xiv] LINKAGE OF FACTORS 223 



suggests that the factors for the coupled characters are borne 

 by some body which behaves in gametogenesis as a unit, 

 and the only bodies known to behave in the way required 

 are the chromosomes. This conclusion is greatly strengthened 

 by the facts as they are now known in Drosophila. In D. 

 ampelophila the haploid number of chromosomes is four, 

 and there are four pairs, easily distinguishable from one 

 another, in the diploid cells. Of these four, two are long 

 ordinary chromosomes (autosomes), one is a very short auto- 

 some, and one a sex-chromosome. To the last further refer- 

 ence will be made later; the facts which bear particularly 

 on the present discussion are that in heredity it is found that 

 there are four groups of coupled characters, which may be 

 numbered I, II, III, IV. According to MORGAN and his 

 associates, when a female bearing any two or more of the 

 characters in group I is mated with a male that lacks them, 

 all these characters are found to be coupled in the germ-cells 

 .of the offspring, and the same is true of the characters in 

 groups II, III, and IV, but there is no coupling between 

 characters belonging to different groups. Furthermore, 

 groups I, II and III each contain a considerable number 

 of Mendelian characters, while group IV is small, and only 

 two characters belonging to it have as yet been discovered. 

 On the basis of these facts, MORGAN supposes that the factors 

 for characters in group I are borne by the sex-chromosome 

 (the reasons for this will be given below), those of groups 

 II and III by the two long autosomes, and of group IV by 

 the small autosome. If no other group of coupled characters 

 is discovered, the correspondence between the coupled groups 

 and the chromosomes is so remarkable that it seems hardly 

 possible to doubt the existence of a connection between them. 

 One very important group of facts, however, remains to be 

 explained. Although when a parent bearing the coupled 



