78 MUSGBAVE. 



scattered literature on the subject, notes the promising suggestions that 

 have been given from time to time by great masters, and carefully 

 examines the interesting and often startling results of individual experi- 

 ments, it is surprising that the subject has not been more minutely inves- 

 tigated. The present paper is general in character, much of it is 

 theoretical, but in important points the conclusions are largely based 

 upon facts. 



Nature is filled with examples of diverging biological activities. Fre- 

 quently living organisms derive benefit by association; and again the 

 presence of one organism or its products may be inimical to the life 

 and growth of certain other forms. These facts are not confined to 

 animals and the higher forms of plant life, but also exist so low down 

 in the scale as to be found among the bacteria. They manifest them- 

 selves in many ways ; for example, in the study of immunity the symbiosis 

 of microorganisms has an intimate, important and much neglected place. 

 Immunity in any instance is a very specific substance or condition and 

 its specificity and other properties, both known and unknown, may be 

 modified or destroyed in many ways. Among the unknown class, more 

 light needs to be thrown upon the influence which the metabolism of both 

 host and parasite exerts. There is evidence to show that changes in 

 symbiosis may produce changes in metabolism and also, as a result of 

 this, changes in the pathogenic character of parasites and in the suscep- 

 tibility of hosts. 



It certainly seems reasonable to assume that changes in the sub- 

 stances which are the stimulants calling forth the development of im- 

 munity ivould result in a modification of the type of the immunity 

 itself. In other words, it is not inconceivable that the immunity unit 

 is a molecule, as it were, the composition of which might be influenced 

 by all agents reacting in its production, and that these agents might be 

 somewhat different, even in the same disease, if they are developed under 

 different environments. Theobald Smith 2 probably had some such idea 

 in mind when, in his classical discussion of the parasitism of the tubercle 

 bacillus, he stated that "In our haste to take the animal and bacterial 

 mechanisms to pieces and to test the individual tissues and components, 

 we have crowed out the broader view that the host fights more as a unit. 

 We had almost forgotten to take into consideration the flexibility and 

 adaptability of the microorganisms themselves." 



It is probable that in our study of the manner in which microorganisms 

 produce disease we have not only confined our work too closely to inves- 

 tigating the component parts of the invader and invaded and have not 

 considered the two subjects as a whole, but the weakness probably extends 

 still further and we have failed to consider the environment or symbiosis 

 of both organisms and the influence which this environment or symbiosis 



2 J. Am. Med. Ass. (1900), 46, 1247. 



