56 HEREDITY [ch. 



dominant tallness, he called ' recessive.' More recent 

 work has indicated that a dominant character pos- 

 sesses some factor which is absent in its recessive 

 alternative ; in the present example the stem has 

 the power of continued growth which is absent in 

 the short pea. Dominance and recessiveness may 

 thus be regarded as presence and absence respectively 

 of the factor in question ; but since the presence or 

 absence of the factor may often give rise to the 

 appearance of an alternative pair of characters, 

 such a pair have been named by Bateson a pair of 

 ' allelomorphs.' When a tall pea is crossed with 

 a short, the factor tallness is introduced from the 

 tall parent, and thus all the offspring are tall. These 

 are called the first filial generation, or more shortly 

 the generation F x . When these hybrid (2^) tails are 

 self-fertilised, their offspring (second filial or F 2 

 generation) consist of tails and shorts. Xow it has 

 been seen that if the factor tallness is present it 

 makes itself visible, and therefore the short peas in 

 F 2 should contain no tall factor. And in fact when 

 self-fertilised, or fertilised with the original short 

 stock, they give only short offspring for as many 

 generations as the experiment has been carried to. 

 The tall factor has thus apparently been completely 

 eliminated from these short peas. 



Further, Mendel found that among the tails in the 

 F 2 generation, some breed true to tallness when self- 



