CHAPTER III 



A CONSIDEEATION OF THE EVIDENCE 



Before discussing in detail instances of variation and of 

 " transmutation," it is necessary to consider the value of the 

 evidence offered in support of them and the possible sources 

 of error, in the way of both observation and deduction. 



1. First and foremost must be considered the possibilities 

 of contamination. A single colony, even after repeated 

 subculture and replating, may not represent an absolutely 

 pure culture and cannot be proved not to contain a single 

 bacterium of another species, the appearance of which in 

 greater numbers at a later stage of the experiment might 

 suggest variation. 



The importance of contamination as a source of error is 

 so obvious that eflScient precautions are taken in almost all 

 cases to eliminate it. 



Barber (1908) has described a method by which a strain 

 can be grown from a single organism thus ensuring the 

 purity of the culture. By means of a glass pipette possessing 

 an extremely fine aperture — no larger than the diameter of 

 a yeast cell — a single organism is removed under the micro- 

 scope from a culture which has been repeatedly diluted. 



2. The original infection, however, with which the in- 

 vestigator is dealing may itself be a " mixed " one and this 

 fact may be overlooked in two ways : 



The conditions of cultivation may favour the growth of 

 one organism and inhibit that of another, so that the first 

 may be present in such overwhelming preponderance that 

 the second is for the time being completely submerged, as 

 it were, and undetected. If the conditions change, as a result 

 either of the activity of the organisms themselves or the 

 intervention of the investigator, the balance may be restored 

 and may even swing in the opposite direction, so that the 



