32 



INTROGRESSIVE HYBRIDIZATION 



be considered between N. alata and N. Langsdorffii. It is, 

 therefore, certain that the recombinations which we have 

 obtained are only an insignificant fraction of the recombina- 

 tions possible under free assortment. 



''To a non-mathematical mind this may seem too strong a 

 statement. When the data are presented, as for the most 

 part they necessarily must be in terms of the recombination 



Fig. 2. Extreme recombinations to be expected in the F2 between Nico- 

 tiana Langsdorffii and N. alata if there were no restrictions upon the re- 

 combination of corolla length, limb \^'idth, and lobing of the corolla. 



The letters refer to Fig. 4. 



of two characters at a time, it takes a pecuUar sort of geo- 

 metric imagination to see that the proportion of actual re- 

 combinations to total recombinations becomes increasingly 

 smaller as more characters are considered. Anyone who has 

 examined second generations or back-crosses of species hy- 

 brids will have been so impressed by their variability that it 

 will be difficult for him to accept the conclusion that such a 

 melange is only a small fraction of total recombination. For 

 such biologists, and as a sort of graphical summary of all the 

 data, figures 2 and 3 have been prepared. In figure 2 are il- 

 lustrated the extreme types of corollas wich might be ex- 

 pected in the second generation if there were free recombina- 

 tion of tube length, limb width and lobing. In figure 3 are 

 shown the closest approaches to these extremes which were 



