Experimental Hybridizing 69 
other, their offspring (F,) will be gray or white in the proportion 
of 3:1. If the white individuals are inbred, they will give 
only white, and this is true for all of their descendants. They 
are said’ therefore to be “pure.” The gray individuals, how- 
ever, show themselves to be of two kinds; one third of them, 
if inbred, produce only gray, and all of their descendants will 
be gray. They, too, are said to be “pure.” The other two 
thirds, if inbred, produce both white and gray mice. If these 
offspring are examined by further crossing, it is found that the 
whites are pure and give only whites; that some of the grays 
are “pure” gray, but the others are gray-dominant-white- 
recessives, A(B), and again in these we find the proportion 1 A : 
2AB:1B. The following scheme will show at a glance 
the succession of generations : — 
A B 
/ i ‘i =o a 
Fe ee N 
7 i a ris \ _ : 
a A A LA 2A iB 3B B B 
A practical consideration of some importance is obvious from 
these results. Pure races can be obtained from the hybrid, 
A(B), by selecting the offspring with “pure” germ-cells, A or B. 
On: the other hand, the A(B) hybrids always produce some 
A(B), so that all their offspring do not return entirely to the 
two parental types, but in every succeeding generation they will 
continue to split off some ‘‘pure” A’s and B’s. 
As has been stated, Mendel’s assumption in regard to the two 
kinds of germ-cells has been tested in other ways and found to 
