254 W. B. McDOUGALL 



examples of so called mutualism are in reality cases of recipro- 

 cal parasitism in which each symbiont takes all he can get from 

 the other symbionts. It would seem more logical, furthermore, 

 to include epiphytism, parasitism and reciprocal parasitism un- 

 der conjunctive symbiosis as coordinate with disjunctive symbi- 

 osis rather than to consider them as individually coordinate with 

 disjunctive symbiosis. 



The classification which is given below is believed to be at 

 once simple enough in its conception and broad enough in its 

 definition and scope to include all phenomena that should be 

 embraced under symbiosis. Symbiosis is defined as the living 

 together of dissimilar organisms. Any more limited definition 

 proves unsatisfactory because of the difficulty of drawing lines 

 of separation between kinds of phenomena among living things. 

 Only by including all phenomena of the living together of organ- 

 isms can this difficulty be avoided. Many authors who have 

 taken symbiosis in a limited sense have never-the-less wished to 

 include the leaf cutting ants and their mushroom gardens as an 

 example of symbiosis. But there is really no essential difference 

 between that phenomenon and the phenomenon of man and 

 his vegetable garden: and it is only a short step from this to the 

 phenomenon of a cow and the grass upon which she feeds. 

 Again, many authors have included epiphytism under symbiosis. 

 But if we call ordinary epiphytes symbionts why should we not 

 call pseudoepiphytes, those land plants which happen to be 

 growing in crotches or other places on trees where soil has col- 

 lected, symbionts? And if we call these symbionts, why should 

 we not take one more step and call the violet that grows under 

 the tree a symbiont since it is living in intimate association with 

 the tree? 



No attempt has been made in the following classification to 

 fit in examples of symbiosis in which only animals are concerned, 

 but it is believed that zoologists as well as botanists will find 

 that there is a place for every case of symbiosis that is well 

 enough known to be classified. 



