52 



A. H. STURTEVANT 

 TABLE 6 



TABLE 7 



1^ 



p 



R] 







It will be noted that here also the evidence, so far as it goes, 

 indicated that the occurrence of one cross-over makes another 

 one less likely to occur in the same gamete. In the case of 

 BOPR there was an opportunity for triple crossing over, but it 

 did not occur. Of course, on the view here presented there is 

 no reason why it should not occur, if enough flies were raised. 

 An examination of the figures will show that it was not to be 

 expected in such small numbers as are here given. So far as I 

 know there is, at present, no evidence that triple crossing over 

 takes place, but it seems highly probable that it will be shown 

 to occur.2 



Unfortunately, in none of the four cases given above are two 

 comparatively long distances involved, and in only one are there 

 enough figures to form a fair basis for calculation, so that it seems 

 as yet hardly possible to determine how much effect double cross- 

 ing over has in pulling down the observed percent of cross-overs 

 in the case of BM and (C, 0)M. Whether or not this effect 

 is partly counter-balanced by triple crossing over must also 

 remain unsettled as yet. Work now under way should furnish 

 answers to both these questions. 



2 A case of triple crossing over witliin the distance CR was observed after 

 this paper went to press. 



