CH. Ill] A CONSIDERATION OF THE EVIDENCE 29 



second organism now predominates in numbers to such an 

 extent that the first one is lost sight of. Such a train of 

 events might be misinterpreted and the assumption made 

 that variation or even transmutation had occurred. Horrocks 

 (1911) investigated the urine of a typhoid carrier which "in 

 certain dilutions always gave practically pure cultures " of 

 B. typhosus, although B. coli was present in small numbers. 

 When the urine was diluted, however, an enormous increase 

 in the B. coli occurred and the B. typhosus rapidly disappeared. 

 Klein (1903-4) observed the same sequence of events in the 

 peritoneal cavity on injecting a strain of B. coli which 

 contained typhoid bacilli. 



Smirnow (1908) quotes experiments in support of the 

 opinion that streptococci inhibit the growth of the Klebs- 

 Loeffler bacillus, but only for a time and he explains in this 

 way the appearance in some cases of the bacillus in what, a 

 few hours previously, had appeared to be an almost pure 

 culture of streptococci. 



Two other instances may be given. The sputum of a 

 patient suffering froin pneumonia and a swab from the throat 

 in a case of diphtheria may contain, in addition to the virulent 

 organisms which cause these diseases, avirulent organisms 

 closely resembling them — the saprophytic pneumococcus and 

 the pseudodiphtheria bacillus respectively. The saprophytic 

 bacteria in both cases will grow at a temperature of 20° — 22° C. 

 though the virulent types are both unable to do so. It is 

 evident that, other things being equal, the temperature of 

 the incubator will decide which of the two types, the virulent 

 or the avirulent, will predominate in the culture. The other 

 one, although actually present, may then easily be overlooked 

 unless an alteration in the temperature gives it, in turn, the 

 ascendancy. In the latter event the change in virulence and 

 in other characters would have the appearance of a " variation " 

 brought about by change of temperature. It would actually 

 be due to a "contamination" which had been previously 

 overlooked. 



A second organism may, again, escape detection because 

 its recognition is made dependent upon some one character 



