334 GENETICS IN RELATION TO AGRICULTURE 



entirely regular or continuous. The saltations or jumps revealed by 

 Surface's analysis were directly consequent upon lump elimination of 

 a number of mediocre lines. These results, therefore, are in entire har- 

 mony with the known nature of allogamous populations. This conclu- 

 sion is further corroborated by the recent report of Reitz and Smith on 

 the statistical study of indirect effects of selection for high and low pro- 

 tein and oil. These authors state: 



"It is found that four distinct types of corn as regards length, circumfer- 

 ence, weight of ears, and number of rows of kernels on ears are so well estab- 

 lished that we may assign orders of values to the means of these characters that 

 persist with but a few exceptions in such changes of environment as have been 

 experienced in 11 years of planting, from 1905 to 1915. 



"While a few slight but progressive changes have been noted, the selections 

 for chemical composition from 1905 to 1915 have not changed decidedly the dif- 

 ferences in mean values of these characters. In fact, we are unable to assert with 

 any high degree of probability that the strains differ more or less with respect to 

 these characters during the second half of the period 1905 to 1915 than during the 

 first half." 



The italics are ours. It is of especial significance that careful biometrical 

 study has failed to reveal any progressive change as a result of continued 

 selection in these strains of corn. For the results of these experiments 

 have been cited as evidence par excellence by Castle in support of his 

 hypothesis of factor variability. 



The ear-to-row method has been modified in various ways but it 

 still forms the basis of most systems of commercial corn breeding. A 

 popular feature of systematic corn improvement is the use of score cards 

 in judging. A special development of the score card method of selection 

 is the use of selection index numbers as advocated by Pearl and Surface. 

 In this plan arbitrary values are assigned to various characters of the 

 corn ear, for example, such as absolute size of the ear, average percentage 

 depth of the grains, etc. The idea is to combine in a single numerical 

 expression the values of a series of variable characters with regard to 

 all of which the breeder wishes to practice selection at the same time. 

 The index numbers of different varieties are not directly comparable 

 but for a given variety they may be useful as an adjunct of the score 

 card method. However, their use requires more attention to details 

 and hence greater expense than most breeders can afford to give. Their 

 use in plant breeding will probably be limited to experiment stations 

 (see Chapter XXXI). 



The danger of continued ear-to-row selection or "narrow breeding" 

 within a variety was pointed out in 1909 by Collins, who emphasized 

 the importance of "broad breeding" in such crops as exhibit loss of 

 vigor when closely inbred. About the same time Williams inaugurated 



