260 WM. REES B. ROBERTSON 



to occur in Drosophila ampelophila. Is it not possible that the 

 extra rod-hke appendage to the apex of one of the V sex chromo- 

 somes described by Miss Stevens may be a rod segment of some 

 V that had previously broken at the apex? 



Besides these sex-linked factors, Morgan has found ''other 

 factors to cross over to various degrees; in the extreme cases 

 the chance is one to one that they cross over." Metz has shown 

 for Drosophila ampelophila two pairs of V-shaped chromosomes 

 in addition to two rod-shaped and two ilf -chromosomes. While 

 he has not shown variation from this configuration in D. ampelo- 

 phila, he has shown it in other species of the genus, where it is 

 perfectly clear that either one pair of V's may be broken up into 

 two pairs of rods, or, similarly, both pairs of V's, making four 

 pairs of rods. While he has not shown a single V alternating with 

 a pair of rods, as occurs in Jamaicana subguttata, I feel reason- 

 ably certain that it will be found. It seems likely that a strain 

 could be built up, having either all rods, as in the all-rod type of 

 J. subguttata, or all V's, as in J. unicolor. I also believe that 

 breaks in the V's may occur at any time and that varying de- 

 grees of this tendency to break might exist, ranging from the 

 condition in those compound V's where no break seems to occur, 

 such as we find true of all species of the genus Chorthippus 

 (Stenobothrus), to that where only occasional linkage occurs, 

 as in Chortophaga (McClung '14), or merely irregular associa- 

 tion, as in the members of the 14 and 16 pairs in two specimens 

 of Jamaicana flava (compare A, nos. 14 and 16, in Woolsey 

 '15, figs. 1-4 with 5-8). From a comparison of such V's, I think 

 it reasonable to suppose that races might be built up in a species 

 with V's of any grade in respect to this tendency to break. 

 Such, again, might form a basis for the varying degrees of cross- 

 overs Morgan obtained in coupling factors. 



6. The chromosomes: a mechanism furnishing a basis for variation, 

 heredity and evolution? 



The observations presented in this paper seem to be but 

 additional links in the chain of evidence going to show that in 



