160 THE THEORY OF EVOLUTION 



tor R and those of the white-flowering race the factor 

 W. The female hybrids produce egg-cells, one-half of 

 which contain R and one-half W; likewise the male 

 hybrids produce pollen-cells, one-half of which contain 

 R and one-half W. Fertilization consists in the fusion 

 of the nucleus of the male cell with that of the female 

 cell, as has been very often directly observed in all 

 kinds of animals and plants which have the sexual 

 method of reproduction. After fertilization the germ, 

 which is now ready to begin its development, contains 

 factors derived from both parents and, according to 

 the mathematical laws of probability and combina- 

 tion, there would be four combinations: RR, WR, 

 RW, and WW. The factor from the male parent is put 

 first, so that the combination RW means that the 

 red factor is derived from the male and the white 

 factor from the female; in WR this is reversed, 

 though, in practice, the result is usually the same in 

 either case. In the Jalap, where neither white nor 

 red is dominant, both RW and WR give the pink 

 flowers and, obviously in the proportion of one-half 

 the entire number of plants in the second hybrid gen- 

 eration. The germs having the factors RR give the 

 segregated red flowers and, as there is no W, it is 

 plain that all the descendants of the RR seeds, 

 generation after generation, if crossing is avoided, 

 will produce only red flowers. For the same reason, 

 the WW germs and all their pure-bred descendants 

 can give only white flowers. If the unit-characters 

 do not actually blend and become inseparable, it is 



