INHERITANCE, VARIATION AND SELECTION. 43 



IN-AND-IN BREEDING. 



Closely related to methodical selection, as practiced by man 

 on domestic animals, is the process of in-and-in breeding. In-and- 

 in breeding is the mating of closely related animals, often parent 

 and offspring, or brother and sister, for the purpose of fixing a 

 particular character upon a breed. Thus, when a new character 

 appears, the chances of having that character reappear in the off- 

 spring are very much increased if the character is common to both 

 parents. As new characters appear rarely, and as the probabilities 

 of the identical character simultaneously appearing in two unre- 

 lated animals of opposite sexes are extremely remote, the breeder 

 carefully watches the progeny of the newly varied individual until 

 he finds one of the opposite sex having the same peculiarity, and 

 then mates parent and child. Some, if not all, of the progeny of 

 a couple so mated are quite certain to also have the new character- 

 istics. By the careful selection and mating of these last offspring 

 the new characters are fixed firmly within a few generations. By 

 this process we have a new breed of animals, and as long as there 

 is no cross with animals outside the breed we have what is known 

 as "pure blood." As a consequence, all of our fancy breeds of 

 animals are the product of in-and-in breeding. Darwin considered 

 in-and-in breeding to be injurious when carried to a considerable 

 extent, and this is true when the evil effects are not eliminated by 

 judicious selection of the best individuals and the rejection of the 

 poorer ones. That selection is a full cure for any ill effects of 

 in-and-in breeding is abundantly proved by the superiority of pure 

 breeds over mongrel stock. 



EFFECTS OF MATING RELATED ANIMALS. 



The evil effects of in-and-in breeding arise from the fact that 

 in many animals there are certain dormant defects. When unrelated 



