05^ - 



■^ • 1. 1 B R A Pi \ 



Foreword 





This little book is concerned with hybridization under those 

 circumstances which we so glibh^ refer to as "natural conditions," 

 that is, with the results of hybridization outside the laboratorj^ 

 and the breeding plot. It passes no judgments on the importance 

 of hybridization in evolution but attempts to take this whole prob- 

 lem outside the area of argument and opinion and into the zone 

 of measurement and analysis. It is verj^ largely concerned with 

 how the effects of hybridization can best be measured in natural 

 populations and with a discussion of the forces at work in such 

 populations. 



Most of the techniques presented here are comparatively simple 

 ones that have been developed for analyzing interspecific and 

 intraspecific variation. Observation and measurement are used 

 much as in traditional taxonomic work but refined to a point where 

 thej^ can be employed for anah^sis as well as for description. By 

 means of such techniques it is now possible for a trained observer 

 to work intensively with a hybrid population in a region completely 

 new to him and from it to deduce exact descriptions of the hybrid- 

 izing species, even when he has never seen that species (see pp. 43 

 to 48 and 92 to 99). 



An}^ field of stud}^ that is in the process of shifting from the 

 descriptive phase to the analj^tic phase is certain to suffer from 

 growing pains. This one is no exception. The first methods used 

 were crude, and the ones described below need further improve- 

 ment. 



This book is a step forward in that the relevant literature is now 

 brought together for the first time. Previous presentations have 

 been piecemeal. The basic theory- appeared in genetic journals 

 (Anderson, 19396); applications to taxonomic problems, in taxo- 

 nomic journals (Anderson and Turrill, 1938; Anderson and Hu- 

 bricht, 1938); and practical applications to plant breeding prob- 

 lems, in still other places (Anderson and Hornback, 1946). This 

 previous division of the subject matter was not capricious. It 

 resulted from the fact that the concept of introgression was merety 



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