No. 636] 



GENETIC SEGBEGATION 



7 



pamila persicifoJia, e^^pednWy (;il)()iit iiidit'- lii-ln. :i 

 plant so strikingly different fn.in tlio tyi..- ( L* ;! Icrt 

 ■ high), that it is sold as a speeies, 'T. mli^l,,." Tlic 

 leaves of the dwarf are an intense, dark .urei'ii. 'V\\\< 

 variety is, of course, a recessive and segregate- without 

 interaiediates, yet, if the qualitative distinctions wcic 

 less evident it might easily be classed as a \ ai'iety m 

 quantity. But tlie critical distinction is certainly quali- 

 tative and the great difference in size is consetjuential. 

 Though in such cases segregation is complete, it may 

 almost be said to he cliaracteristic of jmrely quantitative 

 distinctions that one or other of the original parental 

 types fails to reappear in its extreme form after a cross. 

 So usual is this feature in ciuantitative segregation tliat 

 the phenomenon must have special significance. 



Another fact is beginning to emerge which tnu>t con- 

 tribute to the shaping of a conception of the physio- 

 logical nature of segregation. We liav<' \v\\v\\\ that 

 groups or complexes of factors nia\ -c-rcuatc wliole. 

 To such a complex the distinction in sex is due. l>ul in cer- 

 tain cases it may break up. The occuri-em-e of a large 

 spur in fertile hens (Legliorns for instance) nuist be re- 

 garded as due to the absence of that part of the sex-com- 

 plex wiiich in tlie normal inhil)its the growinu" of tlie 

 spur. In ord.inar\' fowls the whole inliil)itina' uron]> i-e- 

 mains on the female -idc. hut the -i)ur-inhil)itiim .■Icnu-ut 

 can evidently separate from the rest. On the other 

 hand, when in the cocks ot' certain bi'ced- (a- occa-i(tn- 



