THE BIOLOGY OF DAILY LIFE. 87 



animal the same micro-organisms should again be 

 found." 



" A particular micro-organism may probably be the 

 cause of a particular disease, but that really and un- 

 mistakably it is so, can only be inferred with certainty 

 when every one of these desiderata have been 

 satisfied" (p. 3). 



Now first notice condition 4, which I give fully in 

 Klein's own words. And one question must arise. 

 How can you prove that the so affected new animal 

 was affected solely by the micro-organisms introduced, 

 and not by the dangerous assault on the life, which, as 

 Klein's own book has proved, the very operation of in- 

 oculation in itself is. Again, though it may be 

 possible to sterilize the artificial media of cultivation, 

 how can the body, blood, and tissues of a living 

 animal be effectually sterilized ? 



But let us suppose that all these difficulties are 

 successfully met, and that every possible source of 

 error within the limits of these four conditions has 

 been completely excluded say by repetition of ex- 

 periment, so that, by the doctrine of chances, the 

 origin of the disease from that particular pathogene, 

 and nothing else, becomes a moral certainty ; I say, 

 granting all this, there is a FIFTH CONDITION, unmen- 

 tioned by Messrs. Klein and Koch, which must be 

 complied with before the experiments come within 

 measurable distance of a satisfactory proof. 



It is quite true, as Klein shows, that these four con- 

 ditions rigidly enforced would simply decimate the 

 most popularly celebrated pathogenetic "proofs," in- 

 cluding the evidence for Pasteur's hydrophobia 

 microbe and Koch's own comma-baccillus of cholera, 



