GENETIC BASIS 43 



of chromosomes, with 2 to 3 chiasmata per chromosome and 

 with at least a sHght tendency for these chiasmata to occur 

 more frequently in certain parts of the chromosomes. Under 

 such conditions, with about 100 gene differences between 2 

 species, the cohesive force of multiple-factor linkage would 

 be in the neighborhood of 1/500,000 of free recombination. 



CHARACTER ASSOCIATION AS A CRITERION OF 



HYBRIDITY 



New and powerful criteria for the analysis of hybridization 

 under natural conditions were offered by the demonstration 

 that all the multiple-factor characters of an organism are 

 linked with each other so strongly that in species crosses it 

 would take scores of generations of directed breeding to 

 break all the linkages. Two criteria were pointed out specifi- 

 cally in 1939 (Anderson, loc. cit., p. 692). "1. The intermedi- 

 acy of separate characters will be correlated. Hybrids in- 

 termediate in one character will tend to be intermediate in 

 others. Hybrids which are most like either parent in any one 

 character will tend to resemble that parent in all other char- 

 acters. 2. Variation between individuals will lessen as 

 parental character combinations are approached." The 

 application of these criteria (and similar criteria based on 

 multiple-factor linkage) make it possible to take most argu- 

 ments concerning natural hybridization out of the domain 

 of opinion and into that of measurement. If those who are 

 inclined to argue about the importance or nonimportance of 

 hybridization under natural conditions would only gather 

 precise data on character recombination in natural popula- 

 tions, we should have the facts on which sound opinions 

 could be based. By such methods as those demonstrated in 

 Chapter 6, it is now possible to procure critical data from 

 variable populations, which will demonstrate conclusively 

 the role of hybridization in that particular population. It 

 may be well, therefore, to give a detailed discussion of the 

 theoretical basis for these criteria. 



