442 SCIENCE PROGRESS 



in a right-handed direction, sometimes in a left. But when it is 

 not waltzing, a waltzing mouse is immediately recognisable by 

 the appearance of complete lack of control over the movements 

 of its head, which is every now and again thrown back into an 

 apparently very uncomfortable and useless attitude. A waltzing 

 mouse also has the habit of running or rather shuffling backwards, 

 waving his head vigorously from side to side the while. This 

 mouse is said to have come originally from Japan. They 

 certainly exist there now, but I have not been able to obtain 

 any alive from that country. The depth of the ignorance of 

 the general public concerning the simplest facts of heredity is 

 shown by the fact that the following story of the origin of the 

 waltzing mouse has been printed in a daily paper and, pre- 

 sumably, believed. "A mouse fancier once caught the head of 

 one of his pets in the door of his cage. The mouse nearly 

 died, but on recovery exhibited the waltzing movements now 

 so familiar. Wishing to perpetuate the curiosity, the owner 

 of the mouse carefully preserved and bred from the anomaly, 

 and so originated the breed of waltzing mice ! " What actually 

 happened was probably that the feature arose as a sport which 

 was carefully preserved and bred from. Waltzing is, in fact, 

 a very good example of the kind of character that can arise 

 in a tame breed, and be preserved because it tickles the fancy 

 of man. Nature does not tolerate curiosities of this kind for 

 a moment, and promptly eliminates them. The abnormality is 

 due, not, as was originally believed, to the absence or atrophy 

 of one of the semicircular canals, which have been demonstrated 

 to be all there, but to a deficiency in their nerve supply. The 

 waltzing is a constant character of the race; the children of 

 waltzing mice mated together being all waltzers. 



When such a waltzing mouse is mated with a normal mouse 

 — with an ordinary white mouse, for example — the hybrid 

 obtained is always normal, like the white mouse. Waltzing, 

 therefore, is recessive, in the Mendelian sense, to normality 

 of progression. In F2 we should expect the waltzing to 

 reappear in the proportion of 25 per cent, of the whole 

 fraternity. Reappear it certainly does, but not quite in the 

 proportion expected. The number of waltzers, indeed, falls 

 below 20 per cent., in the many hundreds of F2 mice which 

 I have raised. I do not think that this deficiency is due to 

 any complication of the process of segregation in the germ-cells, 



