SYMBIOLOGY THE BIOLOGICAL RELATIONSHIPS OF ORGANISMS 145 



In fact, no organism is free from the accidental association with other 

 organisms. 



In all of the instances mentioned there is no perceptible evidence of 

 either antagonism or mutualism. Injurious results may result, due to 

 mechanical causes. Slight morphological changes usually result, but such 

 changes seem to have no effect upon the life-history and development of 

 the symbionts. 



To the category of accidental symbiosis also belong the association of 

 climbing plants and their living supports. The symbiotic relationship was 

 at first merely accidental. It is a striking example illustrating how marked 

 and highly specialized morphological adaptations favoring one function 

 may initiate widely different morpho-physiological changes. In the case 

 of climbing plants, it is impossible to know when the symbiotic relation- 

 ship begins to overbalance the function of mechanical support. It is just 

 as difficult to determine when marked symbiotic phenomena began to 

 manifest themselves. It is safe to conclude, however, that the morpho- 

 logical changes favoring climbing and support progressed considerably 

 before any marked symbiotic relationships occurred. 



It is also evident that accidental symbiosis is a condition readily sub- 

 ject to change, since the permanency of symbioses is in direct proportion 

 to the degree of mutualistic specialization. Each plant and animal may 

 enter into accidental symbiotism with other plants and animals. In a 

 given animal this association changes with a change of locality, of tem- 

 perature, or of moisture; in fact, with every change in the environment. 

 The absence of all permanency in morphological and functional relation- 

 ship, characterizes accidental symbiosis. It resembles a form of hap- 

 hazard experimentation on the part of nature to determine whether or 

 not a definite symbiotic relationship can be established. 



2. Contingent Symbiosis. In this form of symbiosis the relationship 

 of the organisms is already sufficiently marked to give the semblance of an 

 elective affinity, although the functional interdependence is as yet not 

 manifest. It is of wide occurrence among widely different organisms. 

 Many phenomena heretofore recognized or variously classified as parasit- 

 ism, perhaps belong to this category. Most of the phenomena recog- 

 nized by the German scientists as Rawnparasitismus also belong here. 

 The [citation of a few examples will suffice to explain the nature of contin- 

 gent symbiosis, and to distinguish it from mere accidental symbiosis as 

 well as from the more highly specialized forms of symbiosis. 



There is a difference between the bacterial flora of the digestive tract 

 of man and that of the chicken or dog. Certain bacteria, which have not 

 yet become markedly antagonistic or mutualistic in their symbiotic as- 

 sociations, show a preference for one digestive tract which indicates that 

 10 



