74 BREEDING 



The result, then, of crossing a pure albino mouse 

 with a fawn-and-white, pink-eyed (waltzing) mouse is a 

 mouse which difiers very little from the common house- 

 mouse. As a matter of fact, it is usually paler beneath 

 than the house mouse is ; and the wild race which 

 it most resembles exists in St. Kilda, an island far 

 out in the Atlantic, to the west of Scotland. Be 

 this as it may, the result is a typical instance of 

 reversion on crossing. These reversionary mice con- 

 stitute the first hybrid generation. 



When these hybrids are mated together they 

 produce a generation (the second hybrid generation) 

 which consists of a variety of forms which can be 

 classified in three groups (A, B, and C), which corre- 

 spond roughly to the albino, the fawn-and-white, pink- 

 eyed (waltzing) mouse, and the hybrid. Let us 

 examine them more closely in conjunction with the 

 figures of them on Plate II. For the present, mice 

 Nos. 4, 6 and 10 will be left out of account, and 

 attention will only be paid to those individuals in 

 each group which are below the label indicating the 

 group. 



Mice included in Group A correspond exactly to 

 one of the parental forms, namely the albino ; there 

 is, therefore, only one mouse (below the label) in 

 Group A. These mice are indistinguishable from the 

 pure albino mouse. 



Mice included in Group B correspond to the 

 reversionary forms which constitute the first hybrid 

 generation ; but the correspondence between analo- 

 gous types in the two generations is much less 



