78 CROSSING. 



father and the normal progression of the mother, and among 

 their children we will meet with all four possible combinations, 

 with the combinations of colour and behaviour common to the 

 grand-parents, but also new combinations, black normals and 

 chocolate waltzers. In this case, where we are dealing with only 

 two genes, the remaining genotype being identical, and where 

 the action of each of the two does not interfere with the action 

 of the other, we have an instance of recombination of genes 

 which goes parallel with a corresponding recombination of pa- 

 rental characters. But if we mate a yellow-agouti mouse to a 

 black one, we will obtain two new forms, an agouti in F x and 

 tortoise in F 2 , and it will not be possible to speak of these new 

 forms as of recombinations of parental characters. 



Just as the Japanese waltzing mice differ from other domes- 

 tic mice not only in their most striking character, but also in 

 several others, the domestic rats, and the domestic guinea-pigs 

 and rabbits do not only differ from wild species in their most 

 striking and valued characters. They show traces of their ori- 

 gin by hybridization in certain characters, which are either 

 new, or which the}' have in common with different related wild 

 species. 



The ordinary albino, and hooded black tame rats, bred for 

 laboratory purposes are commonly looked upon as varieties of 

 the Norway rat, differing from it in a few genes. It is true that 

 they will hybridize with this wild rat, and that the hybrids are 

 fertile. But they are nevertheless consistently different from 

 the Norway rat, unless recently crossed with it. These hooded 

 rats originated in Japan, and the way in which they have been 

 produced is obscure. They may have originated from a cross 

 between Mus norvegicus with its oriental representative or with 

 some other oriental rat. We have examined a strain of albino 

 rats bred from wild albino variants of the Norway rat, caught 

 in the sewers of Paris. These albino rats were true Norway rats 

 in everything but colour, and they were very different indeed 

 from the real pure-bred laboratory rat from Japan. Nobody 

 could mistake them. The laboratory rats, when pure, are very 



