Species crosses in Rats. 99 



When we see, that the difference between a certain yellow rat 

 and one of agouti colour is caused by presence in one and absence 

 from the other of one single gene, this gene is not an agoutimaking 

 gene per se. For we find, that it is the same gene, which produces 

 the difference between silver and black, between orange and chocolate. 

 On the other hand, we know, that not all agoutis or blacks distinguish 

 themselves from corresponding yellows by the presence of the same 

 gene; different agoutis may differ from yellows by the possession of 

 different genes. The habit of using letters that recall characters, to 

 designate genes is helping to perpetuate the conception of genes as 

 determinants for characters. 



New hereditary characters are due to novel combinations of genes. 

 A gene which has no appreciable influence upon the development of 

 the individuals of species A may have quite an appreciable influence 

 upon B. To give an example, the gene B, which is present in white- 

 bellied agouti, but not in darkbellied agouti rats, has no influence upon 

 the colour of black i-ats. When we mate a darkbellied agouti species, 

 e. g. alexandrinum with a black possessing B, e. g. rattus the result 

 will be the production of whitebellied agouti young in F2, those which 

 contain B. 



The dominant yellow in the housemouse series, is due to a gene, 

 which is not present in housemice of the other colours. But it may 

 have been derived by crossing from a related non-yellow species, whose 

 further set of genes was such, as to give the animals a colour not made 

 yellow by our gene. 



In the genetic work with colours in the housemouse, the authors 

 have generally striven to use the same symbols for the same genes, 

 or at least, when using different symbols, to point out what these stood 

 for in terms of the symbols of the other authois. 



This, however is not entirely defensible. If we could be sure 

 that the gene, which in the work of one author diffeientiated coloured 

 from albino mice, were identical with that which produced such a 

 difference in the material of a second author, it would be altogether 

 right to denote this gene by a common name. Of this, however, we 

 can not be sure without actually making breeding tests. And if we 

 omit these tests we do more harm than good by trying to use identical 

 symbols for these two genes. For such a procedure must actually hinder 

 us from recognizing the fact, that in different series, different genes 

 may produce similai- effects. A case in point as illustration. In the 



