Species crosses in Rats. 117 



was impossible to uiulerstaiul how new eharacters could ever oii<rinate 

 from crossing. It is evident, that crossbreeding can recombine characters 

 which are each determined by a certain gene, if by the cross these 

 genes which until now had been present in different forms, could now 

 l)e brought together in one organism. 



The three novelties, four, if we count waltzing, can- have taken 

 their origin from ;i cross. Theoretically this is probable. The little 

 group may have consisted mainly of ¥■> animals. Whenevei- in these 

 wild rats an occasional hyljrid is produced, its chances for survival are 

 as those of its purebred relatives. In every generation only a fraction 

 of the rats born, can ever be parents of the next generation, unless 

 we are dealing with a suiall colony in exceptionally favourable 

 circumstances, which is expanding. The chances that tw^o hybrids mate 

 together are very small indeed, generally a hybrid will mate to an 

 animal of one of the parentspecies, and the genotypic aberration is lost, 

 the hybrid is swallowed up into the multitude. As a result of such a 

 cross, occasional matiugs between related rats may pro'duce a recessive 

 variety. In our case, the novel characters were not found to occur as 

 they usually do, one among a mass of normal animals. Five abei-rantly 

 coloured animals, of three colours were caught in one field. This makes 

 it more than probable, that they were produced in a colony descended 

 from hybrid rats. 



Hybrid rats are rare in nature: we do not remember ever having 

 caught a rat which could with any probability be taken for an Fi 

 animal. But we have seen that in our cages treerats and even one 

 time a fieldrat mated to our complex hybrids and produced offspring. 

 One of us has even observed the mating of houserat males with a female 

 treerat in a state of freedom. The unknown non- fieldrat ancestor of 

 our aberrant fieldrats must have been either a houserat or a treerat. 

 The production of so many novelties makes it improbable, that it can 

 have been a more closely related rat, such as a fieldrat from some 

 other locality. 



We saw, that the hybrids between the albino female and the 

 Sumatra fieldrat male had white bellies. Even among our wildcoloured 

 rats of the albino series, some had white bellies and some had the 

 normal belly colour of fieldrats, greywhite with darker underfur. This 

 means, that the albino female carried factor B. From our breedingwork 

 we know, that thß houserat lacks this factor, and that the treerat 

 possesses it. This points to the latter as to the species whose crossing 



