

 formation of colonies, i.e., the first step towards complexity of 

 organisation, depends upon the supply of "appropriate nutri- 

 tive media," and it is for my purpose important to note that 

 these are mineral salts, i.e., the proper food of the plant, thus 

 again emphasizing the value of cross-feeding as a vital factor 

 of development. 



Neither the quantitative nor the qualitative distinctions 

 that I have so far drawn have as yet been recognised in 

 Biology. Yet that a beginning is at last being made is evident 

 also from Prof. Klebs's protest against the constantly repeated 

 expression "good or favourable nourishment," which, he 

 cannot help realising, is both "vague" and "misleading," 

 seeing that circumstances favourable to growth differ from 

 those which promote reproduction. He expresses the view that 

 " for the production of every form there are certain favour- 

 able conditions of nourishment, which may be defined for every 

 species," which, in my view, means that nutritive specialisa- 

 tion varies with bio-economic specialisation. 



