28 Perspecfives in Microbiology 



tion of various bacterial genes. Not only a rise in tempera- 

 ture, but also such metabolic variables as tryptophane 

 deficiency or adenine excess accentuate the mutation rate 

 (40). Perhaps the most provocative finding was that ribo- 

 nucleosides would not merely reverse the effect of excess 

 adenine but would reduce the "spontaneous" rate by half 

 (39). Biophysicists once speculated, and then rejected, 

 cosmic radiation; we now observe that the causation of 

 spontaneous mutation is a problem incidental to inter- 

 mediary metabolism. 



This conclusion converges with the explicit study, 

 largely with microorganisms, of mutations induced by ion- 

 izing radiations and chemical reagents. "Induced" may be 

 as misleading as "spontaneous"; it implies contrived in- 

 crease of the over-all mutation rate, in a sense, an aug- 

 mentation of "spontaneous" change. Between 1928 and 

 1940, the only accepted mutagen was radiant energy, many 

 chemicals having been inconclusively or at least uncon- 

 vincingly tested (3, 36). But starting with the war gases, 

 and now including such domestic articles as hydrogen per- 

 oxide, formaldehyde, soporific drugs, and caffeine, so ex- 

 tensive a catalog has been shown potent (3, 10) that one 

 wonders what, besides rigidity of concept, can have hin- 

 dered the germination of chemical mutagenesis for so long. 

 Now, from a free interchange of results concerning micro- 

 organisms and macroorganisms, ionizing radiations also 

 are inferred to work through chemical intermediaries: 

 free radicals from oxygen and water (45). And, first with 

 Streptomyces (22), then ^vith other organisms, the dysgenic 

 effects of ultraviolet have been found reversible by visible 

 light, suggesting an obscure photochemical intermediation. 



Mutagens have, of course, helped to furnish variants, the 

 raw material for other experiments. But, particularly in 

 industrial applications, there has been exaggerated empha- 

 sis on the technology of provoking genetic variation (which 



