VARIATION AND HEREDITY 131 



admirable Evolutionary Exhibition of the 

 British (Natural History) Museum. When 

 the peculiar 'waltzing mice >: are crossed 

 with normal mice, the offspring (Fj) are all 

 normal. The waltzing habit is recessive. 

 But when the offspring are inbred, their 

 progeny (F 2 ) are normal mice and waltzing 

 mice in the proportion of 3:1. The reces- 

 sive waltzers of this generation might be 

 sold as pure waltzers; with others of their 

 kind they will produce only waltzers for as 

 many generations as one likes to breed them. 

 But the normals of the same generation turn 

 out to be of two kinds though they are all 

 alike in appearance: one-third of them (pure 

 dominants) will yield only normal mice; the 

 other two-thirds (impure dominants) will 

 split up again, when inbred, into normal mice 

 and waltzing mice in the old proportions of 

 3:1. Another form of the scheme may be 

 used : 



D 



F 2 



Fa 



ID 



(R) : 1R' 



1D:2D(R):1R R 



w 



:D 



D ID : 2D (R) : -1R 



