22 



AN ANALYSIS OF THE EFFECT OF SELECTION. 



two lines both came in large part from the sepia, spineless, kidney, 

 sooty, rough, and peach, spineless, kidney, sooty, rough stocks, and 

 therefore selection presumably had similar material to work with in 

 both cases. That the result was the same is, then, only a somewhat 

 unexpected coincidence. It may be pointed out that the identity of 

 the two lines is borne out by their very similar behavior after the 

 seventh and tenth generations, respectively. (See figs. 3 and 4, above.) 



CROSS OF PLUS AND MINUS LINE. 



When two races that differ in quantitative characters are crossed, 

 the usual result is an increased variability in F 2 and an increased 

 F 2 -F 3 parent-offspring correlation. This result was obtained in the 

 present case, as is shown by table 18 and figures 11 and 12, which 

 give the data for a cross of the 1002 plus and 1331 minus lines. 



Table 18. — Cross of Inbred Plus and Inbred Minus Lines. 



'Calculated after elimination of aberrant culture 3077. 



Such a result is capable of explanation in either of two ways. It 

 may be due to the segregation of modifying factors, or it may be due 

 to contamination of unlike allelomorphs in the Fi individuals. 



The contamination hypothesis presents some unusual features in 

 the present case; for the Fi Dichsets were not heterozygous for one 

 plus Dichset gene and one minus one; homozygous Dichsets always die. 

 Half of them had one plus selected Dichset gene and one minus selected 

 normal allelomorph of Dichset (i. e., not-Dichset) , the other half had 

 one minus selected Dichset and one plus selected not-Dichset. Both 

 not-Dichsets, when homozygous, give for the most part 8-bristled 

 flies, which are more "plus" than any Dichset race. Nevertheless, 

 on the contamination view, each must contaminate its mate in the Fi 

 fly, in the direction in which it has been selected. Even the minus 

 selected not-Dichset, that makes for 8 bristles, must contaminate the 

 plus selected Dichset, that makes for 6 bristles, in such a way that 

 the resulting Dichset gene makes for only 4 or 5 bristles. That is, 

 "plusness" or "minusness" and " Dichsetness " must be separable, 

 and a degree of "minusness" that affects the result produced by a 

 not-Dichset gene only very slightly must nevertheless be capable of 



