THE INFLUENCE OF SYMBIOSIS. 87 



the cause of smallpox, which are due to other influences, are shown in 

 the marked difference in the severity of the disease in various epidemics. 

 Not infrequently the disease in certain outbreaks may appear to be so 

 mild as fairly to justify the statement that the infection is harmless, 

 and again under apparently similar conditions or during the same 

 epidemic, the reverse condition of severe infection and high mortality 

 may obtain. Such changes are probably not due to a difference in the 

 condition of the host as is the ease in vaccinia, but to some other symbiotic 

 or environmental influence acting with or upon the parasite, and as 

 Professor Councilman has pointed out, the part played by the constantly 

 present symbiotic bacteria in these lesions has not yet been determined, 

 but it is probably an important one. 



CONCLUSIONS. 



It seems to me that the evidence is fairly conclusive in favor of a 

 more or less definite symbiosis, for example between amoebae and other 

 microorganisms, whether they be found in a saprophytic existence or in 

 pathologic lesions, with the exception of the true animal parasitism 

 found in certain bacteria-free abscesses in the animal host, and this 

 parasitism is evolutionary in character. 



In other words, a condition of parasitism exists even under outside 

 conditions, a parasitism between bacteria and amoeba? which is not only 

 a changeable one, but which may be evolutionary, gradually passing from 

 the extraneous surroundings to be found in water and other saphrophytic 

 environment, through the mixed infections encountered for examjde in 

 dysenteric ulcers, where the elements of nutrition of the amoebae may 

 consist of animal secretions or tissues, modified or reenforced by the 

 bacteria, to a condition of true animal tissue-parasitism found in certain 

 abscesses of the liver and other places in the body, or even to the still 

 more exquisite parasitism of amoebaemia, two cases of which have been 

 observed by Mr. Clegg and myself. 



There does not appear to be anything particularly radical in such a 

 hypothesis, especially when one considers what has been proved about the 

 action of mixed cultures of bacteria in respect to symbiosis and when 

 it is remembered that at least that part of this evolution of parasitism in 

 which amoebae pass from the mixed environment of the intestinal lesion 

 to a more specific tissue parasitism is a matter of routine observation in 

 the tropics. 



I am convinced, as has been stated above, that such an evolution of 

 parasitism from an amoebic and bacterial symbiosis in water or elsewhere 

 in nature, through a beginning or mixed tissue and bacterial parasitism 

 in amoebic ulcers of the colon, to a true tissue parasitism in the internal 

 organs of the body, or even to a blood invasion itself, is a matter of 

 frequent occurrence. However, the extent or generality of such a process 



