Species crosses in Rats. 121 



If in I'eality this explanation is tlu; corrert ono in some eases, it 

 is clear, that the authors, whose main interest lies in the h)ealization 

 of the g-enes, and not in the orig'in of th(> novolties, miss the chance 

 of studyinp: and localizing a great many genes. 



Is it possible to test this hypothesis? In the first place, it is 

 evident, that if a method of Itrothersister matings in siug'le pairs were 

 substituted for masscultures, in maintaining stockbottles, eventual new 

 "novelties"' could be recognized as singlerecessives or doublerecessives 

 immediately, by the proportion in which they occurred among normal 

 brothers and sisters. In the second place it would certainly be very 

 much worth while systematically to collect a great many wild strains 

 of this fly. Drnsophila inelmxigaster. and to cross them in all possible 

 ways, in order to see whether in F2 new forms, either uudescribed or 

 identical with forms already described, would not be a relatively common 

 occurrance. It has been already conceded, that wild flies may diffei- 

 in what these authors call "specific modifiers", genes which influence 

 characters of varieties, rather than those of wild flies. We venture to 

 prophesy from oui" experience with rats and mice, that th(^ more satis- 

 factory explanation for the origin of the novelties would at least in 

 many instances be found applicable. 



Berkelev Cal. 1921. 



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