6o Outline of Genetics 



producing red, but lacked the necessary oxidase, it would remain 

 colorless. If the other white strain contained the oxidase but no 

 chromogen, it too would be colorless. In crossing them, however, 

 chromogen and oxidase would be brought together and a red- 

 grained hybrid would be the result. In breeding, such red-grained 

 individuals would of course give red and white progeny in a ratio 

 of 9:7, as explained in connection with East's corn. 



It should be realized that the foregoing is no more than a 

 suggestion. So far as the genetics of the situation is concerned, 

 complementary factors may be regarded as an established fact; 

 but what either one of these factors actually amounts to, in 

 physiological terms, has as yet only been guessed at. In fact, it 

 would be safe to state that there is no known case where the exact 

 physico-chemical nature of any factor or determiner has been 

 demonstrated. In the foregoing instance, neither chromogen 

 nor oxidase may be the effective units involved. There are, how- 

 ever, plenty of possibilities within the field of chemistry where 

 the bringing together of two inert substances initiates a reaction 

 sufficient to result in a new plant "character." Within limits, 

 hybridizing is very much like mixing chemicals in a test tube. 



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 Darwinian idea of usefulness as 

 explaining the occurrence of everything in a plant is frequently 

 inadequate. One must think rather of a plant as a complex physi- 

 co-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. 



The other alternative is to suppose that these factors origi- 

 nated independently in the plant's history. In this case, of 

 course, the first to be produced would remain functionless until 

 its complement came into existence. This might be an explana- 

 tion of what have been called "latent" characters. Also, not 

 only might they have originated at different times but in different 

 varieties or species. In this case, if natural hybridizing should 



