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 relatignship. The Drosophila workers 
had studied the possibilities of such formule and 
had worked out several that applied under diverse 
conditions. Such formule 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. 
