Changes in Factors through Selection

Most interesting of all the mutations that are now engaging the attention of students of mutation are those in which genetic factors or genes occur, or appear, whose most obvious action is to enhance or diminish some other more conspicuous character. These genes may be called specific modifiers. They do not differ from ordinary genetic factors in any essential respect, but, since their presence can not be detected, except when other factors are themselves producing some particular effect on the individual, it is convenient to give them a special designation.