272 PHYSICAL BASIS OF HEREDITY 



tion, the localization of the gene causing the modihcation 

 shows them to be different. 



The method of localizing genes olfers an opportun-* 

 ity for obtaining evidence m regard to iike-mutants in 

 related species that cannot be crossed, and a step forward 

 in this direction has been taken by C. W. Metz for other 

 species of the genus Drosophila. In one species, D. virUis, 

 he has found 12 mutants, and these fall into three 

 groups of linked genes. Three of them, yellow, forked 

 and confluent, resemble externally characters of D. 

 melanog aster. Yellow and forked are sex-linked and look 

 like the same characters in ynelanog aster. Confluent is 

 like a second chromosome character of the same name in 

 melanogaster in three respects : first, in that the structures 

 are similar; second, in that the character is dominant in 

 both forms; and, third, in that it is lethal in the homo- 

 zygous state. The terminal position of yellow and the 

 large amount of crossing over with forked are, roughly 

 speaking, the same in both. 



Even in this case further work is needed, first, because 

 within the same* species the occurrence of similar-looking 

 characters due to different factors is known, e.g., there 

 are two genes for yellow color (yellow and lemon) in the 

 first chromosome of D. melanogaster and in the same part 

 of that chromosome, and second, because it is not to be 

 expected that the number of crossovers would be identi- 

 cally the same between the same loci in different species, 

 since marked variations are known within a single species. 

 Unless such species can be crossed, the only convincing 

 evidence that we can hope to get will be to establish 

 the same linear order in the chromosome for several 

 genes whose characters appear to be the same or similar. 



Other evidence of a different kind also helps to make 

 probable that the same mutations occur in different 

 species. For example, in cases where a mutant gene pro- 

 duces a number of changes in different parts of the body, 

 the probability that it is the same as one in a different 

 species that causes the same modifications, is in propor- 



