464 T. H. MORGAN AND C. B. BRIDGES 



eosin pink female heterozygous for vermilion. White-eosin 

 vermilion female heterozygous for pink (Xw V P/Xw^^ v p) is 

 of the same color as the male eosin vermilion heterozygous for 



pink (X w^ V P/ p) but both are lighter than the eosin ver- 



mihon female heterozygous for pink (X w v P/X w« v p). This 

 is the only opportunity to dilute the eye of the male by hetero- 

 zygosity because pink is the only factor employed that may 

 exist in diploid in the male, and may therefore be halved, while 

 the eosin vermilion is the only eye color of the right degree of 

 pigmentation to be sensitive to such a dilution. 



Accepting the eye color of the wild fly as the standard, we can, 

 as we have seen in the preceding pages, describe all results ob- 

 tained in terms of dilution, that is, as darker or lighter. The fac- 

 tors differ in their effectiveness as diluters. The dilution effect 

 of each factor is specific, and having once determined the order 

 of effectiveness of a series of factors (diluters) we can predict the 

 relative effect of the same factors starting with any other stages 

 of the same series. There are obviously several possible physical 

 conditions which could give such a series of effects, for example, 

 dilution might be accomplished by a decrease in the number or 

 size of the colored granules, or by the addition of white granules, 

 or the colors might be stages in a chemical process. Again, there 

 might be several series of distinct pigments which would give dif- 

 ferent effects according as one or the other is changed in amount, 

 and so forth. 



The color of the eye might be compared to stature in man, 

 which is the sum of the length of the legs, back, neck, head, etc. 

 A shortening in any one of these parts would decrease the height. 

 Obviously height is here only a common physical measure with- 

 out intrinsic significanGe. Thus it may be with eye color, and » if 

 so, any particular eye color may be the summation of several 

 internal effects and 'dilution' refers only to the actually observed 

 results. 



In Mendelian literature certain factors are spoken of as diluters 

 or intensifiers, as though their peculiar action were only to mod- 

 ify, up or down, certain other characters which by implication 



