154 THE BREEDING OF ANIMALS 



In regressive mutations the animal actually loses 

 some hereditary character that it has formerly possessed. 

 An animal may be born with less than the normal number 

 of digits, and this may be transmitted to offspring. The 

 so-called tailless breed of cats is probably descended 

 from an individual born without a tail. A polled animal, 

 the offspring of horned parents, would also be an example 

 of regressive mutation. 



The term degressive has been applied to such varia- 

 tions as have existed in the previous ancestral history of 

 the animal or plant. When such variations reappear 

 after many generations, they are known as degressive 

 mutations. 



145. Importance of the mutation theory. The im- 

 portance of the mutation theory to the animal-breeder 

 lies in the fact that many of the most important and useful 

 qualities present in the domestic animals have probably 

 arisen through sudden variations or mutations. Such 

 mutations are likely to occur at any time. It is indeed 

 probable that such mutations do occur frequently. The 

 intelligent breeder who understands the laws of evolution 

 will clearly recognize the importance of such mutations. 

 The fact that they are certainly transmitted by heredity 

 has given to the breeder a most important method in 

 the permanent improvement of the domestic animals. 

 It is, of course, quite as true that mutations may be 

 degressive and thus actually be of less value than the par- 

 ent forms. Certain breeds are probably more variable 

 than others, and this fact may be both an advantage and 

 a disadvantage. It is a desirable condition in that mu- 

 tations may occur and some of these may be improve- 

 ments over the parent species. Such a tendency to vary 

 may be a disadvantage in that the variations may some- 



