MYERS : DISCUSSION OF SESSION III 297 



ability in the population. Therefore, within limits, the greater the 

 appropriate genetic variability, the better the average chance and 

 magnitude of success. 



Before considering the tools available to the plant breeder for 

 developing populations within which he practices selection, it will 

 be desirable to consider the kinds of genetic variation, from the stand- 

 point of source, available to him. For purposes of consideration, these 

 may be divided into the following four classes: 



1. Spontaneous mutations that were in the wild ancestor of the 

 species or that have occurred and been preserved since its 

 domestication, 



2. Spontaneous mutations that have occurred and been preserved in 

 related species, 



3. Spontaneous mutations that arise in the breeding cultures, 



4. Induced mutations produced by any of the mutagenic agents. 



From the considerations given above, it becomes clear that the 

 only basic difference between so-called "mutation breeding" and 

 "conventional breeding" is in the source of genetic variability used 

 by the breeder. It would seem preferable, therefore, not to speak of 

 "mutation breeding" as a distinct method, but simply to recognize 

 induction of mutations as a tool for obtaining genetic variability. 

 For the population with which he works, the plant breeder may accept 

 those available, either from natural sources or from mutagen-treated 

 materials. On the other hand, he may build populations by cross- 

 breeding, using parents that differ either because of natural muta- 

 tions, induced mutations, or both. Finally, he may produce new 

 populations with more desirable frequencies and combinations of 

 genes by such tools as backcrossing or recurrent selection. Here again, 

 he will be using mutations as the sources of genetic variability and 

 these mutations may be spontaneous in origin, induced by mutagenes, 

 or both. Induction of mutations as a breeding tool must be considered 

 for efficiency or value, therefore, not in comparison with "conven- 

 tional" breeding methods, but rather in terms of induced versus 

 spontaneous mutations. It is appropriate, therefore, to consider in 

 more detail the kinds of variation outlined above. 



No one who has worked with the crop plants can help but be 

 impressed with the enormous natural variation that exists in most 

 species. Doctor Quinby has pointed out the variation in sorghum. The 



