86 THE ROYAL SOCIETY OF CANADA 



responsible for even these slight differences. In this case the diffi- 

 culties of the multiple determiner hypothesis are greatly multiplied. 

 In general it appears that no matter how great or small the parental 

 difference, the ^^2 variation is very uniform, covering nearly or en- 

 tirely the whole range of parental variation. The number of heredi- 

 tary determiners which it is necessary to assume as distinguishing the 

 earliest from the latest parents may thus be increased indefinitely. 



One might put the argument in the converse form. The recovery 

 of the parental conditions in a small number of F2 plants of crosses 

 involving a wide difference limits the possible number of factors to 

 a very few. Subtracting two pairs of factors for each 5 or 6 days 

 difference, we find that all the possible factors are used up long before 

 the smaller differences are reached. 



What has been said concerning the ripening period applies also 

 to the heading period except that the differences are smaller. 



The conclusions just 1 cached should be tested more thoroughly 

 by raising F3 and F4 generations and thus determining the exact 

 constitution of the F2 plants. They can further be tested by crosses 

 involving on the one hand wider differences than 20 days and on the 

 other hand differences of less than 5 days. But the data already 

 given when taken as a whole seem to be quite unexplainable on the 

 multiple determiner hypothesis. On the basis of this hypothesis 

 AB + BC + CD + DE is greater than AE. 



(3) Economic Application. 



The economic value of early ripening varieties in Western Canada 

 has been explained in the introduction. In the F2 generation of a 

 cross between an early and a late variety there are found combinations 

 of earliness with various characteristics of the late parent and of late- 

 ness with various characteristics of the early parent. The experi- 

 mental results have shown apparently that no valuable qualities are 

 correlated with lateness and no undesirable qualities with earliness. 

 Early segregates are often quite as large and productive as late ones. 

 Earliness can therefore be brought into association with other desirable 

 qualities by the usual Mendelian methods. But on account of the 

 large proportion of intermediate plants in the second hybrid genera- 

 tion it would be necessary to grow much larger numbers than is the 

 case in simple examples, in order to secure any desired combination 

 of characters which includes extreme earliness. And the greater the 

 parental difference the more plants must be grown. Of course it 

 would often be of great value to substract a few days only from the 

 ripening period, and in this case the intermediate plants themselves 

 would serve. 



