CROSSING. 69 



some way essentially alike, that they may be even part of one 

 pre-existing whole. But as soon as we know a little more about 

 such a set of genes, and especially from a biomechanic point of 

 view, that is, if we begin to see in which way, physiologically, 

 each tends to bring about its effect on the given quality, we 

 see that fundamentally they may have no more in common, 

 than they have with genes which influence quite different qual- 

 ities. In the mouse we know a good many genes which influ- 

 ence the colour of the eye. Some of these tend to make the colour 

 lighter, and some tend to make it darker. These genes have all 

 been studied in their effect upon the coat-colour, and as they 

 all materially affect this colour, it has been possible to study 

 them, to distinguish them, one from the other. Only incident- 

 ally it has been found, that they modify eye-colour. 



If someone had set himself to study eye-colour in mice, with- 

 out attention to coat-colours, he would have had enormous 

 difficulties. He might have found the approximate number of 

 the genes concerned in the variation of eye-colour in his mater- 

 ial, but without recourse to a study of the effect of his genes 

 upon coat-colour, he would never have been able to make a 

 complete analysis. Very probably he would have concluded 

 that these genes must be fundamentally analogues. 



Philippe de Vilmorin has studied quite a number of different 

 genes in the pea, which influence the degree of coherence of the 

 seeds in one pod. The influence of different genes proved to be 

 very different. In the first place, one gene was found to be 

 present in most peas, whose presence so modified the texture of 

 the seed-coat that even in the most favourable combination of 

 others, adjacent grains were almost always wholly free from 

 each other. In the absence of this gene, coherence of adjacent 

 seeds was a common phenomenon, but the degree of sticking 

 together was greatly influenced by other genes, in both direct- 

 ions, at least by six. 



The genes which in certain types produce the difference 

 between un-pigmented, yellow or green, and pigmented seeds, 

 and the difference between plants with rose and purple flow- 



