No. 601] MENDELIAN FACTOR DIFFERENCES 37 



the locus has a particular place and function in the reac- 

 tion system, that it bears a specific relation to the other 

 elements of the system. 



It is true that investigation in plant breeding has not 

 yet progressed far enough to furnish a definite confirma- 

 tion all along the line of the work with Drosophila. The 

 reasons for this are very obvious and they lie in the tech- 

 nical difficulties involved in such w ork rather than in fun- 

 damental disagreements in principle. It has not as yet 

 been possible to study as many factors in any plant spe- 

 cies as have been investigated in Drosophila, nor to carry 

 Mie work through as many generations nor to employ as 

 large populations. Moreover, most plant material is 

 more difficult to handle from the standpoint of a chromo- 

 some analysis on account of the longer period of time 

 necessary to secure data and the greater amount of atten- 

 tion which must be given the cnltnros and tlio larger num- 

 ber of chromosomes which aro iwiiall\- involved in such 

 species. It must also be borne in iiiind tiiat practically all 

 the Drosophila differences have arisen under observation 

 as simple factor mutations, and it has therefore been rela- 

 tively easy to determine their relations to the other fac- 

 tors in the system. In plants, on the other hand, the ma- 

 terial has presented itself as a confusing array of varieties 

 containing for the most part a large number of recessive 

 factors, and usually the original form from which they 

 were derived, corres]iondini>' to tlio iioruial fly of ^Morgan's 

 work, has not been o])laiMn1tl(> aiid would Tiot have been 

 'particularly useful, had it been available. Xcvcrtlicloss, 

 there appears to be no real (liflici'.lty in the wa\ of accept- 

 ing the conception derived fi'oin iIk^ hiosophtln studies as 

 a definite, consistent working livj M»t liois : tor it is difficult 

 to believe that the behavior of ]sl;;nt niatci ial sliould be 

 fundaiiKntally different, an.l in.l-cd pnint^ of correspond- 

 ence aro not lackln- to wari-ant n- in vicwin-- <oin."what 

 sc(.],ti.-alK anv nnduo ompliaH^ nlacrMl n])on th.> ditTer- 



We 7nay now" rotnrn to tlie coTU'ci^tion of the reaction 

 system as a iniit in itM'll' in the mmi-c that it is nia<le up of 



