106 Hagedoorn. 



We will call E and F the two genes, which imist l)e absent 

 simultaneously to allow an animal to pi'oduce a white tailtij). (ir and H 

 are the two genes, whose simultaneous absence is responsible for the 

 waltzing character. Waltzing rats have originated repeatedly in our 

 experiments. Whenever they appeared first, it was always in a pro- 

 portion recalling one in sixteen rather than one in four. Rut, as will 

 be seen later, the matter must have been still more complicated in 

 fieldrats. 



This origin of waltzing rats throws light upon the origin of the 

 waltzing mouse. Until now, waltzing has never yet appeared in our 

 cultures as a novelty in mice, after a cross between two groups both 

 pure in respect to a set of genes consistent with normal progression. 

 But this may happen any day when we cross housemice of different 

 provenience. 



Whereas waltzing housemice are absolutely deaf, we found that 

 waltzing rats can hear apparently normally. Waltzing houserats, or 

 rather waltzing rats produced from houserats and treerats, were more 

 nearly helpless than the waltzing fieldrats we obtained later. For in- 

 stance we never once succeeded in obtaining a single living young from 

 a waltzing houserat, the mother iuvarial)ly killing them at the moment 

 of parturition, notwithstanding all our precautions. Waltzing houserats 

 can not climb in the least. Care has to be exercised in selecting nest- 

 material, as they will often entangle their hindfeet in fibrous material 

 when waltzing in the nest. It was found impossible for normal males 

 to mate with the rapidly twirling waltzing females. Waltzing males 

 however did succeed in mating with waltzing females, as they could 

 follow them around as fast as they could go and gradually caught up 

 with them. Waltzing males could breed with normal females. 



I and J we will call the two genes, which must be absent, for an 

 animal to be chocolate instead of black, or cinnammon instead of agouti. 

 We had both whitebellied and darkbellied cinnammons, but all our 

 chocolates were darkbellied, like the blacks and the silvers. 



We experienced considerable difficulty in breeding rats of this 

 group. They are sensitive to low temperature, and w^hereas young 

 females born in spring will sometimes breed the same season, those 

 born later in the summer might be too young to breed that year, and 

 would be apparently past their prime next summer. In the beginning 

 the rats would not breed in cages at all, and we had to devise small 

 ratproof rooms full of suitable material, such as baskets and straw, to 



