244 GENETICS IN RELATION TO AGRICULTURE 
the recombination series obtained in the F, 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. tabacum 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 F;. Pollinated by N. tabacum those gametes at 
the N. tabacum end of the series produce some plants which closely resem- 
ble the NV. tabacum parent and are fertile, and others less fertile and resem- 
bling the NV. 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 F; 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 /’; and perhaps where there is any missing link 
in the chain of tabacwm 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 Cnothera—Curious results have been 
obtained in Gfnothera 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- 
Digitized by Microsoft® 
