No. 605] SHORTER ARTICLES AND DISCUSSION 



306 



7. White 



Three of these seven grades have been made known to us within 

 the last six months. It would not require a bold prophet to pre- 

 dict that as the years pass we shall come to know more of these 

 gradations, till all detectable dififercnces of shade have been made 

 out and each shown to be inherited as a Mendelian unit. Con- 

 sidering that the work on Drosophila has been in progress but 

 eight or nine year-s. we have already remarkable progress toward 

 a (IvitioiisU'ation tluit a single uiiil character may present as 



seven grades, due to alterations of a single unit factor, there is 

 a secondary series containing seven more grades, all affecting 

 the central grade (eosin) of this primary series, but due to al- 

 terations of other parts of the germinal material. ITow much 

 more does the selectionist require? 



This situation in Drosophila is not exceptional. To mention 

 one or two other examples, Castle and Wriglil (V,n(\\ find a 

 large series of such diverse conditions of ii sinuh' fnriof ( ••mul- 

 tiple allelomorphs") determining various sli.idr-^ ((..it ri»lors 

 in rodents. Emerson (1917) in his reenit ,i.'r..ui,! uf iL,. cxTra- 



on for evolution through lli. 



factors" show ihiW 

 finest gradations by 

 material. The objot 

 change through sele( 

 thoroughness of the 



