38 GENETIC VARIATIONS 



ways; that is, by breaking or union of certain chromosomes. 

 This is for example the case in different species of Drosophila 

 and other closely related genera. Metz has given excellent 

 figures of the chromosome groups in different species of 

 Drosophila, some of which we have copied in Figure 8. Type 

 A, in the common fruit fly, has four pairs of chromosomes, 



)> . II. M. il. 

 II , llT II . ~ 



H 



Figure 8. Diagrams showing the chromosome groups of eight differ- 

 ent related species of flies, after Metz, 1916. 



one pair straight, two pairs F-shaped, one pair small. Type B, 

 in another species, is like Type A except that the small chromo- 

 somes have united with some of the larger ones, leaving but 

 three pairs. Type C, found in a third species, is like Type A ex- 

 cept that the chromosomes of one of the F-shaped pairs have 

 broken at the point of the V into two straight chromosomes; 

 so there are in this five pairs. In Type F, both the F-pairs of 

 Type A have broken into two pairs of straight chromosomes, 

 giving six pairs instead of four. The various other types result 

 from similar rather simple changes. There are many other 

 cases of related species having chromosome groups that thus 

 differ by slight but definite modifications of a common type. 



