IS-J Sliull. 



(leteniiiuers involved in any given cross liave been based. NiLSSON- 

 Ehle (1911) lias described a case in which the range of variation in 

 the length of heads of wheat in the Fs considerably exceeded the com- 

 bined ranges of the two parents. Hayes (1912) has found a similar 

 case in the number of leaves in tobacco, and Emerson and East (1913) 

 have seen the same phenomenon in the length of internode and total 

 length of stalks in maize. It seems probable that such transgressive 

 vaiiation may be the rule rather than the exception when very complex 

 characters are investigated-; for it is hardly to be expected that a large 

 number of plural determiners, affecting such a character, .shall all act in 

 the same direction, or that the i)aient having the highest development 

 of the given character shall generally contain all the genes which the 

 otin^i' ciiosen parent possesses. Wlienevei' such transgressive variability 

 is producible by the genotypic recoml)inations of parental characters, 

 the freipiency witli which Fo individuals simulate either parent, gives 

 no clue to the total number of plural determiners which have been 

 brought together, with respect to any character under consideration. 

 Till' difficulty of making an estimate of the number of genes which 

 afffct tiie same cliaracter will be more fully appreciated when it is 

 ki'lit cleaily in mind that these plural determiners need net lie duiilicate, 

 and Ihat conseiiuently there i.s no reason for assuming that the in- 

 fluenre of the several determiners is quantitatively equal. 



(,)iialitative and quantitative differences in the effects individually 

 ]tr(i(liice(l by the several plural fartors for a character will assist in 

 interpreting certain phenomena for which less simple hyi)Otheses have 

 lieeii (iffered. The now celebrated hooded I'ats may serve to illustrate: 

 Because the hooded -pattern reappears in all crosses as a Mendelian 

 recessive to the s{>lf-(!oIored pelages, in the simple monohybrid i)roportion, 

 it is accepted by Castle (1912) as a case in wliich a single de- 

 terminer is involved. Selection of high and lew ('xtremes of this 

 pattern during a s(>ries of generations has resulted in increasing the 

 size of the pattern in the one series and in diminishing it in the other, 

 jnst as CuKNOT (1907) found te be true in regard te tlie iiieliald- 

 pattern of mice. When hooded rats from either the plus ei' tlie minus 

 selected series are crossed with self-colored rats the hooded -i)attern 

 still acts in each case as a simple monohylirid iccessive, though the ex- 

 tracted pattern is somewhat larger when an individual of the plus 

 series has been used in the cross, and somewhat smaller when the 

 hooded jiareiit was taken from the minus series. Castle concludes, 

 therefore, tliat seh'ction does not sim])ly s(U-t out variations already 



