MUTATIONS IN PLANT BREEDING 371 



which gave rise to existing varieties. Factor mutations conform in 

 their manner of occurrence with de Vries' mutation theory; they arise 

 suddenly, they occur in all directions, they are heritable, and some of 

 them are advantageous to the species and are preserved by natural 

 selection. When so preserved they give rise to new forms or races, and 

 when fostered by man they make possible new horticultural varieties of 

 plants or new breeds of animals. But as yet we have no ground for 

 asserting that factor mutations are caused by geographical differences 

 or by any specific elements of the environment. From his study of varia- 

 tion in tobacco Hayes reached the conclusion that while environment is 

 of great importance in breeding tobacco as well as in growing the commer- 

 cial product, yet change of environment "does not cause a breaking up of 

 type, and whatever variations occur due to environment appear alike in all 

 plants of a particular type." Thus it appears that mutations arise quite 

 independently of conditions of culture, and it is probable that they are 

 somewhat more frequent than has generally been supposed. It is certain 

 that mutations which are undesirable for agricultural purposes are quite 

 as apt to occur as are desirable ones. For this reason neglect of seed selec- 

 tion has caused the "running out" of many good varieties. The breeder 

 who would improve the best existing varieties which are adapted to a 

 given location must either resort to hybridization between the varieties or 

 else search for the most desirable biotypes within each variety. Even 

 though hybridization is clearly necessary from the first, it may well be 

 preceded by a systematic search for the best forms within the varieties to 

 be crossed. 



One of the most successful attempts to take advantage of the desirable 

 mutations which had occurred within cultivated varieties was carried out 

 by Surface and Zinn at the Maine Experiment Station in their experi- 

 ments on oat breeding. Oats being self-fertilized, they assumed that 

 any new characters which had originated would breed true. The two 

 most desirable improvements in the commercial varieties of oats culti- 

 vated in Maine are increase in yield and in strength of straw. Accord- 

 ingly individual oat plants were selected with these two points in mind. 

 This work began in 1910 when 460 plants were chosen from 18 different 

 commercial varieties. Of these only 188 were selected for planting in 

 1911 and on the basis of the results obtained 80 were continued for test- 

 ing in duplicate 1-2000 acre plots in 1912. Of these pure lines 34 were 

 sufficiently promising to be continued into field tests in 1913. Thirty- 

 one of these were again tested in 1914. In 1915 all of these pure lines 

 were discarded except 12 and these were tested in quadruplicate plots in 

 1915. In each of the 3 years 1913-15 these pure lines were grown along 

 with a number of the best commercial varieties obtainable. In 1914-15 

 the pure line plots alternated in the field with commercial variety plots. 



