viii FOREWORD 



a by-product of my long-continued (and still continuing) absorp- 

 tion with the genetics of multiple-factor characters. Therefore, 

 not only has a well-rounded discussion of the work on introgression 

 never previously been attempted, but also a good deal of what is 

 presented below has never appeared in print. On the other hand, 

 the bibliography is limited to cited works, since an inclusive bib- 

 liography on introgression by Dr. Charles Heiser is shortly to 

 appear. 



This is largely a book about methods for studjdng hybridization 

 in the field. It is to be hoped that application of these methods 

 and their consequent refinement will produce data from which 

 eventually we can estimate the relative importance of hybridiza- 

 tion in evolution. 



In this book the more usual methods of analyzing hybridization 

 (transplant studies, cytological analysis, pedigree culture, repe- 

 tition of suspected hybridization) receive little more than passing 

 mention. It goes without saying that these methods should be 

 used whenever the facilities for them are at hand. All these tech- 

 niques were employed in the special studies of Tradescantia, Iris, 

 and Nicotiana, from which these newer methods derive their theo- 

 retical and experimental verification. It should be emphasized, 

 however, that from a corollary of the demonstration of multiple- 

 factor linkage (see p. 43) we have a new and powerful criterion for 

 hybridity. 



Furthermore, the general method (pp. 92 to 99) of extrapolated 

 correlates (and the more specialized techniques here described as 

 '^pictorialized scatter diagrams," radiate diagrams, standardized 

 photographs, etc.) have proved to be of wide adaptability in ana- 

 lyzing the effects of such hybridization. Though these methods 

 are here described in full for the first time, they have been rather 

 widely used by my students and colleagues. 



Edgar Anderson 

 Missouri Botanical Garden 

 St. Louis, Mo. 

 January, 1949 



