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THE AMERICAN NATURALIST [Vol. XLIV 



The case of color blindness in man appears to follow 

 the same rule, for here also females may transmit the 

 character without developing it, while males, if they have 

 it at all, develop color blindness. One dose of color 

 blindness in males makes the male color blind ; one dose 

 in females is insufficient. The rarity of color-blind fe- 

 males is explicable on this view. 



These results may be significant for the chromosome 

 hypothesis since the interpretation seems to imply that 

 the amount of a given material, or chromatin, perhaps, 

 is an important element in the determination of the de- 

 velopment of characters. If the interpretation is cor- 

 rect it means that a character will not develop even 

 when its primordia or forerunners are present, unless a 

 sufficient amount of that material be present. And, on 

 the other hand, in other individuals a smaller amount of 

 the same material suffices to call forth the unfolding of 

 a given structure. 



The same interpretation seems to have a wide applica- 

 tion to the characters of the first generation of hybrids, 

 and in all heterozygous individuals that are in nature 

 identical (i. e., heterozygous) with the first generation 

 hybrids. It is known that in several cases the dominant 

 character does not reach its full development in the first 

 generation, as Correns showed for Minibilis Jalapa. 

 Such cases can be explained on the ground that one dose 

 is not enough. The reappearance in the second genera- 

 tion of individuals with the full dominant character is 

 in harmony with this assumption, for in one fourth of 

 the individuals two doses of dominance are expected. 



In mice, too, the heterozygous form between black and 

 chocolate often shows black or chocolate areas in the 

 fur, and in the same mouse a region may be at first black 

 and later chocolate, or vice versa. It appears that the 

 condition of the mouse at the time when the molt takes 

 place determines whether the hair contains the one or 

 the other pigment in excess. Thus external conditions or 

 internal states may regulate dominance in hybrid forms. 



Such facts lead to a consideration of how far quanti- 



