During the mid-forties bacteria and phages were dis- covered to be suitable objects for the study of genet- ics. Genetic phenomena such as mutation and recombina- tion, which had already been known in eukaryotes for a long time, were now shown to exist in bacteria and phages as well. New phenomena as lysogeny and trans- duction were discovered, which gained great importance beyond the field of microbial genetics. Bacteria and phages are of small size, multiply rapid- ly, and have chemically defined growth requirements. Many selective procedures can be applied to screen for rarely occurring...