CORRESPONDENCE 495 



This is of the utmost importance, not only so far as heredity 

 is concerned, but also as indicating how symbiotic momenta 

 generally are the mainstay of health, of " endowments," and 

 of progress, whilst failure duly to maintain these momenta 

 causes the organism to drift from Symbiogenesis into Patho- 

 genesis. 



The most important case of Symbiosis in the organic world 

 is constituted by the complementary interaction between plant 

 and animal which takes place on a grand scale in Nature. 



Recurrent friction notwithstanding, this plant-animal sym- 

 biosis has established itself successfully and effectively in course 

 of evolution. Obviously it must have involved, on the part of 

 those mainly engaged, definite " symbiotic " activities, definite 

 " symbiotic " relations and definite " symbiotic " duties. That 

 reciprocity between the " kingdoms " really constitutes a case 

 of Symbiosis has of course been recognised. For some reason 

 or other, however, only " attached " cases have been labelled 

 Symbiosis, which is regrettable, for it caused it to be over- 

 looked that Symbiosis involves the very principle evolutionists 

 were in search of : the definite and persistent principle, capable, 

 as Samuel Butler would say, of acting as a rudder and compass 

 to the accumulation of variations, and of making them keep 

 steadily on one course for every species, until eventually many 

 havens far remote from one another are safely reached. 



It also caused it to be far too little realised to what an 

 extent organisms are, in Robert Louis Stevenson's words, 

 " condemned to some nobility." In the Encyclopedia Britannica 

 it is stated " 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." [Italics mine.] 



Symbiosis is there actually treated under the head of 

 Parasitism. Surely the time has now come to mark off the 

 reciprocal from its opposite, the non-reciprocal. Surely the 

 definition of Symbiosis is narrowed down too much if it is to 

 apply only to those doubtful cases which hover near the border- 

 land of parasitism. 



The evidence of how much in evolution is due to and de- 

 pendent upon beneficial reciprocal differentiation is enough to 

 show that the chain of symbiotic relations is the medium not 



