No. 592] THE MECHANISM OF CROSSING-OVER 213 



while undertook experiments with a view to determining 

 the order of these factors, their frequencies of separa- 

 tion, and the manner in which these frequencies vary, and 

 also sought to correlate with the results the data previ- 

 ously obtained. 



It has developed from this work that group III is of 

 the same " order of magnitude" as the first and second. 

 This is the result required by the cytological facts. To 

 complete the parallel, it should be found that the third 

 group is longer than the first and, in fact, of just about 

 the same length as the second group. Whether this is 

 true can not yet be stated definitely, but the results indi- 

 cate that it is. It is certain that the length of the group 

 of factors dealt with is at least 55, but another estimate,, 

 which, for reasons given below, would seem more prob- 

 able, gives the length as over 100. It should also be 

 borne in mind that not as many factors have as yet been 

 worked with in this group as in the other two, and it may 

 well be that other factors will be found to lie beyond any 

 of the twelve which have so far been approximately 

 " placed." Thus, even if 55 should be the normal value 

 for the factors dealt with, the whole group may very well 

 be considerably longer. In the first and second groups,, 

 factors lying well beyond all the others were discovered 

 after the positions of more than a dozen had already been 

 determined. 



The reason for the uncertainty in regard to the total fre- 

 quency of separation among those factors which have 

 been worked with is to be found in the linkage variation. 

 Sturtevant had discovered that certain races, containing 

 the mutant factor for ebony body color, gave extremely 

 low frequencies of separation ; that is, ebony flies, when 

 crossed to those with pink eyes (pink is also in group 

 III), gave a hybrid in whose germ cells very little recom- 

 bination between pink and ebony occurred. Other races 

 (e. g., those containing sooty, an allelomorph of ebony), 

 when crossed to pink, gave higher values. He therefore 

 concluded that the ebony flies contain a factor (let us call 



