Amongst the new mutant factors there may be some whose chief effect is on the character that the breeder is already selecting. Such a modification will be likely to attract attention. Superficially it may appear that the factor for the original character has varied, while the truth may be that another factor has appeared that has modified a character already present. In fact, many or all Mendelian factors that affect the same organ may be said to be modifiers of each other's effects. Thus the factor for vermilion causes the eye to be one color, and the factor for eosin another color, while eosin vermilion is different from both. Eosin may be said to be a modifier of vermilion or vermilion of eosin. In general, however, it is convenient to use the term "modifier" for cases in which the factor causes a detectable change in a character already present or conspicuous. [Illustration: FIG. 82. Scheme to indicate influence of the modifying factors, cream and whiting. Neither produces any effect alone but they modify other eye colors such as eosin.] One of the most interesting, and at the same time most treacherous, kinds of modifying factors is that which produces an effect _only_ when some other factor is present. Thus Bridges has shown that there is a factor called "cream" that does not affect the red color of the eye of the wild fly, yet makes "eosin" much paler (fig. 82). Another factor "whiting" which produces no effect on red makes eosin entirely white. Since cream or whiting may be carried by red eyed flies without their presence being seen until eosin is used, the experimenter must be continually on the lookout for such factors which may lead to erroneous conclusions unless detected. As yet breeders have not realized the important rôle that modifiers have played in their results, but there are indications at least that the heaping up of modifying factors has been one of the ways in which highly specialized domesticated animals have been produced. Selection has accomplished this result not by changing factors, but by picking up modifying factors. The demonstration of the presence of these factors has already been made in some cases. Their study promises to be one of the most instructive fields for further work bearing on the selection hypothesis. In addition to these well recognized methods by which artificial selection has produced new things we come now to a question that is the very crux of the selection theory today. Our whole conception of selection turns on the answer that we give to this matter and if I appear insistent and go into some detail it is because I think that the matter is worth very careful consideration. ARE FACTORS CHANGED THROUGH SELECTION?