150 PHARMACEUTICAL BACTERIOLOGY 



outcome, the parasite adapted itself to the dead substance and used it as 

 food. In some instances saprophytism no doubt originated as such. 

 Dead organic matter occurs plentifully everywhere and forms a suitable 

 substratum for a number of animal as well as vegetable organisms, having 

 special morpho-physiological adaptations for utilizing such as food supply. 

 This preference was no doubt gradually acquired. 



III. Mutualistic Symbiosis 



This form of symbiosis differs from the preceding in that the relation- 

 ship of the organisms is mutually beneficial. Each symbiont possesses 

 or has developed a specific character which is useful for the other symbionts. 

 As in the preceding forms of symbioses, widely different organisms may 

 enter into its formation. The morphological changes accompanying the 

 functional adaptations may be very marked or scarcely perceptible, nor 

 is the adaptation quantitatively and qualitatively equal for all the sym- 

 bionts. The adaptation is complementary, one organism supplies a 

 deficiency (morphological or physiological) of the others. Theoretically 

 there is no limit to the degree of specialization and perfection which this 

 form of symbiosis may attain. In fact, mutualistic symbiosis implies 

 that there is an increased specialization and fitness to enter into the struggle 

 for existence. This is most beautifully illustrated in the case of lichens. 

 These plants are of wider distribution and possess greater vitality and 

 physiological activity than either of the symbionts. They occur in the 

 tropics as well as in the extreme north; in the lowest valleys as well as on 

 the highest mountain peaks. Bonnier has shown that their vitality is 

 greater than that of any other morphologically similar plants. Likewise 

 the mutualistic symbiosis occurring in the Leguminosae, adapts these 

 almost equally well to rich and poor soil, thus giving them a great advan- 

 tage over other plants. Our knowledge of the higher forms of mutualistic 

 symbiosis is as yet too incomplete to permit us to make statements as to 

 the full benefits resulting therefrom. 



i. Nutricism. Nutricism establishes a connecting link between the 

 lesser marked symbioses and mutualism. It may be defined as a form of 

 symbiosis in which one symbiont nourishes the second symbiont without 

 receiving any benefit in return. It might therefore be designated as one 

 sided or incomplete mutualism. Absolute nutricism, as above defined, 

 does perhaps not occur, for, as already indicated, it is not reasonable to 

 assume that any symbiotic relationship exists in which all of the sym- 

 bionts are not more or less mutually affected. There are, however, a few 

 instances in which one symbiont is very materially benefited, while the 

 other is not materially benefited. The most marked example is met 

 with in the mycorhiza of the Cupulif erae. A mycorhiza is the association 



