278 CALVIN B. BRIDGES 



THE PURPLE 'EPIDEMIC 'MUTATING PERIODS' 



Shortly after the discovery of purple, purples or eye colors 

 closely resembhng purple began to be found in stock and experi- 

 ments everywhere. In the interval of six months following the 

 discovery of purple such occurrences numbered fourteen and 

 furnished the first as well as the most striking of the 'epidemics 

 of mutation' that seemed to sweep over our material at this 

 period. From later and well-authenticated cases (e.g., ver- 

 milion, cut, notch, etc.) it appears that certain mutations do 

 recur, and in the case of cut. four independent occurrences 

 followed one another so closely that the term 'epidemic' is 

 descriptive of the condition observed. However, in the early 

 eases (purple, jaunty, arc, etc.) it is certain that a large majority 

 of the apparent cases were not true reoccurrences of the mutative 

 change, but were due to several other conditions. Thus, the 

 1st, oth, 6th, and 13th apparent purples proved to be maroon, 

 a third chromosome eye color practically indistinguishable from 

 purple in appearance. That is, 'mimic' mutations were not at 

 first distinguished from the original type. Xor were new 

 mutant allelomorphs distinguished from types already' known 

 unless the difference was striking. Certain others of the occur- 

 rences were proved not to be of independent origin, thus purples 

 S and 9 were both shown to ha^'e been descended from a certain 

 conm:ion stock, and purples 10 and 11 were traced to a second 

 common stock. It is undoubtedly true that in many cases 

 where no connection can be traced such connection really 

 existed, especially in the case of recessives, which might be 

 distributed \s-ithout giA'ing sign of their presence. The psycho- 

 logical element, too. is important — it is exceedingly difficult to 

 recognize a mutative change, even a striking one, before one 

 becomes 'sensitized' to that particular mutation. Some of our 

 mutant characters had long been present in stocks or experi- 

 ments so that many flies showing the character must have been 

 seen before attention became sharply focused upon the differ- 

 ences shown. Contamination and errors of one sort or another 

 have also inflated the number of .apparent reoccurrences of 

 mutations. It is therefore to be doubted if more than two of 

 the apparent reoccurrences of pm'ple were genuine remutations. 



