84 PLANT-BREEDING 



a thousand separate types of combinations do not require 

 more than ten mutually independent changes. Or, in other 

 words, the wide range or variability observed in grain fields 

 may be the effect of the production of a few novelties, com- 

 bined with a sufficient degree of intercrossing. Hence, it 

 follows that one real change of some character in a year, or 

 even in ten or more years, must be considered as wholly suf- 

 ficient for an explanation of the observed variability. If we 

 take it that these changes appear suddenly, or in other words, 

 are mutations, then an ordinary degree of mutability, such 

 as is quite common with horticultural plants, seems to be 

 all that is required to explain the numerous types observed 

 at Svalof. The main difference would be that in horticul- 

 ture all profitable varieties have been observed and isolated 

 as soon as they have appeared, but that in agriculture they 

 have been allowed to pass without observation, or at least, 

 without appreciation. Consequently, the larger horticul- 

 tural groups, such as asters, carnations and dahlias, now 

 contain hundreds of well-defined, pure and uniform varieties, 

 but the agricultural varieties are still almost everywhere 

 mixtures awaiting the process of sifting and testing. 



The experiments of Svalof, however, give at least some 

 evidence concerning the probable origin of the variability of 

 the cereals. Up to this time I have described the pedigree- 

 cultures as constant and uniform, and only excluded the case 

 of the hybrids. But even within the purest races deviations 

 may occur from time to time, though rarely, and these may 

 be compared with what probably happens in the field. When 

 a race is started from one selected mother plant and multi- 

 plied during some years so as to cover hundreds of acres, it is 

 ordinarily seen to keep wholly pure, all the thousands of in- 

 dividuals displaying the same characters and qualities. This 

 is precisely the special feature and the advantage of the cul- 

 tures after the Svalof principle. But, from time to time, one 



