244 GENETICS IN RELATION TO AGRICULTURE 



the recombination series obtained in the Fi hybrid on the assumption that 

 the chromosome mechanism is operating normally and there is no crossing- 

 over. Neither of these assumptions is correct, but the table will show 

 the principles involved in the production of the progeny by back-crossing. 

 If it be assumed that the presence of not more than five N. sylvestris 

 chromosomes in a system containing mostly N. tdbacum chromosomes 

 or correspondingly not more than five N. tabacum chromosomes in a 

 system containing mostly N. sylvestris chromosomes will not com- 

 pletely disturb the relations within the systems to the point of failure to 

 function at all, then about 0.7 per cent, of the gametes will be functional 

 and 99.3 per cent, non-functional. This accounts for the high degree 

 of sterility displayed by FI. Pollinated by N. tabacum those gametes at 

 the N. tabacum end of the series produce some plants which closely resem- 

 ble the N. tabacum parent and are fertile, and others less fertile and resem- 

 bling the N. tabacum parent somewhat less. Conceivably some of these 

 give abnormal forms such as have been observed in the cultures. The 

 N. sylvestris end of the recombination series pollinated with N. tabacum 

 gives sterile hybrids approximating the FI in their characters and some 

 of these might likewise be abnormal. On the other hand when the N. 

 tabacum end of the series is fertilized by N. sylvestris, sterile individuals 

 result which resemble the F\ and perhaps where there is any missing link 

 in the chain of tabacum chromosomes, the resulting individuals are ab- 

 normal. The N. sylvestris end of the series, however, gives fertile in- 

 dividuals closely resembling N. sylvestris and perhaps abnormal indi- 

 viduals which have a tendency to resemble N. sylvestris. The high pro- 

 portion of fertile individuals resembling the parents in either case depends 

 on the selective elimination of the greater proportion of the gametes 

 which contain elements derived from both parents. The conception 

 then that recombination gametes must form harmonious reaction sys- 

 tems in order to function accounts in these nearly sterile hybrids for 

 the high degree of sterility, for the quick recovery of either parent by 

 back-crossing, and for the recovery of full fertility in subsequent genera- 

 tions upon return to the parental type. It is a curious consequence 

 of these phenomena that it is easier to recover the exact parental types 

 from hybrids of N. sylvestris and N. tabacum than from intervarietal 

 hybrids of N. tabacum, which are fully fertile and display all manner of 

 recombinations. 



Species Hybridization in (Enothera. Curious results have been 

 obtained in (Enothera in which genus considerable attention has been 

 given to the results of hybridization of a large number of different species. 

 Since these results have often been cited as evidence of non-Mendelian 

 behavior, it is well to consider some of them in detail. De Vries par- 

 ticularly has made a thorough study of almost every conceivable com- 



