72 



INTROGRESSIVE HYBRIDIZATION 



fused with sativa and with utilis. Had population samples 

 of these ^titles been examined, however, it would have been 



N. LangsdorffU 



F^ (Langsdorffii X alata) 



Fj (Langsdorffii X alata) 



Plates 4 and 5. The basic facts of the genetics of species crosses, graph- 

 ically summarized. Shown to scale are representative flowers of Nico- 

 tiana Langsdorffii^ N. alata, their Fi and F2 hybrids, and backcrosses of 

 the Fi to each parent species. Note the uniform and intermediate Fi, 

 the highly variable F2, and the generally close resemblance of each back- 

 cross to its recurrent parent. 



possible to define these entities exactly and to distinguish 

 between them. Furthermore, by such methods as those 

 outlined in Chapter 6 one could have considered the dy- 

 namics of the whole group. He could have demonstrated 



