104 THE THEORY OF THE GENE 



known, and sometimes these lie near together in the same 

 linkage group. ^ 



In two other sjoecies of Drosophila the work has prog- 

 ressed to a point where the comparisons are at least very- 

 interesting. In Drosophila virilis, Metz and Weinstein 

 have determined the location of several mutant genes, 

 and Metz has compared the order of the series with that 

 of D. melanogaster. The chart (Fig. 60) shows that there 

 are five apparently similar mutants in the sex-chromo- 

 some that stand in the same order as those of melano- 

 gaster, viz., yellow (y), cross-veinless (c), singed (si), 

 miniature (m), forked (f). 



Another species, Drosophila obscura, has a genetic sex- 

 chromosome twice as long as that of melanogaster (Fig. 

 61). It is probably significant that the four characteris- 

 tic mutant types, yellow, white eyes, scute, and notch 

 wings, that lie in the middle of this long sex-chromosome, 

 are identical with the same mutant characters of D. 

 melanogaster that lie at the end of the shorter sex-chro- 

 mosome of melanogaster and simulans. The interpreta- 

 tion of this relation is still being carefully studied by 

 Lancefield. 



These and other results should make us extremely cau- 

 tious in drawing phylogenetic conclusions from inspec- 

 tion alone of the chromosome groups ; for, it follows from 

 the Drosophila evidence that very closely related species 

 may have their genes arranged in a different order in the 

 same chromosomes. Similar groups of chromosomes may 

 at times contain different assortments of genes. Since it 

 is the genes, and not the chromosomes as such, that are 

 important, the final analysis of the hereditary construc- 

 tion must be determined by genetics rather than by 

 cytology. 



1 By taking into account more than a single effect of each gene the 

 identification may he made more probable. 



