198 OUTLINES OF EVOLUTION AEY BIOLOGY 



plants are again cultivated and allowed to fertilize themselves we 

 find that every one of the R's breeds true, and no matter how 

 many generations we raise they will always remain of the 

 recessive type. These are now called "extracted recessives." 

 When, however, we cultivate the plants showing the dominant 

 character in the same way we soon find that, though similar 

 to one another externally, they are not in reality all alike, 

 for one-third of them will yield nothing but D's, while the 

 remaining two-thirds will yield D's and R's in the same 

 proportion of 3 to 1 as the original hybrid. Moreover, the 

 D's obtained from the one-third will continue to breed true 

 from generation to generation, and do what we will we can 

 never get an R out of them again. They are called " extracted 

 dominants." 



It is obvious then, from these experiments, that the apparent 

 D's are not all true D's, but that two-thirds of them contain the 

 recessive character in a latent or dormant condition, and are, 

 therefore, still hybrid in composition. If we indicate those which 

 we can thus prove by breeding to contain the recessive character 

 by the letters D (R), we can sum the whole story up in the following 

 simple diagram : 



D x R (Original Parents) 



Generation 



D D D D D D D R D D D R R R R R; - - F 3 



extracted dominants Extracted recessiues 



FlG. 79. Monohybridism. 



Thus we see that from generation to generation of the 

 offspring of the hybrid there goes on a constant sorting out into 

 three categories, the two original parental types and the hybrid 

 form being always produced in definite proportions. If we 

 assume that each kind produces on an average an equal number 

 of offspring, we shall see from the diagram that in the course of a 

 few generations of undisturbed reproduction by far the greater 

 number of the plants will have finally reverted, in equal propor- 

 tions, to one or other of the original types, but a certain number 



