142 THE SHORTER SCIENTIFIC PAPERS 



of the cases mentioned the germinal purity of the stock 

 was beheved to be without question. 



Such results are not to be attributed to a continuous 

 series of mutations, to progressive changes, or to genetic 

 losses. They clearly suggest that the gene, if it is the 

 individual gene which is involved, is made up of smaller 

 combinational units which through their permutations 

 give rise to the characters in question. Presumptive evi- 

 dence is certainly furnished against the idea that anything 

 new has developed in the organism to form the particular 

 characters. Furthermore, one may well believe that any 

 particular mutation under observation sufficiently long, 

 will exhibit recurrent changes. 



The second argument {b) , to the effect that the gene is 

 comparatively stable and that "mutations" are only 

 transitory combinational changes, is based on the main- 

 tenance of apparently identical genes through long periods 

 of time. Thus Metz (1917) found that the three muta- 

 tions which had, up to that time, occurred in Drosophila 

 virilis Sturt. appeared exact duplications of the mutations 

 in Drosophila melanogaster Meig.^ In both species "con- 

 fluent," a modification of the wing venation, is similar in 

 form, dominant over "normal" and "lethal," when the fly 

 is homozygous for the character. The characters "yellow" 

 and "forked" are sex linked in both species and otherwise 

 alike so far as the evidence exists. Inasmuch as the earliest 

 representative of Drosophila thus far known is a species 

 not decidedly different from those now existing as noted 

 by Low (18 50), who described it from the amber of the 

 Baltic Sea, and belongs to the Lower Oligocene of the 

 Tertiary Period, with an age of from two millions to three 

 millions of years, one must infer that the genes common 

 to the two species mentioned have been preformed for a 

 long period of time, and that nature has paid little atten- 

 tion to such mutational changes as occur in connection 

 with multiple allelomorphs. 



There are certain investigations widely separated as to 

 their content, but apparently closely correlated as to the 



■'^The species are distinctly separated not only in external appearance but also by 

 their chromosome number. D. melanogaster has four pairs, while D. virilis has five 

 pairs of chromosomes. 



