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 ij 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 4 pure' : as 

 regards dwarfness. 



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 



