MENDELISM 443 



but believe it is merely due to the greater mortality which almost 

 certainly takes place among waltzing individuals than among 

 normal ones between the time when the}' are born and the 

 age — from ten da^'s to two weeks — when the characters of the 

 mice are first registered. Waltzers are much more delicate than 

 normal mice; and it is likely that this delicacy is more fatal in 

 youth (perhaps even before birth) than in middle age. 



The mice afford an excellent example of reversion. The 

 Japanese waltzing mouse is almost an albino, and would be 

 were it not for the patches of fawn-coloured fur on the 

 shoulders and haunches. It has pink eyes. It was crossed 

 with an albino mouse. The result, where the albino used 

 was a pure-bred one, was a mouse hardly distinguishable at 

 first sight from the common house mouse. A closer inspection 

 reveals the lighter colour of the belly. Their eyes are jet 

 black. And this is the most remarkable feature of the cross 

 — the production of a black-eyed form from two parent forms 

 both of which had pink eyes. These hybrids are not merely 

 wild in their coloration, but are always noticeably healthier, 

 stronger, and wilder than their albino parent, and infinitely 

 more so than their waltzing parent. As already stated, they 

 never waltz. When they are mated together they produce a 

 generation w^hich is composed of the three colour categories 

 we have just mentioned in the proportions indicated by the 

 percentages prefixed to them — viz. 25 per cent, pink eye and 

 coloured coat (that of the Japanese waltzer), 50 per cent, dark 

 eye and coloured coat (that of the hybrid), and 25 per cent, 

 pink eye and colourless coat {i.e. complete absence of pigment 

 or albinism.) The distribution of waltzing over these various 

 colour categories is interesting. It is distributed, as it were^ 

 at random — that is to say, the waltzing character is not 

 necessarily associated with that colour category with which 

 it was associated in the pure waltzer, but may be associated 

 with any of the three colour categories. So that we get in 

 F2, besides waltzers coloured like their pure grandparents, 

 waltzers with the black eyts and the grey coat of the house 

 mouse, and albino waltzers. It may be noted in passing that 

 these albino waltzers correspond to the green wrinkled peas 

 we have already discussed — that is to say, they represent 

 the association in F2 of the recessive character of one parent 

 with the recessive character of the other parent in one individual 



