148 DOMESTICATED ANIMALS AND PLANTS 
shown in Fig. 25. Here we have a hybrid offspring from an 
albino sire and a black dam. The offspring are all black, so 
black is dominant over white. Their offspring are, however, of 
two kinds, both black and white, but in the proportion of 3 to 1. 
Of this group of four, therefore, only one, the white, can with 
certainty be counted upon to breed true. Some of the blacks 
will also breed true, but only the breeding test will determine 
which they are. 
This whole matter is up in full force in all attempts at im- 
provement by crossing, whether among plants or animals, which 
is the reason why animal breeders especially avoid this form of 
breeding, though it is a favorite method in the improvement of 
plants, which can be produced in large numbers.! 
When the parents differ in two unit characters, the case is 
more complicated, but the principle remains the same, namely, 
that all possible combinations will occur and a perfectly definite 
number of each may be expected. Again, Professor Castle’s work 
with guinea pigs illustrates the point especially well. 
In Fig. 26 are shown a dark-colored smooth-haired and an 
albino rough-haired parent. Their offspring were a// dark and 
rough as shown in the middle figure, but some of their progeny 
were smooth and white as shown in the lower figure, while others 
were like each of the original parents, and still others like the 
first hybrid; that is, all possible combinations had been made. 
In this case the Mendelian expectation is 3: 3:9: 1. 
Pure races may spring from crossing. The facts just pre- 
sented show that for characters that blend, the hybrid will breed 
pure as a single new race, but that for characters that do not 
blend, the individuals may or may not be pure and may or may 
not breed true. 
All the facts go to show that whether the offspring of hybrid 
parents consist of three groups as when only one character is 
involved, or whether they consist of many groups as when two or 
1 The student of breeding should understand, however, that crossing is 
equally effective with animals and plants, except that the very large numbers 
involved makes it too expensive for most individual animal breeders. 
