THE CHROMOSOMES 171 



between map-distance and crossover values. From 

 such careful, detailed, region-by-region studies a 

 more general statement and formulation should 

 emerge. Until such studies are completed there is 

 nothing to be gained from a priori attempts to 

 formulate the relationship. The Drosophila workers 

 had studied the possibilities of such formulae and 

 had worked out several that applied under diverse 

 conditions. Such formulae were recognized as too 

 partial and tentative to be worth publication as yet. 

 Moreover, it is evident from the foregoing that 

 within a given region the exact function of distance 

 represented by crossover values depends directly 

 upon the magnitude of the interference acting at 

 each given distance. The amount of interference 

 may be expressed by the index called *' coincidence," 

 which is obtained by dividing the number of double 

 crossovers actually observed by the number that 

 would be expected if crossings over were independent 

 of one another. Several series of results relating 

 distance and coincidence have already been pub- 

 lished, but it will require extended and special 

 experimentation before the relationship for each 

 region can be precisely determined. A formula for 

 the relation of distance to linkage has little meaning 

 in reference to the underlying chromosome processes 

 unless thus expressed, in terms of the actual coinci- 

 dences involved. 



