FIXING GOOD TRAITS 29 



These white, smooth-coated individuals are 

 obviously different from their parents, and 

 different also from either of their grandparents. 

 They constitute a new race, sprung into being 

 in a single generation, and a race that will neces- 

 sarily breed true as to the character of smooth 

 coat and white coat, because they are "homozy- 

 gous" as to the factors for both these recessive 

 characters. 



Their progeny cannot be black because their 

 germ plasm contains no hereditary factor for 

 blackness; nor can their progeny be rough- 

 coated, because their germ plasm contains no 

 hereditary factors for rough-coatedness. 



Yet, side by side with this new fixed race of 

 smooth-coated white guinea pigs, there are, as 

 we have seen, twins of the same fraternity that, 

 instead of being white and smooth in color, are 

 black and rough. And these also constitute a 

 new race that will breed true because they 

 contain, as regards the unit character for color 

 and for condition of hair, only the dominant 

 factors of blackness and roughness. They also 

 are "homozygotes," but they are of the opposite 

 type dominants instead of recessives. 



Meantime, we must not overlook the other 

 members of the fraternity, twin brethren of these 

 new races, which are individually black and 



