No. 586] THE INHERITANCE OF D UBLENESS 



631 



difference there being very much greater than that shown 

 in the tables in the paper cited. 7 Fifth, in the cultures 

 just mentioned the doubles had larger leaves than the 

 singles and evidently were decidedly larger as young 

 plants. It seems that the double form (ss) is superior to 

 the heterozygous single (Ss) of this double-throwing race 

 in general vegetative vigor, and a similar difference may 

 exist between s and S gametes; on these facts probably 

 depend the peculiarities of the observed ratio. 



In order to make the case of doubleness in Mattltiola 

 as clear as possible, let us consider a brief summary of 

 the formulations that have been proposed. There are 

 two essential points to be explained, namely: (1) the fact 

 that the singleness factor or set of factors of the double- 

 throwing races can not be carried by functional pollen, 

 although the corresponding factor or factor-group of the 

 pure single races so far tested is normal in relation to 

 pollen, even in single-double hybrids; (2) the fact that 

 the double-throwing races show a small but fairly con- 

 stant excess of doubles over 50 per cent. 



Miss Saunders gives a formally adequate but rather 

 complex factorial hypothesis for (2). She leaves (1), 

 however, essentially unexplained ; she evidently relegates 

 it to the realm of somatic segregation, and in any case 

 makes no suggestion as to the real cause of the uniform 

 elimination of singleness. 



Goldschmidt, on the other hand, gives for (1) a hypoth- 

 esis of selective sterility which is adequate, though of 

 obviously unnecessary complexity, but fails with (2) 

 about as completely as Miss Saunders does with (1). 



It is here maintained that an extension of the general 

 idea of selective elimination or viability, in any one of 

 the several forms consistent with the evidence, complies 

 with all the requirements, adequately explaining both (1) 

 and (2). It might seem, at first thought, that the as- 

 sumption of a difference between S and S 1? or of the exist- 



' 'Mutation in Matthiola." 



