V. GRADUAL ALTERATION OF THE GENE 



b(b) 



in Table 10, the changed property is maintained in a usual broth, but 

 in a broth prepared with Liebig's meat extract it will soon return tO' 

 the original property ; this may indicate that back mutation is induced 

 by some factor present in the extract. When phage is developed in a 

 medium containing neither calcium nor magnesium, the property to- 

 produce plaques of extremely small size is given rise to, and this 

 property tends to be retained when the phage is incubated in an or- 



Table 10. 

 Inheritance of Acquired Character in Phage 



Test tube 

 (Phage and bact. are incubated) 



Agar plate 

 (Both large and small plaques exist) 



Large plaque 



Extract-broth 



test tube 



(Incubated 

 with bact.) 



Agar plate 



(Small plaques as 



well as large 



plaques appear) 



Ordinary broth 



test tube 



(Incubated 

 with bact.) 



Agar plate 



(Large 

 plaques onlyi 



Small plaque 



Extract-broth 



test tube 



(Incubated 

 with bact.) 



Agar plate 



(Large plaques as 



well as small 

 plaques appear) 



Ordinary broth 



test tube 



(Incubated 

 with bact.) 



Agar plate 



(Small 



plaques only) 



dinary medium, although frequently the original property is recovered 

 (65). Thus it appears that there is no fundamental difference between 

 the irreversible and reversible change in phage. 



3. The Production of Fitted Characters by Adaptation 



In addition to the reversibility, there may be another great differ- 

 ence between mutation and the gradual, reversible alteration. The 

 difference exists in that the gradual alteration will generally give rise 

 to favourable effects upon the organisms, whereas the characters 

 raised by mutation are mostly harmful. For example, although the 

 possibility of using radiation to induce useful mutations has been 

 extensively investigated, especially in crop plants, almost all of the 

 experimentally produced mutations have proved to be deleterious in 

 character {66). Further, it has been generally recognized that muta- 



