Xo. 592] THE MECHANISM OF CROSSING-OVER 21 7 



The above classification of the flies, in respect to where 

 separation of factors occurred, is based on the assump- 

 tion that the factors are linked in the order: se-dic-sps- 

 so-r, as on any other arrangement the above results 

 would show many more coincident separations between 

 certain factors, than single separations. The reader may 

 convince himself of this by working out the numbers of 

 the different kinds of separations on any other scheme. 

 We may say, then, that the above results prove that the 

 factors are linked in the order just given. Turning to 

 the individual separation frequencies, it will be seen that 

 se and die separated 25 times when there was no other 

 point of separation, and 14 times when there was a coin- 

 cident separation, i. e., 39 times in all. As there was a 

 total of 432 flies this means that these factors separated 

 in 9 per cent, of cases, i. e., are 9 units apart. In a 

 similar way the results show that the distance between 

 die and sps is 10, between sps and so is 11, and between so 

 and r 19.5, giving a total distance between se and r of 

 9 + 10 + 11 + 19.5 = 49.5, which agrees well with the 

 value 55, obtained by combining all the records of crosses 

 of this general sort. 



The following summary of * 1 distances" between vari- 

 ous factors of the third group (considered two at a time) 



