IMPROVEMENT OF WHEAT 35 



portion of this chapter is exclusively devoted to the plant, the 

 treatment of which naturally comes first. 



Variation. It has been recognized for at least a century 

 that wheat is capable of variations. These may be peculiar to 

 the plant itself, and may occur although the environment re- 

 mains constant. Variation in this sense became established 

 only with the theory of evolution, and refers to those changes 

 which tend to become permanent through inheritance. Such 

 variations are assumed to be the manifestations of a natural 

 tendency inherent to all organic life. 



The theory of common descent for all living beings found 

 its first great advocator in Lamark at the beginning of the 

 nineteenth century. Fifty years later Darwin assembled enough 

 evidence in support of the theory to enable it to gain general 

 acceptation. Darwin assumed that the great variation in- 

 volved in the theory proceeded in the main by slow and gradual 

 changes. He recognized, however, that species may also origi- 

 nate in nature by leaps and sports. The theory that all varia- 

 tion occurs by sudden mutations has been held by a minority 

 of scientists. Cope and De Vries 1 are among those who have 

 most recently increased the evidence in this direction. A de- 

 fence of discontinuous evolution has also been made by various 

 other scientists, such as the paleontologist Dollo, the zoologist 

 Bateson, and the botanist Korshinsky. In general, it may be 

 said that if the followers of Darwin have been open to the 

 criticism of under-emphasizing sudden change, the supporters of 

 the theory of mutations have certainly erred more widely in the 

 opposite extreme. 



Variations may also be induced. In this process two different 

 methods may be used, hybridization and change of environ- 

 ment. Only those variations which may occur or be induced 

 independently of environment are considered in this chapter. 

 Others are treated in subsequent chapters. Variations may 

 include differences in habit of growth, chemical composition, 

 periods of development, appearance, form, yield, prolificacy, 

 vigor, hardiness and stability of type. Whatever his concep- 



1 An able criticism of the theory of mutations has been made by 

 Prof "W. P. R. Weldon, "Professor De Vries on the origin of species," 

 Biometrika, 1:365, 1902. A study of this theory is interesting in 

 conjunction with the more elaborate theory of homotyposis devel- 

 oped by Prof Karl Pearson in his work at University College, Eng- 

 land, but space forbids a discussion of the matter here. 



