138 MENDELISM chap. 



the members of different groups there is said to be 

 no evidence of this phenomenon of linkage. The 

 inference is that these four groups of factors corre- 

 spond to the four pairs of chromosomes. This view 

 is supported by the following considerations. In 

 the first place, the four groups of factors are far 

 from equal in number. Three of them contain 

 many more factors than the fourth one, in which but 

 three have so far been identified. This is what 



n rxYxxxxxxxyxxxxxxxixxj 

 iii r-XYxyxyYXXXXXXxxxxxxx) 



IV OZ) 



Fig. 40. 



Diagram to illustrate the linear arrangement of the factors in Chromosomes I. -IV. 

 of Drosophila. The number of separate factors must lie supposed to be very 

 much greater than is represented. 



might be expected from the fact that one of the 

 pairs of chromosomes (Chromosome IV.) is a very 

 small one. Secondly, one of the groups consists 

 of factors which all exhibit sex-linked inheritance, 

 while factors of this nature do not appear in any 

 of the other three groups. This is comprehen- 

 sible if it is assumed that this group of factors is 

 carried by the sex-chromosome (Chromosome I. in 

 Fig. 40). 



An important point is the arrangement of the 

 factors in the chromosome. This is assumed to be 



