130 EVOLUTION 



rediscovery of the buried paper and to a period 

 of very active experiment, in connection with 

 which Bateson, Castle, Cue'not 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, } to 1$ feet 

 high, the offspring were all tall. The charac- 

 ter of tallness which appeared in the hybrid 

 generation (F^, to the exclusion of dwarfness, 

 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 inbreeding 

 in animals, and in their progeny there were tails 

 and dwarfs in the average proportions 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 dwarfness. 



But when the tails of the F 2 generation were 

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

 two kinds : one-third of them (pure dominants) 

 produced tails only; two- thirds of them (impure 

 dominants) produced tails and dwarfs in the 



