The Factor Hypothesis 69 



ture of genetics is full of such things as "intensifying" factors, 

 ''diluting" factors, and "(listril)ution" factors, all of which follow 

 the supplementary factor mechanism. 



Complementary factors are also common, quite as we should 

 expect. Any complex machine contains numerous parts, capable 

 of applying numerous dififerent forces, each one of which may be 

 quite functionless in itself, but, in interaction with some of the 

 others, will produce a visible result. The living organism is just 

 such a complex machine. 



It is not necessary that complementary factors exist only in 

 pairs. In corn itself there is an additional complementary factor 

 A for color of grain (Emerson 4). R, C, and A must all three be 

 present for the grain to have color. (A cross between the red 

 type AARRCC and the white type aarrcc would give a red Fi, 

 AaRrCc, and an F2 which would show a ratio of 27 reds 137 whites.) 

 Many sizable sets of complementary factors doubtless exist in the 

 organism without our knowing it. One can of course never iden- 

 tify a factor wdthout discovering a race in which this factor is 

 lacking. Where the character involved is an "indifferent" one, 

 races lacking one of the complementary factors are frequently 

 discoverable. Where, however, the character involved is vital 

 to the existence of the organism, it is impossible to discover a race 

 lacking one of the necessary complementary factors, since such a 

 race cpuld not live. In this connection a word should be said of 

 "lethal" factors. 



Lethal factors, of which a surprisingly large number have been 

 identified during the last few years of investigation, are by defini- 

 tion factors which bring death to the organism. One might 

 conclude from such a definition that these are inhibitory factors 

 which are inhibiting some vital function of the organism, but this 

 is practically never the case. In almost all cases, it is the homozy- 

 gous recessive condition only which brings the lethal eft'ect, so 

 that it is really the absence of the factor that is lethal rather than 

 the factor itself. How are we to interpret this behavior in terms 

 of the mechanisms which we have already described ? One might 

 assume that merely a simple determiner is involved, a determiner 

 for some vital function, so that its absence brings the lethal eft'ect. 

 On the other hand, it is altogether likely that more than one gene 



