VARIATION AND HEREDITY 129 



period of very active experiment, in connec- 

 tion with which Bateson, Castle, Cuenot and 

 their collaborateurs have been especially 

 prominent. 



Mendel worked chiefly with the edible pea, 

 Pisum sativum, which has many well-marked 

 varieties and is habitually self-fertilized. 

 When he crossed a giant variety of 6 to 

 7 feet with a dwarf variety, f to 1^ feet 

 high, the offspring were all tall. The charac- 

 ter of tallness which appeared in the hybrid 

 generation (Fj), to the exclusion of dwarf ness, 

 was called by Mendel the "dominant" 

 character, the other being "recessive." 



The tall cross-bred peas were left to self- 

 fertilize, which corresponds to close inbreed- 

 ing in animals, and in their progeny there 

 were tails and dwarfs in the average pro- 

 portions of 3:1. 



When the dwarfs of this F 2 generation were 

 allowed to self-fertilize, their offspring (F 3 ) 

 were all dwarfs, and further generations bred 

 from them were also all dwarfs. They may 

 be called pure recessives, being "pure" as 

 regards dwarf ness. 



But when the tails of the F 2 generation were 

 left to self -fertilize, their offspring (F 3 ) were of 

 two kinds: one-third of them (pure domi- 

 nants) produced tails only; two-thirds of 

 them (impure dominants) produced tails 



