Nos. 622-623] ORGANIC EVOLUTION 



527 



results on the basis of multiple genes instead of a con- 

 tinually varying gene. 



It would thus appear evident that the theory outlined 

 by Castle is open to quite the same objections that oc- 

 cur in connection with the mutation theory of DeVries, 

 and that there is little evidence for believing that it has 

 any fundamental value in explaining evolution. 



The mutation theory of Morgan and his associates, 

 based primarily on results obtained in studies of the 

 small "fruit-fly" Drosophila, apparently presents quite 

 another view of the subject. Here it is clearly indicated 

 that evolution has taken place through the incorporation 

 of mutant changes, and that these changes are due to dis- 

 continuous "mutations" of genes as exemplified in mul- 

 tiple allelomorphs. 



Assuming the validity of the arguments based on link- 

 age relations in respect to the localization of the genes, 

 the conclusion follows that the "mutation" results either 

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

 plete linkage of a series of genes. If the latter proposi- 

 tion 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 



