TYPE AND VARIABILITY 445 



the actual type of this crop, though the different characters 

 involved would vary differently from this type or mean. 



SECTION VI EFFECT OF SELECTION UPON TYPE 

 AND VARIABILITY 



The whole purpose of selection is to influence type. In the 

 minds of some it is also to reduce variability. The real effect of 

 selection is well brought out in data (see table, page 446) from 

 Dr. Hopkins's experiments 1 in endeavoring to influence chemical 

 composition of corn by the method of selection. It is to be 

 noted that all four strains sprung from the same original stock 

 (163 ears), and that each seed selection was made from the 

 highest (or lowest) ears within the several strains ; that is, the 

 high-oil stock, for example, originated from those ears show- 

 ing the highest oil content in the original stock, and was 

 developed by successive selection of the highest oil ears, always 

 within the high-oil stock, and similarly for the other strains. 



Discussion of data. A critical study of the columns of means 

 shows a steady rise in the means of both high-protein and high- 

 oil strains and a corresponding decline in low-protein and 

 low-oil strains, indicating a prompt response to selection. An 

 inspection of the columns of standard deviations and coefficients 

 of variability, however, reveals the fact that the variability 

 is practically unchanged. This agrees with the mathematical 

 theory, to be developed later, namely, that the effect of selection 

 is to shift the type but not greatly to reduce variability. 



The peculiarity of this sort of selection is that it is progres- 

 sive ; that is to say, with each response to selection a new 

 standard is set up still more difficult to meet than was the 

 old one. Under these conditions of continuously advancing 

 standards and with rapidly advancing types, in only two out of 

 the four cases was the variability apparently reduced, and this 

 whether we regard the standard deviation or the coefficient of 

 variability. This power of response to the demands of an ad- 

 vancing standard of selection is immensely suggestive and will 

 be considered further under " Heredity." 



1 See Bulletin Xo. 119, University of Illinois Agricultural Experiment Station. 



