52 2 BIOLOGY AND HUMAN LIFE 



parents in several ways, but has many useful qualities. He can 

 stand more hardship than the horse, can do much more work on 

 less food, can stand rougher roads, and is very sure-footed. He 

 has some disagreeable traits also, but his good points are enough 

 to make people tolerate him. It is impossible to establish a pure 

 race of mules, however, for the simple reason that this hybrid 

 is incapable of reproducing itself ; each generation of mules is 

 raised anew by crossing a mare and an ass. If the mule were 

 not sterile, it is probable that his offspring would ''split" into 

 several kinds of individuals showing various combinations of 

 horse and ass characters. 



380. Menders principles apply to animals. As we have al- 

 ready seen, Mendel's principles of heredity apply to animals as 

 well as to plants. A knowledge of these principles has com- 

 pletely changed the practice of animal breeding during the past 

 twenty-five years. The fact of dominance is well illustrated by 

 the horn of cattle. Occasionally there appears an animal with- 

 out horns ; the Polled Angus was a mutation of this kind. The 

 absence of horns is a desirable quaHty. If a polled, or hornless, 

 individual is mated with one that has horns, all the offspring 

 will be without horns ; a pure-bred hornless bull may thus be- 

 come the ancestor of whole herds of hornless cattle; but if 

 hybrid polled animals are mated, the following generation will 

 show segregation in the way already described for the yellow- 

 green pea and other plant characters (see section 373). The 

 table on the opposite page shows how succeeding generations of 

 sheep or cattle will behave with respect to the appearance or 

 non-appearance of horns. 



In some species of animals the presence of pigment is domi- 

 nant over the absence of pigment— as in rabbits, mice, and 

 guinea pigs. Among the white leghorn fowls, however, white- 

 ness is dominant to pigmentation. In the Andalusian fowl, as 

 we saw above, there is no complete dominance either of white- 

 ness or of pigmentation, but the segregation is complete. The 

 short-haired coat of guinea pigs is dominant to the long-haired 

 coat (see table on page 524). 



