No. 604] SHOBTEE ARTICLES AM) nis< UssloXS 



245 



to some very probable conclusions as to the time and method of 

 segregation of these determiners; though it does not yet enable 

 us to identify with absolute certainty the actual physical unit of 

 segregation, the cytological and genetic evidence indicates 

 strongly that such a unit exists. 



In the current chromosome hypothesis, as developed especially 

 by JNIorgan and his collaborators (Morgan, Sturtevant, :Muller, 

 and Bridges, 1915), the material unit of segregation is assumed 

 to be a part of a chromosome. Breaks in two homologous chromo- 

 somes at meiosis, with consequent exchange of parts by the pair, 

 presumably occur at certain definite points only. ITow close these 

 points may be we can not say, but the general stability of ]\Ien- 

 dclian characters indicates that the number of points is limited. 

 Ou these assumptions, the portion of a chromosome between two 

 adjacent points of possible breaking is the ultimate physical unit 

 of genetic segregation — essentially a locus as defined by ]\Iorgan 

 (1915, p. 419).=^ 



It is now widely reeogtiized that, in efTeet. a single real unit of 

 segregation may influence very diverse characters of tlie soma. 



Very possibly one physical unit of segregation may affect, say. 

 flower color and height in w.iys jusl as distinct physiologically 

 as may two distinct units of s("..ri'e<r:ition, although transmission is 

 different in the two cases. !n ih,. lat;cr case we say that two 

 genetic factors are conccfiu'd : are we compelled in the former 

 case to admit only one'/ 



As a matter of fact, as will he eviiiont on further considera- 



factor accepted. 



If by "factor" we mean a <h rrlopnu ,>JaI pofrnfialitu, the de- 

 limitation of a iiartieular factor is largely a matt(^r of convenience 

 iti analysis. On tlie oiher liatid. if the term is used to designate 

 a supposed actiM] phvsical unit of segregation, a factor has a 

 definite objective extent. 



1'lie foi-iiKT vifw is tliat of the presence-and-absence terminol- 



factor is nni an ideiueut of tlu^ germ-plasm: it is rather a prop- 

 gcrm-plasm. Ti- Aur:<-\rr. ..f ati orLratiiMii. as C.itcs , IDU, 



