132 EVOLUTION 



admirable Evolutionary Exhibition of the 

 British (Natural History) Museum. When the 

 peculiar " waltzing mice " are crossed with 

 normal mice, the offspring (F x ) 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 recessive 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 appear- 

 ance : 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 : 

 p D x R 



\y 



F! 



F f ID : 2D(R) : 1R 



D ID : 2 



:1R R 



\ 



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



