94 EVOLUTION OF BRITISH CATTLE 



plants, but, as similar results have been obtained 

 again and again with animals, we may substitute 

 animals for plants. When white cattle are mated 

 with red, their progeny (first crosses) are roan ; 

 but when these roans are mated together, a quarter 

 of their progeny return in colour to the white 

 parent race and a quarter to the red parent race, 

 while the remainder are roans again, like their 

 parents. Mendel's theory explains such phe- 

 nomena. He conceived the idea that an animal 

 carries, from its very beginning, determinants 

 which are going to decide, one its eventual colour, 

 another its size, another its length of limb, and 

 so on ; and that a half of each determinant is 

 inherited from each parent. Each determinant 

 is therefore, as it were, bicellular, bouble-barrelled. 

 A roan animal's red parent carries a double- 

 barrelled determinant for redness, which we may 



represent thus : , while its white parent carries 

 another for whiteness, which we may represent 



thus : 



O 



When a red animal is mated with a white, 

 their determinants meet, and the young, taking 

 a half of that offered by each parent, starts off 

 with a determinant, one half of which is for red- 

 ness, the other half for whiteness, thus: , and 



