176 HEREDITARY CHARACTERS 



accounts for the many cases where dominance is imperfect, 

 and where traces of the cross remain in extracted dominants 

 and recessives. Such cases are common. 



Although dominance appears to be perfect in the case of 

 some plants, we find that in animals the first generation of 

 hybrids usually differs from both parents. Instead of only 

 the dominant character appearing in the impure dominant, 

 reversion to the character of a more or less remote ancestor 

 frequently occurs. Thus when waltzing mice are crossed 

 with albinos, the first generation of hybrids are coloured like 

 the wild house-mouse. 1 In some cases the first generation 

 of hybrids are something in the nature of a blend between 

 the two parents. Thus when black and white fowls are 

 crossed, a blue Andalusian is sometimes produced, with a 

 minute patchwork of black and white. Again, the dominant 

 character may show to a great extent, but the recessive also, 

 though much less. When white Leghorn poultry are crossed 

 with brown Leghorn, most of the offspring have some ticks 

 of colour. 2 Very often the extracted dominants and re- 

 cessives, that is, those individuals in the second and third 

 generation, which in the case of peas appear to have segre- 

 gated completely and possess only the dominant or recessive 

 character, show traces of the cross. " Very frequently, if not 

 always, the character that has once been crossed has been 

 affected by its opposite with which it was mated and whose 

 place it has taken in the hybrid. It may be extracted there- 

 from to use in a new combination, but it will be found altered. 

 This we have seen to be true for almost every character suffi- 

 ciently studied. . . . Everywhere unit characters are changed 

 by hybridism." 3 



With regard to the characters that follow Mendel's law, 

 the assumption that discrete entities representing these 

 characters exist in the individual and are transmitted to 



1 Darbishire, "On the Results of Crossing Japanese Waltzing Mice with 

 European Albino Races," Biometrilca, ii. pp. 101-65 and 282, 1902. 



2 Punnet, R. C., Mendelism, Cambridge, 1905. 



3 Davenport, Inheritance in Poultry, p. 80. 



