BIOLOGICAL SCIENCE 59 



later. G. J. Mendel (1822-1884), Abbot of Briinn, 

 not satisfied with the theory of natural selection of 

 small variations as a sufficient cause of evolution, 

 investigated the effects of crossing different kinds of 

 green peas. 



Mendel's discovery consists essentially in the dis- 

 closure that in heredity certain characters are trans- 

 mitted as indivisible units. In peas the tall and 

 dwarf varieties show these phenomena. Tallness and 

 dwarfness here are opposite unit qualities, and the 

 products of a cross between tall and dwarf parents 

 are not intermediate in character. In the first gene- 

 ration all the seedlings are outwardly like their tall 

 parents : hence tallness is said to be " dominant " 

 over the " recessive " dwarfness. But, when self- 

 fertilised, these tall hybrids reveal their difference 

 with their dominant parent. Instead of breeding 

 " true," and giving consistently tall offspring, the 

 second generation is found to consist of tails and 

 dwarfs in the ratio of three to one. All the dwarfs 

 breed true in future, but of the tails one-third only 

 breed true, while two-thirds reproduce in the third 

 generation the mixed result of the second. 



These relations are explained directly and easily 

 by the supposition that the germ cells of the plants 

 are in character either " tall " or " dwarf," so that 

 a hybrid has some " tall " germ cells and some 

 " dwarf " but no intermediate variety. When a 

 dominant (D) tall plant is crossed with a recessive 



