18 SYMBIOSIS 



It should now be clearly possible to draw a distinct line 

 between Parasitism and Symbiosis, and this is of the utmost 

 consequence in biological interpretation. 



Parasitism is the precise antithesis to Symbiosis. It is, in 

 fact, an extreme form of that " misere physiologique," of that 

 diathesis which, as we saw, characterises our domesticated 

 " productions," many of which, though they gain in size, in 

 fatness and in " variability," yet lose capital in a true evolution- 

 ary sense. 



We shall therefore have to differ from the view expressed 

 in the Encyclopedia Britannica that such terms as symbiosis, 

 commensalism and mutualism cannot be sharply marked off 

 from each other, or from true parasitism and must be taken as 

 descriptive terms rather than as definite categories into which 

 each particular association between organisms can be fitted. 

 Symbiosis in that work is actually treated under the head of 

 Parasitism, and writers so advanced even as Geddes and 

 Thomson would seem to look upon Symbiosis as "an instance 

 of a parasitism which is reaching equilibration."* So far from 

 attaching particular importance to Symbiosis, these writers 

 show a predilection to base their theory upon the modifications 

 due to extreme Parasitism. 



There is very little doubt, I think, that the self-limitation of 

 naturalists in their consideration of Symbiosis is due to a mis- 

 understanding, or rather a neglect, of fundamental economics. 

 The fact is that as regards Natural Economics we have scarcely 

 got beyond the general concept of the " modus vivendi," accord- 

 ing to which the strong are credited with so much self-control 

 that they will not devour all the weak so as to prevent the utter 

 destruction of their own food. Apart from the idea of the 

 modus vivendi, some writers have also emphasised various 

 other factors as contributive to progressive evolution. The 

 "appetency" of the organism, i.e., endeavour perpetually and 

 imperceptibly working in effect through an incalculable series 

 of generations ; the union of diverse sexual elements in fertilis- 

 ation as a potent source of change ; the influence of external 

 factors upon the parturient system ; changed " conditions " 

 generally ; " use " and " disuse " ; the ^si-discipline exercised 

 by animate and inanimate nature ; the memory factor ; all 

 these have been urged. They would seem to require proper 

 * Evolution, pp. 106, 107. 





