(>54 



THE AMERICAN NATURALIST 



[Vol. XLVI 



right in the midst of a useful career? The experiments 

 least open to objection (the pure-line experiments) have 

 shown the wisdom of assuming a stable unit factor, this 

 factor being representative of the stability manifested 

 by a character complex when no interfering conditions 

 intervene. Let us accept this simple interpretation pro- 

 visionally, appreciating the fact that the stability of the 

 characters that have been represented by fixed units may 

 be only a static appearance due to limited experiments ; 

 but that this appearance justifies our neglecting any 

 infinitesimal fluency of our factor standards in experi- 

 ments of like duration, since taking them into account 

 would necessitate a change of standard, a new fabric of 

 hypotheses and a more complicated system. Let us take 

 a physiological view of heredity. Factors are assumed 

 to be stable. Characters are somewhat unstable owing 

 to the effect that other factors have upon their expres- 

 sion. Factor A, for example, is potentially able to pro- 

 duce a typical expression in ontogeny under certain defi- 

 nite conditions of environment, but the presence or ab- 

 sence of B or C or D or B, C, and D are responsible for 

 slight changes in the expression of A. This conception 

 gives us a picture of heredity in real accordance with 

 physiological facts, in contradistinction to the non-bio- 

 logical and fixed physical conception — the mosaic organ- 

 ism conception— that critics often say is held by some 

 geneticists. 



One may answer that this conception is all right for 

 quantitative characters, but do the facts uphold it for 

 qualitative characters? They do. I will give examples 

 from my own experiments on the inheritance of the 

 purple aleurone cells in maize. Here one obtains prog- 

 eny by the thousands and sees phenomena that are ob- 

 scured by lesser numbers. 



Crosses of the purple variety with three different 

 whites have given three different results. One shows 

 that the purple may be represented by the schematic de- 

 scription PPRRCC. Crossed with pp rr cc it gives 

 purples, reds and whites in the F 2 generation, as all three 



