536 PROCEEDINGS OF THE AMERICAN ACADEMY. 



obtained by crossing different garden varieties. Yellow color of coty- 

 ledons was found to be dominant over green ; likewise, round smooth form 

 of seed was found to be dominant over angular wrinkled form ; and 

 violet color of blossoms, over white color. Other illustrations might be 

 mentioned both among animals and among plants, but these will suffice. 



2. Peculiar hybrid forms. The law of dominance is not of universal 

 applicability ; Mendel does not so declare, though some of his critics have 

 thus interpreted him. In many cases the cross-bred offspring possess a 

 character intermediate between those of the parents. This Mendel found 

 to be true when varieties of peas differing in height were crossed. 



Again, the cross-breds may possess what appears to be an intensifica- 

 tion of the character of one parent, as when in crossing dwarf with tall 

 peas the hybrid plant is taller than either parent, or as when, in crossing 

 a brown-seeded with a white-seeded variety of bean, the offspring bear 

 beans of a darker brown than that of the brown-seeded parent. 



Thirdly, the cross-bred may have a character entirely different from 

 that of either parent. Thus a cross between spotted, black-and-wliite 

 mice, and albino mice, produces commonly mice entirely gray in color, 

 like the house-mouse. Again, in crossing beans, a variety having yellow- 

 ish-brown seeds crossed with a white-seeded variety yields sometimes 

 black mottled seed, a character possessed by neither parent. 



These three conditions may be grouped together by saying — the hybrid 

 often possesses a character of its own, instead of the pure character of one 

 parent, as is true in cases of complete dominance. The hybrid character 

 may approximate that of one parent or the other, or it may be different 

 from both. There is no way of predicting what the hybrid character in 

 a given cross will be. It can be determined only by experiment, but it is 

 always the same for the same cross, provided the parents are pure. Often 

 the hybrid form resembles a supposed ancestral condition, in which case 

 it is commonly designated a reversion. Illustrations are the gray hybrid 

 mice, which are indistinguishable in appearance from the house-mouse, 

 and slate-colored pigeons resulting from crossing white with buff pigeons. 



3. Purity of the germ-cells. The great discovery of Mendel is this: 

 The hybrid, tvhatever its own character, produces ripe germ-cells which 

 bear only the pure character of one parent or the other. Thus, when one 

 parent has the character A, and the other the character B, the hybrid 

 will have the character AB, or in cases of simple dominance, A(B)* or 



* The parenthesis is used to indicate a recessive character not visible in the 

 individual. 



