Variation and Heredity 481 



crossed should possess differentiating or contrasting 

 characters; that the hybrid offspring should offer the 

 possibility of being readily protected from foreign pollen ; 

 and that the offspring should be, with respect to fertility, 

 unaffected by the inbreeding process necessarily pursued. 



He found in the common garden pea (Pisum sativum), 

 and other related forms, promising material for his work. 

 After testing for two years the constancy of thirty-four 

 varieties, proceeding with great care, he selected those 

 which showed well-defined contrasting characters, in all, 

 seven combinations. We may consider three of the typical 

 cases with a single differential character-pair (later termed 

 simple allelomorph, or allelomorphic pair) as follows : 



(1) Difference in color of cotyledons ; yellow vs. green. 



(2) Difference in the color of the seed coats ; white vs. 

 colored. 



(3) Difference in size ; tall vs. dwarf. 



Like Darwin and others who were interested in a more or 

 less similar line at the same time, he recognized the neces- 

 sity of dealing with large numbers in order that individual 

 errors might be avoided as far as possible. From the 

 crosses obtained with the strains showing the contrasting 

 characters above mentioned, he found that with respect to 

 these characters all of the hybrids resembled one of the 

 parents ; that is, there was no blending of these qualities. 

 The character which appeared in the hybrid of the first 

 generation, known as the F x generation, was termed the 

 dominant of the pair, and that character which was veiled 

 or latent in the F t was termed the recessive. In the three 

 cases above the dominant characters were yellow cotyle- 

 dons, colored seed coat, and tall habit. No transitional 

 2i 



