GREGORY: EFFICACY OF MUTATION BREEDING 467 



other obvious or major effects than to provide variation in expres- 

 sivity consistent with a changing environment. Each chromosome 

 would come into a form of internal balance, as Mather has postu- 

 lated, with respect to the amount of modulating material optimum 

 for the genome to maintain in relation to external environment. 



The relations of self-pollination, mutation, and expressivity 

 permits a genetic explanation for Stebbins' (50) observation that 

 self-pollinators possess, "a relatively high degree of phenotypic 

 plasticity so that the individual is susceptible of tremendous modi- 

 fication in the face of adverse or extreme conditions". 



The homozygote's capacity to adapt to great environmental 

 change combined with the preservation of phenotype suggests that 

 protection against expression of newly acquired mutation may 

 be high (22). It is the possibility that the height of this barrier may 

 permit a sufficient number of simultaneous mutations that makes 

 the mutation breeding of selfbreds attractive. It should be said 

 that not every breeder of self-fertilized crops shares this view (1, 

 27). 



Comparative effects of Irradiation of 

 Pure Lines and Their Hybrids 



Notwithstanding the theoretical possibility that normally self- 

 fertilizing species may utilize mutation as a resource of genetic 

 adaptability in a manner comparable to the use of heterozygosity 

 by cross-fertilizing species, it is still uncertain whether self-ferti- 

 lizers are capable of absorbing to advantage very much larger muta- 

 tion rates than those established in the species by natural selection. 

 The solution of this problem requires knowledge of the effects 

 of mutation on characters equatable to fitness in a number of dif- 

 ferent species. Interest in the effects of mutagenic treatment on 

 quantitative characters, some of which are the most likely indi- 

 cators of fitness, has been slow in its development. As a consequence 

 of this it will be difficult to give a satisfactory assessment of the sit- 

 uation at the present time. 



Papers which have laid the foundation for further experi- 

 ments on quantitative characters have been published by Brock 

 and Latter (6), Daly (10), Gregory (19), Mertens and Burdick (36), 

 Oka (39), and Rawlings, et al. (41). For an assessment of the efficacy 



