GREGORY: EFFICACY OF MUTATION BREEDING 463 



highly susceptible varieties. In the variety bearing some tolerance 

 to the disease significant increases in tolerance have been selected 

 among its progenies. Cooper and Gregory (9) observed an increase 

 in the variability of tolerance to Cercospora leaf spot in peanuts, 

 but this also was in a variety where tolerance was already at a rela- 

 tively high level. 



The conclusion appears inescapable that despite the large num- 

 ber of desirable mutations reported in the literature there have been 

 sought highly desirable characteristics which have not been attained 

 by artificial mutation after extensive trials. Furthermore, since 

 many investigators do not report the negative results of breeding 

 trials, there may be many more instances of inefficacy of mutation 

 breeding than we suspect. It has to be admitted also that with a new 

 character no a priori decision can be made as to (a) whether a certain 

 desired change can be had through mutation, (b) how much mutagen 

 should be applied, or (c) how large a population would be required. 

 One might ask, for example, what it would require of mutation 

 breeding to produce a peanut with tendrils. Miracles of mutation 

 or even the more optimistic predictions of breeders simply cannot 

 be realized because of points of no return attained already in the 

 evolution of species. 



These considerations appear to contradict the thesis that muta- 

 genic agents simply increase the rate of natural mutation and the 

 logical conclusion therefrom that since nearly every conceivable 

 change has occurred in plants and animals, nearly every conceiv- 

 able change should occur if sufficient number of mutations were 

 produced. The fallacy lies in the failure to recognize the evolved 

 relationships between variation at the gene level and the acceptance 

 of that variation by the genome and by the organism. 



Under the various conditions of nature various organisms 

 have found the balances which worked for them in particular environ- 

 ments. They have opposed the conservatism of linkage to the oppor- 

 tunities of crossing over, the possession of a few large chromosomes to 

 having a large number of small ones, self-fertilization to cross- 

 fertilization, and have evolved many of the possible compromises of 

 different levels of these characteristics. 



Mutation, sometimes thought of erroneously as an extra-organis- 

 mal force, is a characteristic of organisms related particularly to 



