LEE BARKER WALTON 141 



the conclusion follows that the "mutation" results either 

 (1) from a change in a specific gene or (2) from the 

 complete linkage of a series of genes. If the latter prop- 

 osition should be the correct interpretation, and it is by no 

 means clear that it is not, the objections urged against the 

 theories of DeVries and of Castle hold equally here. 



Morgan and several others have presented evidence for 

 believing in the specific change of a gene. Granting that 

 this is the actual explanation of the facts presented in 

 connection with multiple allelomorphs, etc., there are two 

 lines of argument leading to the conclusion that these 

 changes are results of combinational sub-units or sub- 

 genes existing in the species, and that progressive evolu- 

 tionary changes are no more represented here than in the 

 previous theories of DeVries and of Castle. 



The first argument (a) rests upon the recurrent "mu- 

 tations" which have been noted in a considerable number 

 of cases. Thus the sex-linked eye colors of Drosophila 

 forming the multiple allelomorph system consisting of 

 white, eosin, cherry, blood, tinged, and buff, and their 

 dominant allelomorph, red, of the wild fly, have their 

 origin from a single definite area or locus in the "X" 

 chromosome, accepting linkage as a criterion. They have 

 not arisen in a continuous series but as sudden changes 

 from one extreme to another at comparatively long inter- 

 vals. The character may remain modified in one direction 

 and then suddenly revert to an original condition. Thus 

 white changed to eosin and later back to white as noted 

 by Morgan (1916). Furthermore, the changes are not 

 extremely infrequent. A similar transformation has been 

 noted by Emerson (1917) in maize where self color 

 apparently changed to variegation and later back to self 

 color. A variation which may be of the same type has 

 been described by Shull (1911) for Lychnis. Quite re- 

 cently Zeleny (1917) in studies on Drosophila melano- 

 gaster Meig. (=ampelophila Low) ^ has noted a reversed 

 mutation where full-eyed flies result from the return of 

 the bar gene to the original full-eyed condition. In each 



■'Sturtevant, mss. 



