THROWING BACK ^7 



to me his conception of the causation of the waltzing 

 habit in the statement that it is due to the absence 

 of the power of becoming giddy. The unpleasant 

 sensations which would soon stop a normal mouse 

 from spinning round are not felt by the waltzing 

 mouse, and what ultimately brings the bout of 

 spinning to an end is not nausea, but exhaustion. 



Let us, now, turn to the inheritance of the waltzing 

 character. When two waltzing mice are mated 

 together the offspring produced are all waltzers. 

 The character breeds true. 



When a waltzing mouse is mated with a mouse 

 with normal movement, as in my own cross repre- 

 sented on Plate II., the hybrid produced never 

 exhibits the waltzing movements. This, at any 

 rate, has invariably been the case in the hundreds 

 of hybrids raised by Dr. von Guaita and by myself. 

 The waltzing character is, therefore, recessive and 

 normality of progression is dominant. Being a reces- 

 sive character, waltzing should reappear in the 

 second hybrid generation in one individual in every 

 four. It does reappear in the second hybrid genera- 

 tion, but not in a quarter of the individuals ; there 

 were 97 out of 6^5, which is less than a fifth. 

 But I do not think that this proportion should 

 lead us to the conclusion that waltzing does not 

 behave as a Mendelian character in inheritance, 

 because I believe that the reason that the number of 

 waltzers falls short of the expected ratio is simply 

 that waltzers are more delicate constitutionally than 

 normal mice ; and that relatively more waltzers 



