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 possibilities 
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, 
