Production of Heritable Variations in Bacteria 93 



when first cultivated on these substances they had not this 

 power. In many cases the cultures so tested have been de- 

 rived originally from a single individual, so that there is no 

 question but that there has been an actual change in the 

 hereditary capabilities of a single race. 



In most cases the change thus brought about is per- 

 manent ; the descendants never lose the capability that they 

 have acquired. But in some cases it has been found that 

 long cultivation under other conditions causes the descend- 

 ants to lose the power which their ancestors had acquired. 2 



The slightness and delicacy of the hereditary changes so 

 induced, and the fact that they increase by gradations, is 

 shown in certain other experiments of Wolf (1909). Cer- 

 tain peculiar organisms known as Myxobacteria form dense 

 swarms on decaying substances. A species known as Myxo- 

 coccus rubescens thus forms circular red patches on culture 

 media. If from a single patch of these, two distinct cultures 

 are made, and the two swarms are later allowed to come in 

 contact, they flow together and form a single swarm. But 

 if the swarms come from diverse but related species they will 

 not unite, but remain sharply separate. Even within the 

 single species named above it was found that swarms from 

 diverse sources refuse to unite, so that there are diversities 

 of race showing in this behavior. A large number of races, 

 diverse according to this test, were isolated from the single 

 species. 



The possibility naturally suggests itself that such differ- 

 ences can be produced within a single race. This was at- 

 tempted by Quehl (1906), and later by Wolf (1909). A 

 single swarm was divided into a number of parts, which 

 were kept under different conditions, on diverse culture 



* An excellent summary and review of all such cases up to 1912, with 

 a helpful account of methods, and important details, is given in the 

 paper of Dobell (1912). 



