Production of Heritable Variations in Bacteria 95 



complete refusal; conditions in which union was incomplete, 

 and the like. 



After the differences had been brought about, the colonies 

 were all restored to the same culture medium and to the 

 same other conditions, and cultivated thus for a long time. 

 They still refused to unite when brought in contact. The 

 change produced was hereditary and permanent. After 

 fifteen transfers under uniform conditions, representing 

 hundreds of generations of the organisms, the diversities 

 still existed. 



Even when the organisms were cultivated separately for a 

 very long time, but without diverse chemicals in the culture 

 media, they ultimately became diverse. In an experiment 

 of this sort, it required fifty-six transfers, occupying a year 

 and a half, to bring about hereditary diversities within a 

 single stock. 



When the diverse stocks thus produced were examined 

 under the microscope, no differences could be detected. The 

 change was evidently in the intimate chemical processes of 

 the organisms, not showing in any visible way. The case is 

 of particular interest because it shows that hereditary 

 changes may arise in most delicate shadings which gradually 

 become more and more marked. 



Besides the work which we have just described, there has 

 been much experimentation upon induced changes in heredi- 

 tary characters of bacteria, with special relation to virul- 

 ence, to immunity, and the like. The production of "attenu- 

 ated" strains of bacteria, weaker in their injurious effects on 

 other organisms, is a not uncommon practice. But most of 

 this work has been done without the precautions necessary 

 for establishing the results, from a genetic standpoint, as ac- 

 tual cases of the inheritance of induced modifications. But 

 the recent critical work of Wolf and others, described above, 



