90 MANUAL OF FABM ANIMALS 



work they may undertake the breeding of pure-bred animals 

 with much better prospects of success. 



The great advantage of grading is that it is cheap, particu- 

 larly when accomplished with the male. For example, a 

 pure-bred bull, in a herd of twenty-five cows, will make half- 

 bloods of the entire crop of offspring, whereas if the grading 

 was attempted in the other way, it would require twenty-five 

 pure-bred cows and the calves would show no more improve- 

 ment. Furthermore, if the improvement were accomplished with 

 the cows, it would be in twenty-five lines, each with its shade 

 of difference, and not in one line as would be the case were the 

 grading done with the bull. This is a very important factor 

 when we consider the value o^ uniformity among all classes of 

 farm animals. 



Crossing, as we have seen (p. 77), consists in mating two 

 distinct breeds or families, and is a powerful means of inducing 

 variability. Because of its strong tendency to produce varia- 

 tion, crossing is very seldom employed as a system of animal 

 breeding. Attention is directed to it in this connection because 

 it is the mode employed in breeding mules, the offspring of a 

 jack and mare. The reciprocal cross, or the stallion on the 

 jennet, gives an offspring called hinny. It is interesting to 

 note that the mule more nearly resembles the jack, having 

 long ears, large head, no hairs in mane and tail, and very 

 small feet, whereas the hinny resembles the stallion, having a 

 rather small head, rather short ears, hairs in mane and tail, and 

 large feet similar to the horse. 



While we have mules and hinnies of both sexes, they are 

 sterile; that is, one cannot produce mules by mating a male 

 with a female mule, nor will the mule breed with either the jack 

 or stallion. The only way to produce a mule is to mate jack 

 and mare. 



Line-breeding consists in mating animals representing a 

 single line of descent. This system of breeding is used either in 



