The Source of Variability 



71 



NON-SETA TYPE 



SETA TYPE 



Fig. 26. A mite character caused by a virus-like factor transmitted through 

 the mite's cytoplasm. Se/a type, normal setae on leg, especially including those 

 marked x; ISlon-seta type, setal pattern lacking the four marked x, caused by 

 virus-like factor. (After Boudreaux. ) 



of their bacterial host. When occurring in the cytoplasm, the phage 

 virus multiplied at a prodigious rate and killed its host. When it 

 occurred as a molecule of virus DNA on the host nucleus, the virus 

 DNA molecule duplicated only in unison with its host and acted 

 as if it were a normal part of the genome of the host. Sager sug- 

 gested from this that perhaps many mutants originate in the 

 cytoplasm and eventually become incorporated into the chromo- 

 somal organization. 



