194 



EVOLUTION AND ANIMAL LIFE 



kernels, white kernels, kernels of intermediate bluish-white tint 

 and kernels streaked with blue and white. The streaked flowers 

 and kernels of these two cases are due to mosaic inheritance. 



Or the apparent 

 dominance of the 

 contrasting charac- 

 teristics may be 

 proved to have 

 something of real 

 dominance about it, 

 as Miss McCracken 

 has so clearly shown 

 in her studies of 

 the inheritance of 

 dichromatism in the 

 little beetles Lina 

 lapponica (Fig. 117) 

 and Gastroidca dis- 

 similis. Here the 



FIG. 116. At left an ear of field corn; at right at) ear of 

 sweet corn; and in the middle a hybrid of these two, 

 showing alternation of kernels resembling those of 

 each different parent. (After de Vries.) 



first two or three 

 generations behave 

 in true Mendelian 

 manner, but with 



successive generations the dominant character is plainly seen to 

 be gradually extinguishing the recessive character in the cross- 

 bred groups, so that in the seventh generation after the original 

 cross-mating the Mende- 

 lian ratio of 2 to 1 in 

 the cross-bred group is 

 changed to 28 to 1. 



There may occur also 

 a breaking up or decom- 

 position of the parental 

 varietal characters, which 

 may mean that the domi- 

 nant and recessive char- 

 acters are not simple 

 ones but are complex- 



i. e., really the resultant of several combined characteristics; or 

 it may mean that there exists a real instability in the parent 

 type and that the stimulus or influence of the cross-mating is all 



FIG. 117. Lina lapponica, showing its two forms, 

 one black and one spotted. (After McCracken.) 



