382 CALVIN B. BRIDGES 



plied to mutations depend largely on circumstances and per- 

 sonalities, and are correspondingly inexact. Thus, students be- 

 ginning work on Drosophila usually complain that there is almost 

 no difference between pink eye color, for example, and the wild 

 form, so that the classifications are made with considerable un- 

 certainty. With more experience these same workers come to 

 regard this difference as very great and are astonished that they 

 should ever have thought otherwise. Again, some workers are 

 able to distinguish with assurance characters that quite pass the 

 resolving power of other workers seemingly equally experienced. 

 In the Drosophila work great numbers of mutations have been 

 encountered whose somatic effect may be roughly described as 

 'slight.' If degree of effect were marked off as abscissae and 

 number of mutations that actually arise as ordinates, it seems 

 probable that the highest point of the curve would be at the least 

 extreme mutations and the curve would fall rapidly and gradually 

 with the most extreme mutations. For most of the problems 

 in which we have been interested precision of classification is 

 essential, and precision is afforded only by the more extreme 

 mutations and by some few of slight degree but definite character. 

 Accordingly, there has been scant mention of the many 'slight' 

 mutations in our accounts, which have dealt in the main with 

 problems in which such characters were of no use as working 

 tools. Likewise, when these mutations are found, they incite 

 little desire to work out the facts concerning their inheritance, 

 chromosome, locus, interactions, etc. However, enough of them 

 have been investigated thoroughly for us to be certain that no 

 departure from normal Mendelian inheritance is involved. Ac- 

 counts of some of these 'poor' characters have appeared in Car- 

 negie Publication no. 237, which deals with mutants whose locus 

 is in the first or X chromosome. Thus, for example, dot, bow, 

 depressed, green, chrome, and facet may all be fairly described 

 as of 'slight' somatic effect. Accounts of many others will ap- 

 pear in the publications dealing with the second, third, and 

 fourth groups. Even a larger number will be treated in a section 

 on miscellaneous mutations, while scores will never be referred 

 to at all. 



