100 - CROSSING. 



be said to have proced the existence of a seventh gene, unless 

 we can prove by all the necessary breeding tests that this 

 number seven is not perhaps really number three, or that what 

 appears as a distinct gene G is not really a combination of B 

 and E. 



We firmly believe, that, if we want to do such work, we 

 must never lose sight of the fact that the action of a certain 

 gene upon the development is not always the same, but differs 

 with the set of other genes present and with the action of the 

 environment. If we meet a gene for the first time in yellow 

 mice, and we find that it distinguishes these yellow mice from 

 non-yellows, it does not follow that all yellow mice are yellow 

 because of the presence of this gene ; or that all mice with this 

 gene must be yellow. In fact, we positively know, that mice 

 without this gene can be yellow as a result of different combin- 

 ations of genes in the absence of the gene studied, and also, 

 that mice with the gene, may be albino or sable or very faint 

 pearl grey. It is bad enough to use the letter Y for this gene 

 (or G, J, K, according to nationality), but if we find an author 

 writing about the localization of Yellow in the chromosome, 

 we feel convinced that he should not be trusted very far in an 

 analysis of the other genes in the same material. For it is evi- 

 dent that, if such an author discovers a gene influencing the 

 number of scales on the tail, or the curvature of the claws, it 

 will not occur to him to make the necessary tests to prove that 

 this new gene is not in reality his gene Y. 



Every new gene can be accepted as such only, if its distinct- 

 ness from every other hitherto described gene is sufficiently 

 demonstrated. It is clear that, according to this standard, only 

 a very slight proportion of the genes name in Drosophila can 

 be accepted, and that the proofs which most of the authors 

 require for the acceptance of the novelty of a gene are abso- 

 lutely inadequate. If we did not know from the work with ro- 

 dents and with peas, how very striking can be the new charac- 

 ters produced by novel combinations of genes, and how very 

 often it is wholly impossible to judge of the particular com- 



