218 THE PERPETUATION OF ADAPTATIONS 



yellow -bearing anther may unite with a yellow -bearing ovule 

 giving a homozygous yellow; or finally a yellow-bearing anther 

 may unite with a green-bearing ovule giving a heterozygous 

 yellow. Thus there will be three yellows to one green or 

 one pure yellow, two heterozygous yellows and one pure green 

 (Fig. 94, F2 generation). 



Mendel found further, that these pure greens if continually 

 self-fertilized never gave rise to yellow peas and that the pure 



PARENTS { 



o 



"0 er o 



FIG. 96. Diagram to illustrate the history of the gametes of crossed white and 

 red Mirabilis. A gamete with factor for white and one with factor for red unite 

 to form the pink zygote of Fi. < The gametes in FI are homozygous for red or 

 white, and these, by random mating, give the Mendelian ratio. (From Morgan.) 



yellows never gave rise to green peas while the mixed yellows 

 and greens on self-fertilization always produced offspring in the 

 proportion of three yellow to one green. 



A similar result is obtained with white and red races of 

 Mirabilis jalapa, the "four o'clock" in which the hybrid (F), 

 is pink (Figs. 95 and 96). 



The same result may be worked out theoretically along the 

 lines of Weismann's hypothesis of the significance of maturation. 

 The chromosomes which unite at pseudo-reduction contain 



