CROSSING. 77 



waltzing mice have looked upon these animals as upon a var- 

 iety of the house-mouse, Mus musculus. Droogleever For- 

 tuyn however, found that the ordinary Japanese waltzing-mice 

 had characters which distinguished them Zoologically from 

 Mus nusculus, and made it appear to be a variety of the 

 Oriental house-mouse, Mus Wagneri. The short tail and small 

 size of these mice are not due to the genotypic aberration which 

 causes the forced movements. We found that in cross-breeding 

 Japanese waltzers with normal mice, small size, tail-length 

 and waltzing were independent characters, which could be 

 found differently combined in F2 individuals. Recently, we 

 found that the ordinary tame mice of the Orient which we 

 imported from Hong Kong and Japan, had the same size and 

 tail-length as waltzers from those regions. It is very probable, 

 that the cause of the production of the first waltzing mice has 

 been a cross between house-mice of different countries, for 

 instance between Mus musculus and native house-mice of 

 Japan. The same cross may have produced other novelties, 

 such as the pink-eyed coloured forms which are derived from 

 Oriental mice, and are frequently met with among oriental 

 stock. 



The de Vries Weismann speculations have intimately per- 

 meated Genetics, and it is not surprising that almost every- 

 body thinks of hybridization only as a way in which existing 

 characters may be recombined. Genes are by the majority of 

 Geneticians still looked upon as determinants for characters, 

 and recombination of genes is thought to be the same thing 

 as recombination of characters. 



If, however, we think of genes as of things transmitted 

 through the germ, which may in given circumstances, and in 

 given combinations of other genes influence the development, it 

 becomes clear at once that recombination of genes often must 

 be the cause of new characters, dominant characters as well 

 as recessive ones. 



If we mate a black waltzing mouse to a chocolate normal, 

 the young will have a combination of the blackness of the 



