THE ECONOMY OF NATURE 9 



sufficient to give rise to long successions of phyla, orders and 

 genera. It has been found that Symbiosis is, for the most part, 

 ultimately connected with nutrition. What is more significant 

 still is this : genuine symbiotic adaptation is everywhere based 

 upon what I have termed " cross-feeding." Be it amongst the 

 symbiotic bacteria, or in the case of the lichen, or in that of the 

 bee and the flower, or in any other case, the significant fact is 

 that we have to do with " cross-feeding," i.e., reliance upon 

 special products of another " kingdom." The animal Convoluta 

 roscoffensis, mentioned above, which in virtue of its " garden " 

 of green cells can live and reproduce very comfortably without 

 need of depredation, presents a relation which is not the excep- 

 tion but the norm of Nature. What is exceptional rather in 

 this particular case is that it shows a retrogressive step in 

 Symbiosis, i.e., from the non-attached to the attached form. 

 The inferiority in the Convolutal arrangement is emphasised by 

 its impermanence. This impermanence is clearly due to an 

 insufficiency of the principle of " live and let live " between the 

 partners, a fact to which I shall presently recur. Meanwhile 

 we are warranted to infer that genuine symbiotic adaptation is 

 not compatible with predaceous ways of living, and that the 

 symbiotic relation requires indeed the utmost discrimination 

 as regards food lest the delicate balance of physiological and 

 socio-physiological services that it entails become disturbed. 

 Needless to say, such discrimination is fruitful also in 

 psychological good effects. I would point out in this con- 

 nection that such mainly cross-feeders as man, the apes and 

 parrots, for instance, rank high in intelligence and status. Had 

 theirs not been a mainly symbiotic history, they would scarcely 

 present, as they do, an almost unbroken tradition of cross-feeding. 

 Obviously, if the land has provided more favourable conditions 

 than the waters towards the acceleration of progressive evolution, 

 this was in no small measure due to the fact that the land presented 

 conditions more favourable to the " sociological " requirements 

 of Symbiosis, such as are indispensable to physiological perfec- 

 tion. In the case of the lichen, the fungus can be a model partner 

 to the alga only on condition that it exercises forbearance 

 and does not prey upon the alga. It may exchange products ; 

 but it must essentially remain a worker and a cross-feeder, i.e., 

 it must draw for sustenance on the soil or on the rocks. The 

 reproductive specialism of the fungus, so useful in the partnership 



