No. 555] SIMPLIFICATION OF MENDELIAN FORMULAE 171 



"presence and absence" hypothesis of Bateson was the 

 second step which led to our present usage. On this hy- 

 potheses gray in mice is not the allelomorph of white, but 

 of no-gray; while the allelomorph of white is color, or 

 more properly speaking white is equivalent to no-color 

 and this is the allelomorph of color. 



Both of these steps have been amply justified by their 

 utility in making possible the prediction of the previously 

 unpredictable consequences of particular crosses. 



It was natural that in applying the presence and ab- 

 sence hypothesis the usage of Mendel should have been 

 retained, in accordance with which capital letters were 

 used as the symbols of dominant characters and small 

 letters as the symbols of recessive characters. But this 

 retention has involved most unfortunate consequences and 

 is, I believe, the real seat of our present difficulty. 



Mendel's small letters stood for realities as truly as did 

 the capitals. His A was a round form of pea, his a was a 

 wrinkled form of pea; his B was a yellow-seeded, his b a 

 green-seeded pea. But the significance of these terms has 

 been changed under the presence and absence hypothesis. 

 A still means a round pea, but a is simply a not-round 

 pea ; it may or may not be wrinkled. Likewise B is still 

 a yellow-seeded pea, but b is nothing but a not-yellow 

 pea; it may or may not be green under the presence and 

 absence hypothesis. For all that b signifies now, the pea 

 may be blue, violet, indigo or carmine. 



It is most unfortunate, therefore, that the small letters, 

 having lost their original significance, were not discarded 

 altogether, for under the presence and absence hypothesis 

 they have done nothing but cause mischief. 



The investigator who employs them starts out well in- 

 tentioned with a clear notion that the small letters stand 

 for negation only, that they are merely signboards to 

 show what characters he is talking about, but presently, 

 unless he is unusually careful, we find him talking about 

 them as if they stood for something, instead of nothing; 

 ne speaks of repulsions and couplings or associations 



