THE GERM-PLASM TIIKOin 309 



suits of hybridization. Tims If \]\v j)()siti()n of a 

 newly discovered factor is deteniiiiicd 1)\ compariMjii 

 with another particular known factor, it i\ pos^flijc 

 to ''calculate the results for all other known factors 

 in the same chromosome." ]\Ior<(an's ideas re^'ard- 

 ing the organization of the chromosomes coincide 

 with those expressed hy AVeismann in one r<'spect, 

 that is, they are assumed ''to have definite strnctnres 

 and not to be simply bags filled with a hoino^^^eneons 

 fluid." Wilson (191^2, p. (),S) also regards the chro- 

 mosomes as "componnd })o(Iies, consisting of diller- 

 ent constituents which undergo dilferenl modes of 

 segregation in different species." 



Students of genetics now consider the iinhvidnal 

 as built up of a number of unit characters represented 

 in the germ-plasm by factors, and when two different 

 germ-plasms unite (amphimixis) the factors do not 

 mix, but remain uncontaminated. The germ-plasm 

 of offspring which develop from fertilized vfi^i^s is 

 supposed to consist of an assortment of factors 

 brought about during synapsis and rednctionas indi- 

 cated in Fig. 84. The factors (or genes) in the germ- 

 plasm occur in pairs called alleloni()ri)hs,' and one of 

 the pair may be regarded as donn'nant, the other re- 

 cessive, as, for exani})le, the yellow and green color> of 

 pea seeds. Thns the appearance of the individual 

 depends upon the character of its dominant factors. 

 Any attempt to account for the origin of new species 



1 According to some investigators, especially in Kngland. tlie pn'senct* 

 of a factor should be considered one ailelonuirph and its absence jus the 



contrasting factor. 



