EYE COLOR IN DROSOPHILA MELANOGASTER 381 



ments of Drosophila we continually observe mutations arising, 

 and are often able to locate the time and place of origin to a 

 particular fly and to within one or a few cell generations. Every- 

 one who believes that mutations are now occurring in his stocks 

 in this manner must also accept the probable occurrence of such 

 mutations in the immediate ancestry of his stocks, since it is not 

 hkely that either the rate or tendency of mutation have changed 

 within any period with which we deal. The process of mutation 

 thus gives rise in any stock to a complexity and heterogeneity 

 which is only that 'static condition of diversity' said by Jennings 

 (Jennings, '17) to obtain for any species or kind of organism, 

 such, for example, as the hooded rat. When selection is started 

 in such a stock immediate progress should be expected and this 

 should continue until the stock is homozygous for all the genes 

 capable of modifying the selected character. One cannot be 

 certain that observed progress is due to fresh mutation until 

 selection has been continued long enough so that the initial 

 diversity has been removed. The initial diversity means 

 that mutations had previously occurred in various loci, 

 and the presumption is that during the progress of 

 selection the stock will not cease mutating in diverse loci and 

 henceforth mutate solely in one locus. MuUer ('14), MacDowell 

 ('16), Sturtevant ('18), and others have shown that the results 

 obtained with selection in the hooded rats are, aside from one 

 definite new mutation, exactly those expected from simple se- 

 lection, in a heterogeneous stock of the kind supposed by the 

 Frenchman Jordan, subscribed to by Jennings, and easily ex- 

 plained as the result of previous diverse mutations. It is inter- 

 esting to observe that the Ust of cases in which unilocal mutations 

 or contamination may be seriously considered is dwindling with 

 the progress of exact knowledge. 



In our opinion, the attempted distinctions between 'saltations,' 

 'mutations,' and 'variations of slight degree' have led rather to 

 confusion of thought than to clearer thinking. To us these are 

 all a single class, 'mutations,' and the term carries no restrictions 

 of degree, covering the most extreme as well as the slightest 

 detectable inherited variation. Distinctions of degree when ap- 



