148 THE BREEDING OF ANIMALS 



I plant the grains from these ears in such a manner that 

 each ear occupies a row by itself. ... At harvest, 

 after the most careful study and comparison of the stools 

 from all these grains, I select the finest one, which I 

 accept as proof that its parent grain was the best of all, 

 under the peculiar circumstances of that season. This 

 process is repeated annually, starting each year with 

 the proved best grain, although the verification of this 

 superiority is not obtained until the following harvest. 



" During these investigations no single circumstance 

 has struck me as more forcibly illustrating the necessity 

 of repeated selection than the fact, that of the grains in 

 the same ear one is found greatly to excel the others in 

 vital power." 1 



Hallett's experience has demonstrated, first, that 

 great improvement may be secured by this method of 

 selection. Second, he advocated the " ear to row " 

 method. Third, it is probable that the results may be 

 satisfactorily explained by Johanssen's pure line theory. 



141. The presence and absence hypothesis. In 

 Mendel's conception of the theory of dominant and reces- 

 sive characters, there existed a definite determiner for 

 both the dominant and recessive characters. Each char- 

 acter appeared in the gametes in a definite form. Later 

 investigations have pointed to the fact that the dominant 

 character may be due to the presence of a specific deter- 

 miner, while the recessive character may be due to its 

 absence. This conception of the behavior of the mende- 

 lian pair of characters is quite different from Mendel's 

 explanation. 



The presence and absence theory has contributed 

 materially to the science of genetics. This hypothesis 



1 Journal of the Royal Agricultural Society, vol. 22, p. 371. 



