444 RADIATION BIOLOGY 



sively and extensively applied, result in the accumulation of minor 

 changes which collectively work in the desired direction, even though it 

 would very seldom be possible by this means to obtain perceptible 

 improvement in characteristics useful to the organism itself while it is 

 living under exactly its natural conditions and retaining in other respects 

 its original genetic constitution. 



The above considerations suggest that microorganisms should on the 

 w^hole present especially suitable material for the practical use of radi- 

 ation for its mutagenic effects. There are already several examples to 

 illustrate its use in this way. One is provided by experiments with the 

 mold Penicillium, initiated by Hollaender (1945) and continued by a 

 whole group of investigators. In this work, the irradiation was carried 

 out in several successive steps. After each exposure that mutant was 

 selected which had the best yield of penicillin and the colony derived 

 from it was then used for the next exposure. In this way a strain was 

 finally produced, incorporating all the mutations together, which had a 

 yield some four to five times as high as that of the original variety. This 

 result was of considerable economic and medical value. Somewhat simi- 

 larly, Hollaender and his co-workers (Hollaender, 1945; Hollaender, 

 Raper, and Coghill, 1945; Lockwood, Raper, Moyer, and Coghill, 1945; 

 Raper, Coghill, and Hollaender, 1945), irradiating an already somewhat 

 suitable variety of the mold Aspergillus terreus in order to obtain a strain 

 with a higher yield of the economically important substance itaconic acid, 

 were successful at the same time in increasing the concentration of it in 

 the cell still further, and decreasing that of contaminating substances, 

 even though they found, as expected, that the great majority of the muta- 

 tions which affected itaconic acid production at all decreased the yield 

 of it. 



There would appear to be an enormous field still open in such work, 

 especially when the wide variety and unlimited succession of steps possi- 

 ble in evolution are taken into consideration. So, for example, it should 

 be possible by successive mutations to adapt bacteria, fungi, protozoa, 

 and viruses to new hosts, or even to make free-living ones parasitic, as 

 well as to increase their virulence for their hosts so as to make them use- 

 ful in the control of insect parasites, predators, noxious weeds, and other 

 inimical species. Tissue specificities also could be developed in parasitic 

 microorganisms, such as have already been claimed to afford an attack on 

 certain malignant tumors. In other cases, not destruction but a specific 

 constructive influence on given host tissues — as in the production of use- 

 ful galls— might be evolved. The opportunities of establishing, in organ- 

 isms not now possessing them, other beneficial forms of symbiosis with 

 microorganisms, such as already exist, for example, in ruminants and ter- 

 mites, seem so far to have been exploited only to a very limited degree. 



But the possibilities are by no means confined to parasites and symbi- 



