VARIATION IN UNICELLULAR ORGANISMS iii 



and favorable kinds of culture media commonly employed 

 seem to have favored this effort; under the conditions sup- 

 plied there was little genetic variation. 



Yet even so, many phenomena were observed that did not 

 agree with this interpretation. In a supposedly pure culture of 

 a single type, there appeared types that were diverse in form, 

 structure or physiology. It was believed that in most cases such 

 phenomena were due to contamination. The supposedly pure 

 culture was held not to be pure. Mingled with the prevailing 

 type were presumably a few of another type, undetected. 

 Under certain conditions these did not multiply, so that they 

 remained inconspicuous. But under changed conditions these 

 hidden bacteria multiplied and became prominent. Thus 

 there appeared to have been a genetic variation, a change of 

 type that was inherited. But it was held that all that had 

 really happened was the substitution of one type for another. 



There were, however, certain changes, particularly in dis- 

 ease-producing bacteria that were very extensively studied, 

 which could not be explained in that way. In old cultures, or 

 under changed conditions, the forms, structures and physio- 

 logical action of the cells were greatly altered, and these 

 changes persisted even when the organisms were returned to 

 the original conditions, so that genetic variation had occurred. 

 These were characterized as abnormal or involution forms, 

 and their significance therefore slighted; although for the 

 problems of genetic variation such changes are of high 

 interest. 



In recent years these changes have been studied systemati- 

 cally and thoroughly for their own interest. The effects of 

 changed environmental conditions have been extensively 

 studied. Improved methods have been devised, for beginning 

 a culture with a single bacterial cell, so that apparent changes 



