420 READINGS IN EVOLUTION, GENETICS, AND EUGENICS 



and the compound the enzyme attacks, as in the oxidase and chromo- 

 gen situation just described. On the other hand, we might be dealing 

 with two chemical compounds that are inert when occurring separately 

 but active when brought together, active in such a way as to produce 

 a distinctly new character. Also two active substances might neutral- 

 ize one another when brought together in a hybrid, and the failure in 

 their activity might result either in a new character or the failure of 

 some parental character to develop. Such are some of the possible 

 mechanisms to explain the behavior of complementary factors. 



Hybridizing, therefore, is much like mixing chemicals in a test 

 tube. We know that very wide crosses cannot be made successfully; 

 but the surprising thing is that certain very close crosses are constantly 

 unsuccessful, even though both parents may cross freely with closely 

 related types. We obtain a glimpse of the possibility of such appar- 

 ently inconsistent behavior when we consider the chemical possibiUties 

 suggested by the behavior of complementary factors. 



The origin of complementary factors is an interesting field of 

 speculation. Did they originate together or separately? A natural 

 inference would be that they originated together, for neither would be 

 of any use without the other. It should be remembered, however, 

 that the idea of use as explaining the occurrence of everything in a 

 plant is being abandoned; one must think rather of a plant as a com- 

 plex physico-chemical laboratory. No one claims that all chemical 

 reactions are useful; they are simply inevitable; and plant characters 

 are the result of chemical reactions and physical necessities. Even 

 though we assume the simultaneous origin of two complementary 

 factors, they would have to be put on separate chromosomes, for the 

 factors are separately inherited. 



The other alternative is to suppose that these factors originated 

 independently in the history of a plant. In this case, of course, the 

 first one to be produced would remain functionless until finally its 

 complement came into existence. This might be an explanation of what 

 are called latent characters. Also they might have not only originated 

 independently but in different varieties or species. In this case if 

 natural hybridizing should bring them together the result would be 

 the appearance of a new character, and this may have been a very 

 important factor in the origin of species. 



This may serve as an introduction to the factor hypothesis, with 

 complementary factors as an illustration, simply because it is the 

 simplest situation. There are many other kinds of factors recognized, 



