NATURAL SELECTION 45 



pure white mice. If two of these white mice were bred 

 together they had white offspring, and the same was true 

 in breeding again from their young, generation after gen- 

 eration, showing that they were of pure strain without 

 admixture from the gray variety, though the original 

 parents in the first cross were one gray and one white. 

 It is of great interest to note that, in spite of the cross- 

 ing of the two varieties, there appeared in the later gen- 

 erations certain individuals which were of pure blood, 

 showing no trace of the admixture which we would expect 

 to find resulting from the cross. Extensive experiments 

 in breeding showed that the results were to be interpreted 

 as follows : a gray mouse, G, bred with a white mouse, 

 W, gave offspring which seemed to be all gray, but were 

 really a mixture of gray and white, the gray character 

 being dominant and the white character obscured, or 

 " recessive," as Mendel called it. That is G x W gave 

 G ( W\ G ( W\ G ( W}, etc., the parenthesis indicating that 

 the white character was recessive. This hidden complex 

 nature of the second generation (the young from the first 

 cross) was clearly indicated when they were bred together. 

 It was found that their offspring were of three sorts, and 

 that these three kinds were in definite and constant 

 numerical proportions. G ( W} x G (W} gave offspring 

 i G + 2 G ( W) + i W, one-fourth being pure gray, one- 

 fourth pure white, and one-half apparently gray, but really, 

 as further breeding showed, gray and white, the white 

 character being recessive and obscured. These numerical 

 proportions held true for an extensive series of experiments 

 in the case of white mice, as they had done in the experi- 

 ments of Mendel upon certain plants. 



