350 



THE AMERICAN NATURALIST 



[Vol. L 



secure stock that contained many mutant, linked factors at 

 the same time. But, as was explained in the account of 

 experiments with the third chromosome, it is necessary, 

 in dealing with linked factors, to make the crosses in a 

 particular way to secure a "multiple stock." Thus, it 

 may be pointed out again here, a stock containing factors 

 A, B, C cannot be obtained ordinarily by crossing stock 

 A to stock C, and then crossing the double stock A C (pro- 

 duced in F 2 , F 8 , or F 4 from the first cross) to stock B ; be- 

 cause it would require double crossing-over for the hybrid 

 fly, containing A and C in one chromosome, and B in the 

 other, to produce a gamete with A, B and C all in the same 

 chromosome (assuming the factors to be linked in this 

 order). If the linkage is tight such double crossing-over 

 will never occur. But by first obtaining stock AB and 

 then crossing this to C, stock ABC may be secured ; for 

 in the hybrid fly that contains A B in one chromosome and 

 C in the other, a crossing-over between B and C will result 

 in a chromosome that contains A, B and C, the link be- 

 tween A and B not having been broken. 



In other words, the factors can only be added together 

 in a certain order, owing to their position in the linkage 

 chain. Just as in adding links to a chain, one or more 

 factors cannot be wedged in between factors in another 

 collection (except by double crossing-over) ; but if they lie 

 beyond this collection, they may be added on, either singly 

 or in a group. The information that had already been 

 gained by Sturtevant, Morgan and Bridges concerning 

 the order in which various factors lay, was therefore of 

 great service in determining how the crosses should be 

 made, to get the factors together, and besides this several 

 double stocks of a sort that could be used in the present 

 experiment had already been synthesized by them. But 

 the progress of the experiment was very considerably re- 

 tarded by the fact that the position of a number of the 

 factors which it was desired to use had not yet been deter- 

 mined. These comprised bifid and forked in chromosome 

 I and dachs, jaunty, curved, arc and balloon in II. (The 

 exact position of jaunty with respect to black, and of 



