No. 511] "PRESENCE AND ABSENCE" HYPOTHESIS 411 



taneously with Correns's paper, but published several 

 months later, used the same expression, 6 and most recent 

 writers on Mendelian inheritance have adopted the 

 method of presenting- the characters in the terms of 

 presence and absence. 7 



But while this change of usage has gradually taken 

 place, little attention has been given to the real signifi- 

 cance of the newer method of statement, except by Hurst, 8 

 who gives a good general discussion of the presence and 

 absence hypothesis in a paper read before the Third In- 

 ternational Conference of Genetics two years ago. 



Hurst showed that of 44 Mendelian characteristics of 

 various plants and animals studied by him, 41, or more 

 than 93 per cent., can be appropriately described in terms 

 of presence and absence. As one reviews these various 

 characteristics, he can not avoid the feeling that in a 

 number of cases the presence and absence could be read 

 quite as well backward as forward, and it will doubtless 

 be impossible in many cases to decide which is the positive 

 character and which its absence. Thus in the contrast 

 between a yellow and a green pea, the yellow is described 



