998 THE PHYSIOLOGY OF REPRODUCTION. 



selected and established. Such a process of selection is simple in 

 the case of the recessive characteristics, but in the case of the 

 dominant it is, of course, more difficult to distinguish between the 

 DD and the D(R). The distinction may be made by breeding 

 with an animal showing the recessive. If the dominant is pure, 

 all of the offspring will exhibit the dominant characteristics. If, 

 on the contrary, it is a hybrid, the offspring will be half dominant 

 and half recessive, according to the formula: 



D R 



| | = DR, DR, RR, RR. 

 R R 



The many attempts to verify this law in breeding have shown that 

 it expresses probably a great truth, although the application of it 

 to the practical purposes of breeding is beset with many compli- 

 cations. The newer experimental work in heredity has emphasized 

 the importance of breeding experiments made with what are known 

 as " pure lines," that is to say, with those plants or animals which 

 are capable of propagation without cross fertilization.* These 

 experiments have tended to prove that the characteristics of 

 each race or species are inherent in its germ plasm and will breed 

 true if not fertilized or mixed with germ plasm from another 

 individual of different origin. When there is cross fertilization, the 

 offspring are hybrids which exhibit some of the characteristics of 

 each parent. According to the Mendelian law, the unit factors 

 conveying these characteristics are sorted out or segregated in the 

 reproductive cells of the parents. If a cross fertilization is effected, 

 for example, between a white and a crimson flower of the same spe- 

 cies the offspring show an intermediate pink color. The fusion of 

 characteristics is, however, only apparent, for in the reproductive 

 cells of the pink hybrids the factors conveying the white and crim- 

 son characters will again be separated. That is to say, one-half 

 of the reproductive cells will have the factor for white and one-half 

 the factor for crimson. Two pink hybrids bred together will 

 produce offspring whose color characteristics may be predicted, 

 according to the Mendelian principle, as follows: 



Pink Hybrid Pink Hybrid 



WC WC 



/\ /\ 



/ \ / \ 



Pollen W C W C Ovules 



12 34 



Combination 1 and 3 will give a pure white. 

 Combination 1 and 4 will give a hybrid pink. 

 Combination 2 and 3 will give a hybrid pink. 

 Combination 2 and 4 will give a pure crimson. 



* For a general presentation see "American Naturalist," February and 

 March, 1911, Jennings and others. 



