218 THE BREEDING OF ANIMALS 



gan. 1 " For species with separate sexes the term ' in- 

 breeding ' is used to express either the union between 

 brothers and sisters or between offspring and parent, in 

 one or more generations/' Unfortunately the literature 

 on the subject of in-breeding has not placed such narrow 

 limitations on the term. 



It must be recognized that there are different degrees 

 of in-breeding. Animals may be closely in-bred as, for 

 example, results from the mating of parent and offspring, 

 or brother and sister. The union of more distant rela- 

 tionships, as third or fourth cousins, would not be expected 

 to show the same good or bad results in the offspring as 

 the more closely related parents. In the literature of 

 the subject, the discussions generally have reference to 

 the most intensive forms of close-breeding. The results, 

 good or bad, therefore, are those which may be expected 

 to follow the most intensive in-breeding. After all, the 

 real question of importance for the practical breeder to 

 answer is not whether any form of in-breeding should 

 be practiced, but to what extent it may be practiced and 

 its known advantages become realized. In a sense, prac- 

 tically every registered improved breed to-day is the 

 result of a certain amount of in-breeding. David Starr 

 Jordan has calculated that if no in-breeding of any degree 

 had taken place in the human race, each person born 

 in the thirtieth generation from William the Conqueror 

 would have had 8,598,094,592 living ancestors at the 

 time when the Conqueror was alive. 



204. Advantages claimed for in-breeding. The mat- 

 ing of animals having the same parentage, has resulted 

 in certain definite advantages to the race or breed. These 

 results are as easily demonstrable as are the results from 



1 Morgan, " Experimental Zoology," 1907, p. 186. 



