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THE AMERICAN NATURALIST [Vol. L 



two stocks, but in opposite directions in the two sexes. 

 The judgment based upon measurements accordingly con- 

 firms the judgments based upon inspection. 



ichs from Uncrossed 



Dachs, Outcrossed 



Summary 



1. Recent results complete the parallelism between fac- 

 tor groups and chromosomes in Drosophila. This 

 strengthens the evidence that separation of linked factors 

 is due to an interchange between chromosomes. 



2. The chief gaps in the information regarding the total 

 frequency of interchange in the different groups have been 

 filled, and it is found that the usual total frequencies of 

 separation correspond to the lengths of the chromosomes. 

 This constitutes specific evidence that crossing-over is the 

 method of interchange between the chromosomes, and that 



