30 INTROGRESSIVE HYBRIDIZATION 



Up to this point our exposition has been concerned with 

 relatively simple cases of a multiple-factor difference af- 

 fecting a single character (such as leaf length, for instance). 

 In nature, of course, we never meet with such simple cases. 

 Species do not differ from one another just in leaf length and 

 nothing else, but in various characters. Some of these dif- 

 ferences are clearly multifactorial in their genetic basis; 

 others, such as flower color or color pattern, are much simpler 

 and result largely from differences in one or a few pairs of 

 genes. 



The genetics of a species cross is, therefore, a far more 

 complicated subject than those examples we have been con- 

 sidering. Both the basic data and the basic theory are chal- 

 lengingly difficult. To catalogue in their entirety the simul- 

 taneous changes in a whole set of characters in an F2 popu- 

 lation, presenting an overall picture of the extent to which 

 each character is independent of the variation in each of 

 the others, is a complex task. No such body of data has yet 

 been published for any species cross. Nor do we yet have a 

 generalized theoretical presentation in genetic formulae, 

 demonstrating the effects of large numbers of genes, or- 

 ganized in linkage groups, in hybrid and in backcross popu- 

 lations. Considering its theoretical and its practical im- 

 portance, a thorough exposition of hybrid segregation in 

 finite and in infinite populations is badly needed. To deter- 

 mine the overall effects of all the gene differences in all the 

 chromosomes upon all the characters of successive hybrid 

 generations, making due allowances for the effects of linkage 

 and of finite populations, is almost beyond the power of the 

 human mind. But because it is so difficult it is a challenging 

 subject. In the following pages we shall not present any 

 such generalized theory but shall attempt to determine (one 

 at a time) the effects of those general forces that operate in 

 all species crosses. Of these the most universal is linkage, 

 and we shall try to estimate its cohesive effect upon the ex- 

 tent of character recombination and upon the comparative 

 frequencies of different types of recombinations. We shall 



