Inheritance in Bacteria 87 



work with races all members of which are derived from a 

 single original individual. It was long impossible to ful- 

 fill this requirement; the so-called pure cultures of bacteria 

 were derived from a large number of individuals. There 

 might be slight racial differences between these original indi- 

 viduals, the different strains being adapted to different con- 

 ditions. Then under given conditions one strain multiplied 

 until the entire population seemed to take on its characteris- 

 tics, the other strains remaining without activity or mul- 

 tiplication. But on a change of conditions this prevalent 

 strain ceased its multiplication, while some other strain 

 became active, multiplying until the population showed the 

 characteristics of this second strain. It appeared as if the 

 changed environment had altered the hereditary characteris- 

 tics of the organisms, but this appearance would be illusory. 

 It seems probable that such impurity of the original stock 

 accounts for some of the apparent transformations that 

 have been described. 



But in recent years a number of methods have been de- 

 vised for isolating a single bacterium, so that an entire 

 stock can be derived from this. 1 From such pure 'races 

 dependable results can be obtained. 



One of the first to isolate pure stocks was Barber (1907) ; 

 he worked both with bacteria and with yeasts. Barber did 

 not attempt to modify the organisms, but merely to deter- 

 mine whether the variations in size and form often observed 

 are inherited. From a pure race he picked out large indi- 

 viduals, long individuals, individuals of peculiar form, and 



1 Apparently the simplest and most effective method is that of 

 mixing a very little of the fluid containing bacteria with a large quan- 

 tity of India ink, then producing a thin layer of this ink between two 

 cover glasses. Single bacteria, owing to the fact that they are sur- 

 rounded by a cloud of gelatinous material, are visible as minute clear 

 specks in the dark ink. A cover glass preparation containing but a 

 single individual is taken as the beginning of a culture. This is known 

 as Burri's method. 



