CHAPTER VII 



EXPERIMENTAL HYBRIDIZING (Continued) 

 Experiments with Mice 



THE ease with which mice can be kept in captivity, their 

 rapid rate of multiplication, and the occurrence not only of an 

 albino race, but of other fancy breeds as well, have made these 

 animals a favorite subject for experimental work. 



The house mouse (Mus musculus) is of a gray, sometimes 

 called cinnamon or agouti, color. The white mouse is an 

 albino race derived without much doubt directly from the wild 

 form. Albino mice have been recorded as occurring in situa- 

 tions where there was no reason to suppose that their origin 

 could be due to the escape of domesticated albinos. It is also 

 known that in many groups of the animal kingdom albino indi- 

 viduals often suddenly appear. 



The color of the hair of the gray mouse is due, according to 

 Bateson and Durham, to three kinds of pigment often asso- 

 ciated in each hair: (i) opaque black, (2) less opaque brown, 

 (3) transparent yellow. In albinos all three colors have dis- 

 appeared as well as the pigment in the eyes. As many as a 

 dozen or more races of fancy mice are known. It appears that 

 the color of the race is determined according to which of the 

 three pigments found in the gray house mouse predominates or 

 exists alone. Thus the golden agouti, of a tawny color, con- 

 tains yellow and brown pigments, but not black ; chocolate mice 

 contain only the brown pigment ; yellow contain only the yellow; 

 black contain black and brown. Variegated mice are those 

 in which irregular small spots of black or chocolate occur on a 



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