CROSSING. 99 



know, and a few hundreds which we do not know, its presence 

 or absence makes the difference between a black and a non- 

 black individual. We can study its inheritance in groups of 

 animals where its presence does not produce black colour at 

 all. In all those groups, in all those genetic formulae of ani- 

 mals with and without this gene, there is no real advantage to 

 be made to remember that this is the gene, whicr would pro- 

 duce black colour if genes number 2 and 3 and 4 5, 8, 9 and 

 12 were present and numbers 6, 7, 10 and 1 1 were absent. Cal- 

 ling it 1, or 13 would be simpler. 



Calling a gene by a name taken from some character which 

 it may help to produce in certain combinations, has its grave 

 dangers. It produces an association between the idea of the 

 character and the gene and it makes for a false simplicity in 

 the work. Some people are even to-day wont to think of genes 

 as of determinants for particular characters. To take a few- 

 examples from the work of the Drosophila authors Notch and 

 Purple and Peach are obviously names of characters of cer- 

 tain flies. But the names are also used to denote certain genes 

 which are thought to be always present, or always absent 

 when these characters are seen. So long as N and P do not 

 pretend to be more than our A and B, symbols for genes de- 

 monstrated in the analysis, the use of the two first letters 

 rather than the two others is wholly a matter of taste. But 

 when we read of the localization in the chromosomes not of 

 genes A and B, or genes N and P, but of Notch, and of Peach, 

 we begin to see that the use of these names for genes has ham- 

 pered the author in his thinking about these genes and their 

 re.ation to the characters of the flies, which he also calls Notch 

 and Peach. 



From experience we know how very laborious is the work of 

 disentangling the several genes which can be demonstrated 

 in our material. 



When we know six mutually distinct genes, the amount of 

 work to be done, before we are satisfied that a seventh is really 

 distinct from all the other six, is apalling. And yet we cannot 



