No. 594] BLENDING AND MEN DELI AX INHERITANCE 333 



gradation of forms connecting one parental mode with the 

 other. The most numerous of these intermediate forms 

 will in F 4 and later generations be that which is midway 

 between the modes of the respective parents. 



This case throws a flood of light on the nature of blend- 

 ing and of Mendelian inheritance and of their relations to 

 each other. In typical Mendelian inheritance determiners 

 of allelomorphic characters may meet each other genera- 

 tion after generation in a common zygote, separating 

 again in gametogenesis without apparent modification of 

 either in consequence of their conjugation in a hetero- 

 zygote. This is well illustrated in the color inheritance 

 of animals and plants. 



In typical blending inheritance the determiners of con- 

 trasted parental conditions apparently blend into a deter- 

 miner of intermediate character, the gametes formed by 

 an Fj individual being practically as uniform in char- 

 acter as those of either parent individual. Blending is 

 illustrated in the inheritance of ordinary size differences 

 in birds and mammals (Castle, 1916). 



A third type of inheritance must now be recognized 

 which is a compromise between these two, for it exhibits 

 Mendelian segregation of the contrasted parental condi- 

 tions but with modification due to partial blending of the 

 unlike determiners in the F x zygote. The blending in- 

 creases and evidences of segregation decrease with every 

 generation during which the contrasted characters re- 

 main in conjugation. Consequently with every genera- 

 tion of inbreeding or self-fertilization following a cross 

 of this sort, a stable intermediate class is more and more 

 closely approached until its realization is complete. See 

 Marshall (1916) on the Corriedale breed of sheep. The 

 existence of this third type of inheritance was pointed out 

 by Castle and Forbes (1906) in. the case of hair-length in 

 guinea-pigs and by Castle (1906) in the case of polydactyl- 

 ism in guinea-pigs. The opinion was then expressed that 

 ''more characters fall in this category than in any other." 

 Hoshino's observations on flowering time in peas (if I 



