222 



THE PERPETUATION OF ADAPTATIONS 



of the hybrid germ cells, whereby the yellow and green factors 

 are separated from one another, so as to produce germ cells 

 having only one factor for green or for yellow. This is known as 

 Mendel's principle of segregation. The hybrid ovule containing 

 both factors is said to be heterozygous, and the two factors for 

 the same character (here color) are called allelomorphs , one of 

 which is dominant, the other recessive. If only one type of 

 factors is present in an ovule, it is said to be homozygous. 



FIG. 95. Example of Mendelian inheritance in which the hybrid (Fi) is 

 intermediate between the two parents, but showing segregation of the two factors 

 in the germ cells giving in the F 2 generation the proportion of i : 2 : i. The cross 

 is made between white and red races of Mirabilis jalapa (the "four o'clock"). 

 The hybrid (F\) is pink, and these when inbred give white, pink, and red flowers 

 in the proportion of i : 2 : i (F 2 ). (From Morgan.) 



Now if the hybrid plants are self-fertilized, i.e., through union 

 of their own germ cells, and if segregation of factors occurs dur- 

 ing the formation of these germ cells in both anthers and ovules, 

 then the following combinations may occur (Fig. 94). The 

 green-bearing anther may unite with a green-bearing ovule, 

 and the result is green (homozygous) ; or a green-bearing anther 



