No. 511] "PRESENCE AND ABSENCE" HYPOTHESIS 417 



Very neat laboratory experiments can be arranged to 

 illustrate this behavior, with any reaction in which a cer- 

 tain excess of one of the reagents is require.!, and while 

 these reactions will probably be in most cases of much 

 greater simplicity than those presented by the interaction 

 of the hereditary units, and they can, therefore, be con- 

 sidered only as presenting analogies, I am convinced 

 that such analogies are not unfair ones. 



It is especially easy to arrange an experiment showing 

 such a result in the case of certain organic substances 

 known as indicators, as litmus and phenolphthalein, for 

 here one needs to assume only that the single, unpaired 

 unit in the heterozygote produces such a quantity of acid 

 or alkali as will not quite change the character of the 

 cell-sap of the negative homozygote with respect to acid- 

 ity or alkalinity. Thus if 1 make an alkaline solution of 

 litmus and add to it as the product of one assumed unit, 

 A such a quantity of any acid as leaves the solution still 

 slightly alkaline, I may allow this to represent the hetero- 

 zygote. Then the homozygote possessing the acid-pro- 

 ducing unit A will have it in double quantity or intensity. 

 AVhen I add this second portion of acid to the solution it 

 is instantly changed from alkaline to acid, as is indicated 

 by a change from blue to red color. The negative homo- 

 zygote lacking the acid-producing unit and the two hetero- 

 zvgotes are alike blue, while the individual which is pure 

 with respect to this unit whose specific external manifes- 

 tation is the production of a red color, alone possesses 

 that character, and this results in a realization of the 

 ratio, 3 absences to 1 presence, or the dominance of ab- 

 sence over presence. This example has the advantage of 

 being conceivably duplicated in the case of many vege- 

 table color-characters, for the very widely distributed 

 anthocvan which gives the red and blue colors is an indi- 

 cator similar to litmus, and could have been used in this 

 experiment instead of litmus. 



Whether the situation here outlined is actually attained 

 in the case of red and blue flowers in nature can not per- 



